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Gas pipe size calculator

Gas sizing has one rule that catches people: the longest-length method reads every section of pipe in one length column, the run from the meter to the furthest outlet, not each branch's own length. Add up every appliance that can fire at the same time, measure to the outlet furthest from the meter, and that load in that column sizes the first section, the one that carries everything.

Gas pipe sizeIFGC 402.4

Pre-filled with a furnace, water heater, range and dryer on a 60 ft longest run.

BTU/h
ft
1"nominal

Size for the full-load section

Demand
197CFH
Length column used
60ft
Table capacity at that length
257CFH
Heating value assumed
1,050BTU / ft³
Size20 ft40 ft60 ft100 ft150 ft
1/2"11881655040
3/4"24717013710483
1"466320257195157
1-1/4"957657528400322
1-1/2"1430985791600482
2"2760190015201160928

How it sizes

The table is indexed by the length to the furthest outlet and read against demand in cubic feet per hour. Natural gas is taken at 1,050 BTU per cubic foot, so a 207,000 BTU/h connected load is about 197 CFH. The length column rounds up to the next published column — 60 ft of run reads the 60 ft column, 61 ft reads 70.

Every other section is read in the same length column against only the load it carries, so the pipe may step down as appliances branch off. Under IFGC §402.4.1 the length never changes; only the load does.

Every appliance at full input

IFGC §402.2 sizes on the total connected load, as if every appliance fired at once. A dryer and a range on the same line both count; a decorative fireplace that is never lit in summer still counts. Size lower only on a load diversity you can establish to the authority having jurisdiction (AHJ).

Worked example

An 80,000 BTU furnace, a 40,000 BTU water heater, a 65,000 BTU range and a 22,000 BTU dryer come to 207,000 BTU/h, or 197 CFH. The dryer sits 58 ft of developed pipe from the meter, so the 60 ft column applies. Schedule 40 at 60 ft carries 137 CFH in 3/4" and 257 CFH in 1", so the section from the meter is 1".

Swap the tank heater for a 180,000 BTU tankless and the load becomes 347,000 BTU/h, about 330 CFH. That is past what 1" carries at 60 ft, and the section goes to 1-1/4", which carries 528 CFH. That is why a tankless swap can mean new gas pipe.

Code notes

NFPA 54 IFGC The Schedule 40 capacities are the same in the National Fuel Gas Code (NFPA 54 Table 6.2.1(b)) and the International Fuel Gas Code (IFGC 2024 Table 402.4(2)); the CSST column here is IFGC Table 402.4(15). Your AHJ decides which fuel gas code applies. CSST is sized by equivalent hydraulic diameter (EHD), not by nominal size, and every manufacturer publishes its own EHD-to-part mapping — check the one in the box. Bonding of CSST is a separate requirement and is enforced.

Code violation

Gas work in most jurisdictions requires a permit, a pressure test and a licensed installer. Sizing on paper is not authorisation to cut pipe.

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