Gas Pipe Sizing Guide: Longest-Length Method for Steel and CSST

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What the Longest-Length Method Actually Does

Fuel-gas codes (NFPA 54 / ANSI Z223.1 Chapter 6, IFGC Section 402, IRC G2413 for dwellings) let you size pipe from capacity tables. The longest-length approach is the conservative field default: find the length from the meter or second-stage regulator to the farthest appliance on the segment, treat that length (plus fitting equivalent length) as the table row for every size on that path, then pick the smallest diameter whose capacity at that row covers the connected load.

That means a short branch to a dryer still gets sized as if it ran the full distance to the distant furnace — unless you switch to branch-length sizing. Longest-length is simple, inspectable, and hard to under-size accidentally. It can also oversize short branches. Walk the tradeoff in longest vs branch length.

Load: Nameplate Input, Not Output After AFUE

Every appliance on the segment contributes its input rating from the nameplate or listing. A 95% AFUE furnace that inputs 100,000 BTU/hr still needs pipe for 100,000 BTU/hr. Water heaters are the same story — size the gas pipe to the burner, then size the tank with the water heater sizing calculator. Furnace load for the house: furnace sizing.

Natural gas Schedule 40 tables are in cubic feet per hour. This site converts at 1000 BTU per cubic foot (need CFH = BTU ÷ 1000). Propane Schedule 40 and propane CSST tables are already in thousands of BTU per hour. Do not mix rows from a natural-gas table into a propane job.

Developed Length: Measured Feet Plus Fittings

Measured length is centerline pipe from the sizing origin (meter outlet, second-stage regulator, or branch takeoff you are designing from) to the farthest appliance on that path. Fittings add equivalent length because they create pressure drop.

This calculator uses field approximations that track common training sheets:

If you already computed equivalent length from the code’s fitting table for a specific diameter, enter that total in the override field. Always round the developed length up to the next published table length (15 ft of steel becomes the 20 ft row, not the 10 ft row).

Which IFGC Table Matches the Gas and Material

Other pressure-drop bands, elevations, and some manufacturer CSST listings use different tables. The AHJ and the tubing maker win over a website. NFPA 54 Chapter 6 is the parent method; IFGC 402 and IRC G2413 are how many jurisdictions adopt it for buildings and dwellings.

Schedule 40 Metallic vs CSST

Black iron and other Schedule 40 metallic pipe are sized by nominal diameter (1/2" through 2" on this sheet). CSST is sized by EHD (equivalent hydraulic diameter) labels such as EHD 18 through EHD 37. You cannot “convert” 3/4" steel to an EHD by eye — use the matching capacity table.

CSST almost always needs bonding per the manufacturer and the electrical code. Strike protection, routing clearances, and termination fittings are listing-driven. Threaded steel joints need proper NPT engagement and listed compound — see the NPT pipe thread calculator. Neither material forgives an undersized trunk when every appliance fires at once.

Worked Examples (Same Math as the Calculator)

Example A. 100,000 BTU/hr natural gas, Schedule 40, 40 ft measured, no fittings. Developed = 40 ft → 40 ft row. Need 100 CFH. 1/2" = 81 CFH (fails). 3/4" = 170 CFH → 3/4".

Example B. 40,000 BTU/hr natural gas, Schedule 40, 15 ft. Round up to 20 ft. Need 40 CFH. 1/2" = 118 CFH → 1/2".

Example C. Same 100,000 BTU load but six elbows on steel: developed = 40 + 6×2.5 = 55 ft → 60 ft row. 3/4" at 60 ft is 137 CFH — still covers 100 CFH, but you can see how fittings push you toward the next row or the next size on heavier loads.