Gas Pipe Size Calculator

Sum all appliance BTU ratings in the house (range + oven + water heater + furnace + dryer + any future additions) — call that BTU_total. Pick a pressure class (2 PSI is the modern minimum for homes with more than two appliances; 5 PSI is commercial). The Weymouth equation gives SCFH from ID, pressure and run length; BTU_total ÷ BTU_per_SCF = required SCFH; solve back for the smallest ID whose SCFH at your run length exceeds the required number. This calculator runs that search for you.

2 PSI works for most residential systems; it keeps pressure drop as a fraction of inlet small and line sizes manageable. Upgrade to 5 PSI if your run is over 200 ft of equivalent pipe, if you have four or more high-BTU appliances on one trunk, or if the supplier delivers to you at 5 PSI. Higher inlet pressure lets you run smaller pipe at the same BTU — but the line and fittings must be rated for 5 PSI too (black iron B16.9 is fine, some CSST is rated to 5 PSI, check the label).

Length is the biggest lever. At 2 PSI inlet, 0.5 PSI drop, 60,000 BTU of natural gas (60 SCFH): 30 ft of 1-inch pipe runs it fine; 50 ft needs 1-1/4 inch; 100 ft needs 1-1/2 inch. Length doubles → required ID grows roughly by √2 ≈ 1.4, because Q ∝ √(1/L). Never guess line length — measure it, or add 15% if you are planning and do not have exact ductwork drawings yet.

Both this sizing tool and the pipe-sizing calculator use the same internal diameter and Weymouth equation — the material decides allowed pressure rating, not flow math. CSST (corrugated stainless steel) is popular because it runs faster and uses fewer fittings, but its corrugation raises friction slightly vs smooth black iron. For the same ID, derate CSST sizing by roughly 10% to account for that extra friction. This tool defaults to black iron; if you pick CSST we show a derated size recommendation.

Air sizing runs at 60-120 PSI and uses isothermal or adiabatic flow models — not Weymouth, which assumes gas velocity well below the speed of sound. Air compressors also express capacity in SCFM vs gas meters' SCFH. Open the <Link href="/pipe-air-flow-calculator" className="text-sky-700 font-semibold hover:underline">compressed air calculator</Link> when the fluid is air, and leave this one for residential/combustible gas.

Compliance is your local fuel gas code (NFPA 54 / IRC / UCC or your country's equivalent) + a licensed fitter's sign-off. This calculator is a planning aid that uses the same physical model the code tables use, but code tables apply pipe roughness, safety factors and material limitations this tool does not enforce. Always confirm the final size with your fitter and have the line inspected.