What is the longest-length method?

It is the simplest NFPA 54 approach: you size every section of the system to the single longest run from the meter to the farthest appliance. It oversizes some branches but never undersizes, so it is fast and code-compliant for residential work.

Why does propane use different capacities than natural gas?

Propane has roughly 2.5 times the heating value per cubic foot, so far less volume flows for the same BTU/h. NFPA 54 publishes separate propane tables; this tool uses a 1.50 specific-gravity flow correction so a given pipe carries more BTU/h on propane than on natural gas.

Is 0.3 or 0.5 in WC the right pressure drop?

Use 0.5 in WC for standard low-pressure residential natural gas off a 7 in WC meter, and 0.3 in WC where the appliance regulator allowance is tighter. Propane systems at 11 in WC commonly use a larger allowable drop; check the appliance and regulator data.

Does CSST really use a different size than iron pipe?

Yes. CSST is rated by an EHD (equivalent hydraulic diameter) flute number, not nominal pipe size, and its corrugations create more friction per inch of nominal bore. This tool carries manufacturer-style CSST capacity values so you do not confuse a 1/2 in CSST flute with 1/2 in iron pipe.

Should I add fittings to the run length?

The longest-length method already builds in a generous fitting allowance, so for residential work you can use the straight measured length. For long runs with many elbows, add equivalent length per fitting from the NFPA 54 fitting table to be safe.

Natural Gas / Propane Pipe Sizing Calculator

Undersized gas piping starves appliances, causes low flames, nuisance lockouts, and unsafe combustion. This calculator applies the NFPA 54 longest-length method to pick the smallest Schedule 40 black iron pipe or CSST that still delivers your total BTU/h demand across the longest run without exceeding the allowable pressure drop.

How it works

Each pipe size has a published capacity in BTU/h that falls as the run gets longer (friction rises with length). The longest-length method sizes the whole system to one number — the farthest appliance:

1. Total demand   = sum of every appliance input (BTU/h)
2. Length         = longest run, meter to farthest appliance (ft)
3. Capacity(size) = table value for that diameter at that length
4. Choose the smallest size where Capacity(size) >= Total demand

Capacity scales approximately with length as C ∝ length^-0.541 (the Pole/Spitzglass exponent NFPA 54 tables are derived from), and propane is corrected by the square root of the specific-gravity ratio so the same pipe carries more BTU/h on propane than on 0.60-SG natural gas.

Example and notes

A house with a 199,000 BTU/h tankless heater plus a 65,000 BTU/h furnace and a 40,000 BTU/h range totals about 304,000 BTU/h. With a 60 ft longest run on natural gas at 0.5 in WC, a 1 in black iron pipe falls short, so the tool steps up to 1-1/4 in iron pipe, which comfortably carries the load. Always verify against the printed NFPA 54 table for your exact pressure and confirm CSST sizing against the specific manufacturer’s listing, since flute geometry varies between brands.