How to use the low-pressure gas pipe sizing calculator
Low-pressure fuel gas piping has very little pressure to spare, so dropping one size below what the load needs leaves appliances starved at the burner inlet. Oversizing costs material and labour and leaves more gas volume in the line. This tool sizes the run straight from the connected load, the equivalent length and the pressure drop you are willing to spend.
Capacity follows the low-pressure form Q = 1,730 × D2.623 × (ΔH ÷ (Cr × L))0.541, solved for diameter. Q is in CFH, ΔH in inches water column, D is the inside diameter in inches and Cr the gas specific gravity. The constant and exponents are set to reproduce NFPA 54 Table 6.2(a) slightly on the conservative side across every size and length. Load converts to flow at 1,030 Btu/ft³ for natural gas and 2,516 Btu/ft³ for propane vapour.
Enter the straight run plus the equivalent length of every elbow, tee and valve. The tool reports the actual pressure drop and velocity at the selected Schedule 40 size so you can see how much margin is left. Always cross-check the final size against the NFPA 54 capacity table and your local code amendments, and have the installation signed off by a licensed gas fitter.
Frequently asked questions
Low-pressure natural gas systems supplied at about 7 in. w.c. are normally sized on a 0.5 in. w.c. drop, which is the basis of the standard NFPA 54 table. Two-pound systems with a regulator at each appliance use different tables.
Count each elbow as roughly 30 pipe diameters and each gate valve as about 10, then add that to the straight run. For a quick check, adding 20 to 30 % to the measured run is usually close enough.