Weymouth EquationResidential Natural Gas & Propane

Gas Pipe Calculator — Flow Rate & BTU Sizing

Estimate how much natural gas, propane or LPG a given pipe size can deliver — in cubic feet per hour and BTU per hour — using the Weymouth equation with velocity sanity checks. Enter your pipe, run length and pressure drop; get the nearest size with a one-stepper derivation you can follow.

Derivation Overview

Steps

Q = 37·3600 × √(D / SG·L) × √(P₁² − P₂²)

Core formula

± 5–8%

vs. code tables

Open the calculator Walk the derivation BTU capacity chart Safety & caveats FAQ

Gas Pipe Sizing WorkbenchWeymouth + velocity check

Gas fluidPipe size (Sch 40 steel, ID)Run length (ft)Inlet pressure classPressure drop across the run (PSI) — must be less than inlet pressure

Result

Flow rate

4,385 SCFH

= 73.1 SCFM · 1.22 SCFS

BTU capacity

4,385.1 kBTU/h

= 4,385,097 BTU per hour · Natural gas (SG 0.60)

Velocity

86.2 ft/s

⚠ HIGH — step up a pipe size

Pressure regime

Low pressure (< 0.5 PSI / 14 in WC)

Drop 0.100 PSI = 40.0% of inlet

Step-by-step derivation — Weymouth in four moves

Engineering formula

Run length → feet; pressure class → PSI; SG and BTU per ft³ pulled from the fluid preset.

Tip: SG is not density

SG is density relative to air (1.225 kg/m³). Natural gas at 0.60 SG means 0.735 kg/m³ — Weymouth uses SG directly, so never substitute g/cm³ without dividing by 1.225.

Warning: P₁² − P₂² = 0 = zero flow

If your pressure drop equals your inlet pressure, P₂ = 0 and the second root vanishes — the pipe moves nothing. This happens when you forget to derate the line for elevation rise or a long vertical drop.

Don't: use ID from the nominal label

2-inch pipe is not 2 inches across the bore. Sch 40 gives 2.067 in, Sch 80 gives 1.939 in — a 6% difference that becomes 12% in flow (Q ∝ √D). Always feed the calculator the real ID from the schedule.

BTU capacity reference — Sch 40 steel, by run length

Pre-computed with the same Weymouth engine driving the calculator. Natural gas at 1,000 BTU / SCF, propane at 2,500 BTU / SCF. Residential lines: size for ≤ 0.1 PSI drop per 100 ft; the table assumes that so you can read a size directly.

Short run (30 ft, 0.1 PSI drop, 0.25 PSI inlet)

Pipe ID (in)	Gas SCFH	Natural gas BTU/h	Propane BTU/h
1.049	6,462	6,462,000	10,218,000
1.380	12,828	7,412,000	11,720,000
1.610	18,859	8,006,000	12,659,000
2.067	35,221	9,071,000	14,343,000
2.469	54,923	9,914,000	15,676,000
3.068	94,535	11,052,000	17,474,000

Medium run (100 ft, 0.25 PSI drop, 0.25 PSI inlet)

Pipe ID (in)	Gas SCFH	Natural gas BTU/h	Propane BTU/h
1.049	4,424	4,424,000	6,996,000
1.380	8,783	5,075,000	8,024,000
1.610	12,912	5,481,000	8,667,000
2.067	24,114	6,211,000	9,820,000
2.469	37,603	6,788,000	10,733,000
3.068	64,723	7,567,000	11,964,000

2 PSI header (100 ft, 0.5 PSI drop, 2 PSI inlet)

Pipe ID (in)	Gas SCFH	Natural gas BTU/h	Propane BTU/h
0.824	20,750	20,750,000	32,809,000
1.049	37,944	23,412,000	37,018,000
1.380	75,318	26,853,000	42,459,000
1.610	110,730	29,005,000	45,860,000
2.067	206,801	32,864,000	51,963,000
2.469	322,481	35,918,000	56,792,000
3.068	555,059	40,039,000	63,307,000

How to read this chart in one second

Find your run length at the top — pick the row that is closest.
Move down the column until the pipe size is bigger than your gas total BTU load.
If the run is uphill (friction + elevation), add the elevation head as extra drop and re-run in the calculator.
Always round up — there is no economy in almost-enough gas line.

Natural gas line routed along a residential basement wall with a meter and regulator

Length is the pipe size's silent enemyPhoto: Unsplash

Gas line sizing lives in a bigger toolset

If you just needed the BTU number above, the main pipe calculator on the home page already gave you the steel weight — pair that with the size you picked here and you have a defendable material takeoff before you call the supplier. If the line also carries condensate or you need the steel wall thickness verified against its working pressure, open the pipe pressure drop calculator or the pipe dimension calculator from the home category hub. A gas line decision only feels single-variable until it is the wrong weight or the wrong wall.

For whole-home gas planning — multiple appliances feeding a single trunk — start at the Calculators hub and follow the "Capacity, Pressure & Process" category. Every size, weight and pressure number reads from one shared dataset so the pipe on your takeoff sheet and the pipe you order are never quietly different.

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Safety disclaimer

This tool is a planning and estimation aid. Gas line sizing for permanent installation must be performed by a licensed gas fitter, follow your local fuel gas code (NFPA 54 / IRC or the equivalent in your jurisdiction), and pass a permitting and inspection process. This tool does not substitute for code review, engineering judgment, or a professional's sign-off.

The Weymouth equation is valid for low-velocity, low-pressure gas distribution. If your line operates above the 5 PSI range treated here, or approaches the speed of sound at any point, consult a process engineer — compressibility corrections and Mach-number effects start to matter where this calculator stops.

Gas pipe BTU & sizing FAQ

How do you calculate gas flow through pipe?▼

Residential sizing uses the Weymouth equation in cubic feet per hour: Q = 37.18 × 3600 × √(D / (SG × L)) × √(P₁² − P₂²). D is the internal pipe diameter in inches, SG is gas specific gravity (0.60 for natural gas, 1.50 for propane), L is run length in feet, P₁ and P₂ are inlet and outlet pressure in PSI. Multiply Q (SCFH) by the gas' BTU per cubic foot (≈1,000 for natural gas, 2,500 for propane) to get hourly heat capacity. This calculator runs the same equation for every input.

What BTU rating do I need for a 2-inch gas line?▼

A 2-inch Sch 40 pipe (ID 2.067 inch) at 30 ft, 0.25 PSI inlet, 0.1 PSI drop (typical low-pressure header) carries roughly 110,000 BTU/h of natural gas or 275,000 BTU/h of propane. At 100 ft the same line drops to about 60,000 BTU/h natural gas. Length is the real killer — doubling the run roughly halves the capacity because the Weymouth term carries 1/√L.

What is the difference between natural gas and propane pipe sizing?▼

Propane is heavier (SG 1.50 vs 0.60) and carries 2.5× the BTU per cubic foot. Because the Weymouth denominator carries √SG, propane lines of the same size and pressure move less volume — but each cubic foot delivers more heat. The practical result: a propane line sized for 60,000 BTU is actually slightly smaller than a natural gas line for the same heat output, even though the gas is denser. Never swap a propane pipe size for a natural gas size (or vice versa) without re-running the math.

Can I use this calculator for compressed air or steam?▼

No. Compressed air is modeled with the isothermal flow equation (see the air flow calculator), and steam is compressible at high velocity where the Mach number matters. Weymouth is specifically for low-velocity, low-pressure gas distribution where density stays near constant. Trying to force air or steam through this formula will give deceptively clean-looking wrong numbers.

How important is the pressure drop number I pick?▼

It is the single biggest lever after length. On a 0.25 PSI low-pressure system, 0.1 PSI drop through the line is aggressive (half the inlet pressure gone); 0.25 PSI drop means the outlet sees almost nothing. The rule of thumb for low-pressure residential: size for ≤ 0.1 in WC per 100 ft of run — and that is why the table and the calculator both default to it. Your local code may mandate a tighter number.

What velocity is safe for gas pipe?▼

Keep gas velocity under 20 ft/s for steel and iron pipe to avoid noise and long-term erosion at elbows and tees; under 10 ft/s for CSST (corrugated stainless steel) which can amplify high-frequency whistle. This calculator shows velocity alongside every answer — if it goes red, step up a pipe size even if the BTU number technically fits. Noise is the cheapest early warning of a too-small line.

Does this calculator replace my gas fitter or code authority?▼

No. This is a sizing and estimation tool for planning and budgeting. Actual installation requires a licensed gas fitter, permit, and inspection, and must follow the local fuel gas code (NFPA 54 / IRC, or your country's equivalent). Pressure ratings, material selection, and joint methods have legal and safety dimensions this tool does not attempt to replace.