Gravity Pipe Flow Calculator
Manning equation capacity for sewer lines, storm drains and culverts — full or partially full pipe, any slope, any n. Slope to flow or flow to slope, with a self-cleansing velocity check on every result.
Gravity flow inputs
Enter the grade and get the capacity and velocity.
Sewer and drain pipe is sized by inside diameter — the list uses the nominal bore, or type any custom ID.
The smooth bore of extruded PVC and ABS gives the lowest common n.
= 1.00% = 0.0100 ft per ft
Results
Self-Cleansing ✓Flow capacity
783 GPM
= 49.4 L/s = 1.745 cfs
Velocity
5.00 ft/s
= 1.52 m/s
Water area A
0.3491 ft²
= 0.0324 m²
Wetted perimeter P
2.094 ft
= 0.638 m
Hydraulic radius R
0.1667 ft
= 2.00 in
Manning: Q = (1.486/n) · A · R2/3 · S1/2 — uniform open-channel flow, water at normal temperature.
Paste straight into a test report or takeoff sheet.
Which flow method fits a gravity line?
Four roads lead to a flow number, and they are not interchangeable. Picking the wrong one is the single most common error in drainage estimates — here is the map before the math.
The pattern behind the table: gravity flow is an energy problem, not a pressure problem. The water falls, gravity pays the bill, and the bill is friction plus geometry. Manning's equation is simply the accounting rule that balances the slope's energy against the pipe's roughness and shape — which is why it owns the gravity column and why Hazen-Williams, a pressure-side formula, does not belong there. When you need the pressurized twin of this calculation — flow driven by pressure instead of grade — the pipe flow rate calculator covers that side of the house.
Five steps to a sewer capacity figure
The cards below re-compute from the calculator above — change the size, material, grade or fill depth and every number updates. The baseline case: an 8 inch PVC sewer main at a 1% grade, flowing full.
Measure the pipe geometry
Take the inside diameter (8" in the worked example) and the fill condition. Full pipe: A = π·D²/4 = 0.3491 ft² and wetted perimeter P = π·D = 2.0944 ft for D = 0.667 ft.
Compute the hydraulic radius
R = A / P = 0.3491 / 2.0944 = 0.1667 ft, which equals D/4 for any full circular pipe. For partial depth the calculator derives the segment area and perimeter automatically.
Pick Manning's n
Smooth PVC is n = 0.009 in the example; concrete or clay is 0.013 and corrugated metal 0.024. The lower the n, the more the same pipe carries.
Set the slope
Convert the grade to ft/ft: a 1% slope is S = 0.01. Flow scales with the square root of S, so doubling the grade increases capacity by about 41%.
Compute the flow
Q = (1.486/0.009) × 0.3491 × 0.1667^(2/3) × 0.01^(1/2) = 1.745 cfs ≈ 783 GPM (49.4 L/s), giving v = Q/A = 5.00 ft/s — comfortably above the 2 ft/s self-cleansing floor.
Reading the result like an engineer: velocity comes first. At 5.00 ft/s this line scours grit on its worst day and stays under the 10 ft/s erosion-noise ceiling, so the geometry survives review. Capacity comes second: 783 GPM full-flow is the ceiling for peak-hour comparison, not the everyday figure — sewers are designed to run well below full so the crown stays ventilated. Half-depth on this grade still moves about 392 GPM at the same 5.0 ft/s, which is why a sewer flowing half-full is a healthy sewer, not an underused one.
Gravity flow charts: sewers, drains and culverts
Three tables that answer most capacity questions without typing: the minimum grade that keeps a line self-cleansing, what a partially full pipe still delivers, and quick culvert figures. All rows are Manning full-flow values on the same basis as the calculator.
Minimum slope for self-cleansing (2 ft/s, flowing full)
Derived from Manning at exactly v = 2.0 ft/s — the standard self-cleansing target. The concrete column (n = 0.013) is the traditional sewer-main basis; the PVC column (n = 0.009) shows how much less grade smooth plastic needs. Slopes shown in percent; flow is the capacity at that grade.
| Pipe size | Min slope — concrete n=0.013 | Min slope — PVC n=0.009 | Flow at that grade |
|---|---|---|---|
| 3" | 1.23% | 0.59% | 44 GPM |
| 4" | 0.84% | 0.40% | 78 GPM |
| 6" | 0.49% | 0.23% | 176 GPM |
| 8" | 0.33% | 0.16% | 313 GPM |
| 10" | 0.25% | 0.12% | 490 GPM |
| 12" | 0.19% | 0.09% | 705 GPM |
| 15" | 0.14% | 0.07% | 1,102 GPM |
| 18" | 0.11% | 0.05% | 1,586 GPM |
Partially full pipe: what fraction of capacity remains?
The ratios below are size-independent — they apply to any circular pipe running by gravity. Depth is measured as y/D, the water depth over the diameter. Note the velocity crossing 1.0 at half depth and peaking near 81%.
| Depth y/D | Flow q/Q full | Velocity v/V full |
|---|---|---|
| 25% | 0.137 | 0.70 |
| 50% | 0.500 | 1.000 |
| 75% | 0.912 | 1.133 |
| 90% | 1.066 | 1.124 |
The two maxima worth memorizing
Total flow peaks at about 94% depth, at 1.076 × full-flow capacity — a brim-full pipe is not the maximum-flow condition, because the added sliver of top area costs more perimeter than it gains area.
Velocity peaks even earlier, near 81% depth, at 1.140 × the full-pipe value — which is why designers often cap peak operating depth at 75–80%: the pipe scours best just below full.
Culvert quick table — concrete, full flow
Manning full-flow figures for round concrete pipe (n = 0.013) at two common grades. Treat these as the outlet-control ceiling; short steep culverts are usually throttled by their inlet instead.
| Culvert size | 0.5% grade | 1% grade | ||
|---|---|---|---|---|
| Capacity | Velocity | Capacity | Velocity | |
| 12" | 1,131 GPM | 3.21 ft/s | 1,599 GPM | 4.54 ft/s |
| 15" | 2,050 GPM | 3.72 ft/s | 2,899 GPM | 5.26 ft/s |
| 18" | 3,334 GPM | 4.20 ft/s | 4,715 GPM | 5.94 ft/s |
| 24" | 7,180 GPM | 5.09 ft/s | 10,154 GPM | 7.20 ft/s |
What is gravity pipe flow?
Gravity pipe flow is water moving through a pipe because the pipe itself is tilted — no pump, no pressure zone, no push. The free water surface inside the line stays open to atmosphere, the depth adjusts until friction exactly consumes the elevation the water gains, and the whole system settles into what hydraulicians call uniform open-channel flow. Every sanitary sewer, storm drain, highway culvert, agricultural drainage line and building drain works this way. It is the oldest form of piped water conveyance — Roman aqueducts ran on the same physics, tuned with nothing but surveying stakes and patience.
The consequence of "free surface" matters more than people expect. A gravity main is almost never full: a typical design runs the daily flow at half-depth or less, which means the flowing area, the velocity and the capacity all change as the flow changes. That is fundamentally different from a pressurized main, where the pipe is always full and the pressure does the varying. It is why gravity hydraulics keeps its own formula family (Manning, Chezy, the gradually-varied-flow profiles) rather than borrowing from pressure pipe practice — and why this page exists separately from the pressure-flow tools.
Gravity also explains the two design instincts every sewer engineer carries. First, slope is life: without grade there is no energy, and without energy there is stagnant, septic, sulfide-generating water. Second, slope is expensive: every additional percent of grade deepens the trench, and a foot of extra excavation on a mile of 12 inch main is real money. The entire craft of gravity design is landing between those two — steep enough to scour, flat enough to afford. The calculator above exists to make that landing precise.
Where the water goes
Sanitary sewers, storm sewers, roof and area drains, culverts under roads, field tiles and irrigation tailwater returns — all free-surface, all Manning territory.
What governs capacity
Three levers only: the bore (area grows with diameter squared), the grade (capacity grows with the square root of slope) and the roughness n. Diameter is the strongest lever of the three.
The Manning equation for pipe flow
Q = (1.486 / n) × A × R^(2/3) × S^(1/2)
Q — flow (cfs) · n — roughness coefficient · A — water cross-section area (ft²) · R — hydraulic radius (ft) · S — slope (ft/ft)
Each term earns its place. The 1.486 is a unit correction — it exists only because the equation was derived in metric with a coefficient of 1.0, and 1.486 converts the result to US customary feet and seconds; if you work entirely in meters and seconds the leading factor is just 1.0. The exponent on R — two thirds — encodes a century of channel measurements showing how drag scales with depth, and the square root on S encodes the energy balance: gravity supplies head proportional to distance, friction absorbs it, and the reachable velocity grows with the square root of the balance.
For a full circular pipe the geometry collapses pleasantly: A = π·D²/4 and R = D/4. Feed those in and Manning becomes a two-variable formula — diameter and slope, with n as the material multiplier. That collapse is what powers the quick tables above, and it is also the source of the most common beginner error: plugging the nominal size where the inside diameter belongs, or forgetting that R is A ÷ P and not simply the radius of the pipe.
What Manning does and does not assume
- Uniform steady flow — the depth and velocity stop changing along the reach.
- Turbulent, rough-boundary flow — true for nearly all practical sewer and culvert sizes.
- A straight, prismatic reach — constant shape and grade for the length considered.
- It does not model backwater: a downstream obstruction, a surcharged outfall or a flat pool at the outlet invalidates the answer for that reach.
- It does not model entrance control: a short steep culvert's capacity is set by its inlet acting as an orifice, not by barrel friction.
Hydraulic radius and wetted perimeter of a pipe
The hydraulic radius R is the single most misunderstood term in the equation, and it is just a ratio: the cross-section area of the flowing water divided by the length of the perimeter that touches it. The word "radius" is historical bad luck — for a full pipe it equals D/4, not the pipe radius. Its meaning is geometric efficiency: how much water cross-section each foot of wetted drag supports. A deeper, narrower stream carries more flow per foot of friction than a wide thin one at the same area, and R is the number that says so.
For a partially full circular pipe the segment formulas do the work. With θ = 2 × acos(1 − 2y/D) the central angle in radians, the area is A = (D²/8)(θ − sin θ), the wetted perimeter is P = θ·D/2, and R = A/P follows. The calculator runs this geometry on every partial-depth input, which is what makes the y/D slider honest — the half-depth case really does return R = D/4, and the 25% depth really does collapse capacity to 13.7% of full-flow.
Full pipe
R = D / 4
12" pipe → R = 0.25 ft
Half full
R = D / 4
Same R — same velocity
Any depth
R = A / P
Segment formulas above
Wetted perimeter deserves its own caution. In a gravity line it is only the surface the water touches — the arc of the bore below the waterline — never the full circumference. Counting the dry crown is a real error seen in the wild, and it understates capacity at low depths badly. The same discipline applies to the area: it is the segment of the circle below the water surface, not the bore area.
Manning's n for pipe materials
The roughness coefficient n is where material choice enters the calculation, and it enters with full force: capacity scales as 1/n, so a material change from concrete (0.013) to PVC (0.009) buys roughly 44% more flow through the same hole. The table lists the values in steady use for design. Age them honestly — the n you design on should be the one the pipe will have in year thirty, not year one.
| Material | n | Design note |
|---|---|---|
| PVC / plastic (smooth) | 0.009 | The smooth bore of extruded PVC and ABS gives the lowest common n. |
| HDPE (smooth bore) | 0.010 | Smooth-wall HDPE is nearly as slick as PVC; ribbed or corrugated HDPE is not. |
| Steel pipe (new, uncoated) | 0.012 | Mill finish steel plate and pipe; riveted or badly pitted steel is higher. |
| Concrete (straight, clean) | 0.013 | The classic sewer-outfall value for well-made concrete and cement pipe. |
| Vitrified clay / sewer brick | 0.013 | Traditional vitrified clay tile with good joints sits at the same n. |
| Corrugated metal | 0.024 | Annular corrugations roughly triple the drag of smooth wall — capacity drops hard. |
| Cast iron (tuberculated) | 0.015 | Old, tuberculated iron grows n over its life; design on the aged value. |
Two traps hide in this table. The first is corrugated plastic: a corrugated HDPE drain tile shares a polymer name with smooth-wall HDPE but hydraulics of n = 0.024 — a 62% capacity penalty against PVC at identical grade. The second is the aging of metal and concrete: tuberculation, sediment film, joint intrusion and root penetration push effective n upward through the pipe's life, which is why conservative designs carry n = 0.015 for old iron even when the catalog says 0.012. When a capacity figure must survive a review, the defensible move is computing with the aged value and letting the new-pipe number be the upside surprise.
Note also what n does not touch: velocity and capacity scale together with 1/n, so the self-cleansing check moves in the same direction. A smoother pipe reaches 2 ft/s on a flatter grade — visible in the minimum-slope table above, where the PVC column sits at roughly half the concrete values.
Full pipe vs partially full pipe
The most quoted curiosity in sewer hydraulics: a circular gravity pipe reaches its maximum flow not when running full but at about 94% depth, where it moves roughly 7.6% more than full-flow capacity. The reason is pure geometry. As the water climbs the last few percent toward the crown, the added sliver of area is narrow while the added wetted perimeter is long — friction grows faster than area — and the hydraulic radius drops. At 94% the area-perimeter trade still nets positive; at 100% it does not.
Velocity tells an even sharper story. It peaks near 81% depth at 1.14 times the full-pipe value, and at exactly half depth the velocity equals the full-pipe figure — because a half-circle keeps the same R = D/4 as the full circle. Flow at half depth, though, is only 50% of full capacity, because the area is genuinely halved. The partial-flow table above summarizes the whole curve.
Design practice builds directly on these facts. Peak operating depth is typically capped at 75–80% of the diameter for two reasons at once: it keeps the crown ventilated (sewer gases need somewhere to move, and a brim-full pipe seals its own airway), and it buys hydraulic headroom for the diurnal peak plus the storm inflow that every sanitary line secretly collects. So while the calculator will happily report full-flow capacity, the professional reading of that number treats roughly 80% of it as the practical working ceiling — and the partial mode exists precisely to check how a real operating depth performs.
Half-depth sanity check
8 inch PVC at 1%, half full: A = 0.1745 ft², P = 1.0472 ft, R = 0.1667 ft — identical R to full — so v stays 5.00 ft/s while Q halves to about 392 GPM. If a vendor claims "half the depth means half the velocity," this is the counterexample.
Sewer pipe capacity in practice
"Sewer pipe capacity" sounds like a single number, but any honest answer has three coordinates: size, material and grade. A 10 inch clay sewer at 0.28% is a different animal from the same size in PVC at the same grade — and both are different from a 10 inch line laid at 0.5%. The calculator plus the tables on this page resolve the trio consistently, because they all draw on the same Manning arithmetic and the same n-values.
Capacity also lives on a schedule. Sanitary flow arrives on a daily curve — low overnight, two peaks morning and evening — so a sewer sized for the peak spends most of its life at quarter depth, checking the low-end velocity limit instead of the high one. That is why self-cleansing matters at average flow, not peak: a line that only reaches 2 ft/s at maximum hour lets grit settle every night. The fix is either a steeper grade (expensive) or a smaller diameter (cheap), which is why good sewer design is as much about not oversizing as anything else.
The third dimension is what the line collects. Storm systems see pulses that sanitary lines never do, and combined sewers see both — so their capacity checks are probabilistic, paired with a design-storm return period rather than a fixture count. The Manning math on this page is identical in all three cases; what changes is the flow you feed it.
Grade in, capacity outPhoto: UnsplashThe velocity window that rules the design
Gravity design is a two-sided velocity test wearing a capacity costume. The floor is 2 ft/s — below it, sand and grit settle, and the line begins building its own permanent constriction. The ceiling is near 10 ft/s, where abrasive solids start eroding inverts and hydrogen sulfide turbulence turns the crown into a corrosion farm.
The calculator badges every result against exactly this window: green at 2–10 ft/s, amber below 2 as a deposition risk, red above 10. A capacity figure outside the window is not a design — it is a maintenance schedule.
Flushing note: lines that cannot hold 2 ft/s on grade are sometimes given periodic flushing regimes instead — a legitimate trade-off where trench depth is impossible, and a poor trade where it is not.
Storm drain and culvert sizing
Storm drains and culverts share Manning geometry with sanitary sewers but face a harder input problem: the flow arrives as a design storm, not a fixture count. The storm side of the work — rational-method Q = C·i·A estimates, time of concentration, return-period selection — produces a peak GPM that you then feed into exactly the calculator above to check whether a candidate size on a candidate grade carries it. That two-step pattern (hydrology decides Q, hydraulics decides size and slope) is the whole trade.
Culverts add one more fork: inlet versus outlet control. A long or flat culvert runs barrel-controlled, and the Manning figures in the quick table are the right ceiling. A short steep culvert under a high fill runs inlet-controlled — the entrance edge, not barrel friction, throttles the flow, and the barrel could carry three times what gets in. Length under 50 diameters and slope above about 1% are the usual symptoms of inlet control; proper treatment needs entrance-loss coefficients and is a step beyond this page's uniform-flow scope. Use the Manning number as the generous bound and verify the inlet separately for short installations.
Storm drain checklist
- • Peak inflow from the design storm, not the average
- • Depth capped near 80% to keep the airway open
- • Junctions and bends evaluated for head loss separately
- • Outfall tailwater checked for submergence and backwater
Culvert checklist
- • Barrel capacity from Manning (tables above)
- • Inlet control check for short, steep installs
- • Fish passage and embedment where required
- • Debris and blockage allowance on small diameters
Minimum slopes and self-cleansing
The minimum-slope table earlier in this page was built by inverting Manning for the grade that yields exactly 2.0 ft/s flowing full — the classic self-cleansing floor. Read it as physics, not regulation: the slopes are what the equation demands, while your local sewer authority publishes enforceable minimums that are usually close, occasionally stricter, and always the ones that govern a real submittal.
Household plumbing keeps a cruder but related tradition: building drains at 1/4 inch of fall per foot (2.08%) for sizes up to 3 inches, and 1/8 inch per foot (1.04%) for 4 inches and larger. Both exceed the self-cleansing requirement for their sizes comfortably — the household rule exists because fixture flows are intermittent, and an intermittent line needs spare velocity to survive its dry spells. The same logic explains why long public sewers can legally run flatter: their continuous base flow never lets the solids sit.
One more interaction worth knowing: slope trades against diameter. Because capacity scales with D^(8/3) at fixed grade (full pipe, fixed n — a consequence of substituting A and R=D/4 into Manning), one step up in size buys capacity that would otherwise demand dramatically more grade. When a design misses self-cleansing, the cheap fix is usually not steepness — it is one size smaller. The Flow → Slope mode of the calculator is built for exactly this negotiation.
Gravity flow vs pressure flow
The same pipe can run in either regime, and the numbers diverge immediately. Gravity flow spends elevation for velocity; pressure flow spends psi. A force main at 40 psi moving 300 GPM through 4 inch pipe and a gravity sewer moving the same 300 GPM on a 0.5% grade are different physics wearing identical hardware — different formula, different failure modes, different vocabulary ("capacity" vs "headloss", "grade" vs "pressure class").
The practical boundary is the water surface. Free surface inside the pipe — open channel, Manning territory. Pipe flowing full under a downstream control or a rising main — pressure territory, and the Darcy-Weisbach friction accounting of the pressure drop calculator takes over. A gravity sewer that surcharges at its outfall has, at that moment, become a pressure pipe: its capacity stops being a Manning question and starts being a backwater one. Designers keep peak depth below the crown precisely so this regime change never happens by accident.
For the pressure side of a pumped system — sizing the force main, checking velocity and friction — the pipe flow rate calculator handles velocity and Q=V·A directly for pressurized mains, and the pipe size calculator works the reverse problem of finding a diameter for a target flow. This page keeps the gravity lane: grade in, capacity out.
Common mistakes in gravity flow calculations
Using outer diameter for the bore
Manning cares about the water channel. For sewer pipe the nominal size is normally the inside diameter, but for pressure-class pipe the ID runs smaller — an SDR 21 8 inch line carries a 7.16 inch bore, about 20% less area than a true 8. Capacity scales with D^(8/3), so that "small" difference costs nearly half the flow.
Confusing percent with degrees or ft/ft
A "2% slope" is 2 feet per hundred — 0.02 ft/ft — which is about 1.15 degrees. Entering 2 as ft/ft (a 200% grade) inflates capacity by a factor of ten. The calculator's slope unit selector converts percent, ft/ft, in/ft and 1:X ratios onto one honest scale.
Designing to full-flow capacity
The full-flow figure is a reference point, not an operating plan. Real designs keep peak depth near 80% for ventilation and headroom — so a line whose peak-hour load computes to 95% of capacity is already overcommitted before the first inflow surprise.
Forgetting the downstream boundary
Manning assumes the reach can actually deliver — no backwater from a submerged outfall, a full receiving ditch, or a downstream bottleneck. When the tailwater sits above the crown, the reach is surcharged and the true answer comes from backwater profiling, not this page.
Mixing n from the wrong pipe generation
Design on the n the pipe will have when it is inspected, not when it is installed. New concrete at 0.013 may be 0.015 by year twenty-five; smooth plastic holds its 0.009–0.010 for life. The conservative entry is the aged one — capacity upside is welcome news, the reverse is a rehab project.
Gravity pipe flow FAQ
How do you calculate gravity flow in a pipe?+
Use the Manning equation: Q = (1.486/n) × A × R^(2/3) × S^(1/2), with A the water cross-section area, R the hydraulic radius (A divided by wetted perimeter) and S the slope in ft/ft. For a full 8 inch PVC pipe (n = 0.009) at a 1% slope the area is 0.3491 ft², R is 0.1667 ft, and the flow works out to 1.745 cfs — about 783 GPM — at 5.00 ft/s. The calculator above runs the same arithmetic for any size, material, slope and fill depth.
What is the capacity of an 8 inch sewer pipe?+
It depends on the material and the slope. An 8 inch PVC sewer main (n = 0.009) flowing full at 1% carries about 783 GPM (1.745 cfs) at 5.0 ft/s; the same size in concrete (n = 0.013) carries about 542 GPM at the same grade. At half-full the PVC line passes roughly 392 GPM, and at a typical design slope of 0.33% even the concrete version still moves 2 ft/s. Capacity scales with the square root of slope, so half the grade buys about 71% of the flow.
What slope does a 4 inch drain need?+
Two answers, two jobs. For household drains the trade rule is 1/8 to 1/4 inch of fall per foot — 1.04% to 2.08% — which keeps 4 inch building drains well above the 2 ft/s self-cleansing threshold. For a public sewer main designed on Manning with concrete (n = 0.013), a 4 inch pipe needs about 0.84% to hold exactly 2 ft/s flowing full; in PVC (n = 0.009) it needs about 0.40%. Steeper is never harmful for small lines — it only costs you trench depth.
What is Manning's n for PVC pipe?+
Design practice uses n = 0.009 for smooth-wall PVC and ABS, and about 0.010 for smooth-bore HDPE. That smoothness is why a plastic sewer outfalls more flow than the same size in concrete (n = 0.013): flow scales inversely with n, so the PVC line carries roughly 44% more water on the same grade. Do not apply 0.009 to corrugated plastic — corrugated walls behave like n = 0.024, a capacity loss of more than half.
How do you calculate the hydraulic radius of a pipe?+
Divide the water cross-section area by its wetted perimeter. For a pipe flowing full, R = D/4 — a full 12 inch pipe has A = 0.785 ft², P = 3.14 ft and R = 0.25 ft. For a partially full pipe, compute the circular segment: with θ = 2 × acos(1 − 2y/D) radians, A = (D²/8)(θ − sin θ) and P = θ·D/2, then R = A/P. A half-full pipe has the same R as a full one — the classic Manning surprise.
How much water can a 12 inch culvert carry?+
On outlet-controlled full-flow Manning numbers, a 12 inch concrete culvert (n = 0.013) at a 0.5% grade passes about 1,131 GPM (2.52 cfs) at 3.2 ft/s, and at 1% about 1,599 GPM (3.56 cfs) at 4.5 ft/s. Real culvert capacity is often set by the inlet instead — the entrance acts as an orifice — so treat these figures as the upper bound and check inlet control for short, steep installs.
Does a half-full pipe flow slower than a full one?+
No — at exactly half depth it flows at the same velocity as a full pipe, because the half-circle of water keeps the same area-to-perimeter ratio (R = D/4 in both cases). Velocity actually peaks around 81% depth at about 1.14 times the full-pipe value, and total flow peaks near 94% depth at about 1.076 times full-flow capacity. A pipe running literally brim-full is not the maximum-flow condition — that is one of Manning's most quoted results.
What is the minimum slope for a gravity sewer?+
Design practice targets at least 2 ft/s of self-cleansing velocity at least part of the day. With concrete (n = 0.013) that implies roughly: 6 inch at 0.49%, 8 inch at 0.33%, 10 inch at 0.25%, 12 inch at 0.19%, and 15 inch at 0.14%, flowing full. Smooth PVC reaches the same 2 ft/s on about half the grade — 8 inch at just 0.16%. Local sewer authorities publish their own enforceable minimums, so confirm before finalizing a design.
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