IFGC 402.4 · Gas pipe sizing

How to read gas pipe sizing tables

Picking the right table by material, pressure, pressure drop, and specific gravity; the longest-length and branch-length methods; and a worked example — with a free link to the official tables.

Need the actual tables? Read the IFGC 402.4 tables free at codes.iccsafe.org →

Licensed work only. Gas piping must be installed by a licensed gas fitter under a permit and pressure-tested per IFGC Section 406 before it goes into service. Use this page to understand the tables — not to design or install a system yourself.

Step 1 — pick the right table: read the header block first

Header item	What it means	Why it matters
Gas	Natural gas or undiluted propane	Propane carries more energy per cubic foot and uses its own tables
Material	Schedule 40 steel, copper tubing (K/L), CSST, polyethylene, and others	Inside diameter and roughness differ — never read a steel table for CSST or copper
Inlet pressure	The system pressure — low pressure (under 2 psi) or elevated pressure	Elevated-pressure systems have their own tables and a regulator ahead of the appliances
Pressure drop	How much pressure the design lets the pipe lose from meter to appliance	A bigger allowed drop means smaller pipe — but only if the appliances still get their minimum inlet pressure
Specific gravity	The gas density the table assumes (natural gas tables assume a typical value)	If the supplier’s gas differs a lot, the table needs a correction or an engineered design

Our own summary, not the code text. Read IFGC Section 402 and the 402.4 tables free at codes.iccsafe.org (2021/2024 IFGC) and verify against the edition adopted by your AHJ. Not engineering advice.

Which length do I read? The three IFGC methods

Method	Length used for each section	Load used	Section
Longest length	The single longest run from the point of delivery (meter or regulator) to the most remote outlet — used for every section	The load that section carries	402.4.1
Branch length	Main run: the longest length. Each branch: the length from the point of delivery to the most remote outlet on that branch	The load that section carries	402.4.2
Hybrid pressure	Higher-pressure piping: longest length to the most remote line regulator. After each regulator: length from that regulator to its most remote outlet	The load that section carries	402.4.3

Our own summary, not the code text. Read IFGC Section 402 and the 402.4 tables free at codes.iccsafe.org (2021/2024 IFGC) and verify against the edition adopted by your AHJ. Not engineering advice.

How to size a gas system, step by step

Step	What you do	Watch out for
1	List every appliance with its maximum input (Btu/h) from the nameplate	Include planned appliances — adding a generator later is where undersized pipe shows up
2	Convert Btu/h to cubic feet per hour (CFH) by dividing by the gas heating value from your supplier	Heating value varies by supplier and altitude; ask rather than assume
3	Sketch the piping and label each section with the load it carries downstream	The first section from the meter carries the total connected load
4	Pick the table that matches gas, material, inlet pressure, pressure drop, and specific gravity	Most sizing errors are reading the wrong table
5	Choose the method (longest length or branch length) and find the length row — round up to the next row	Never interpolate down to a shorter length
6	Read across that row to the smallest pipe whose capacity is at least the section load	Capacity is in CFH, not Btu/h
7	Repeat for every section, then check appliance inlet-pressure requirements and the utility’s meter capacity	Tables assume the drop in the header — they don’t know your regulator or meter

Our own summary, not the code text. Read IFGC Section 402 and the 402.4 tables free at codes.iccsafe.org (2021/2024 IFGC) and verify against the edition adopted by your AHJ. Not engineering advice.

Worked example — main run for a house, longest-length method

Input / step	Value	Source
Furnace + water heater	100,000 + 40,000 = 140,000 Btu/h	Nameplates
Heating value (from supplier)	1,000 Btu per cu ft (assumed)	Gas supplier
Load on the first section	140 CFH	140,000 ÷ 1,000
Longest run, meter to most remote appliance	50 ft	Measured
Table	Schedule 40 steel, natural gas, under 2 psi, 0.3 in. w.c. drop, 0.60 specific gravity	Table 402.4(1)
3/4-in. at the 50-ft row	114 CFH — too small for 140 CFH	Table 402.4(1)
1-in. at the 50-ft row	215 CFH — OK	Table 402.4(1)
Result	1-in. for the first section; size each downstream section the same way with its own load, using the same 50-ft row	Longest-length method (402.4.1)

Worked example values only. Read the full tables free at codes.iccsafe.org. A licensed gas fitter sizes, installs, and tests the actual system.

Common mistakes

Mistake	What to do instead
Reading a Btu/h load against a CFH table	Convert Btu/h to CFH with the supplier’s heating value first
Using the length of each branch instead of the longest length	With the longest-length method, every section uses the longest run
Using a steel table for CSST, copper, or PE	Use the table for the material — CSST uses its own tables or the listed manufacturer’s sizing tables (402.3)
Using a propane table for natural gas (or the reverse)	Match the gas in the header block
Picking the pressure-drop column that makes the pipe smallest	The drop must leave every appliance its minimum inlet pressure
Leaving out future loads	Size the main for planned appliances — upsizing a buried or concealed line later is expensive

Our own summary, not the code text. Read IFGC Section 402 and the 402.4 tables free at codes.iccsafe.org (2021/2024 IFGC) and verify against the edition adopted by your AHJ. Not engineering advice.

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Sizing gas pipe the way the inspector checks it

Read Section 402 and the 402.4 tables free on codes.iccsafe.org. ICC offers free read-only access to the 2021 and 2024 International Fuel Gas Code. This page explains how to use the tables; it does not reproduce them.

Safety first. Gas piping is life-safety work. It must be designed, installed, and pressure-tested by a licensed gas fitter or plumber under a permit, tested per IFGC Section 406 before it is put in service, and purged and leak-checked before appliances are lit. Undersized pipe starves appliances and can cause incomplete combustion and carbon monoxide. This page teaches how the tables work; it is not a substitute for a licensed installer, the manufacturer’s instructions, or your AHJ.

Every table is built for one set of conditions. The header block names the gas, the pipe material, the inlet pressure, the allowed pressure drop, and the specific gravity. Match all five before you read a single number. Then choose your length: the longest-length method (402.4.1) uses the longest run from the meter to the most remote outlet for every section; the branch-length method (402.4.2) uses the longest run for the main and each branch’s own run for that branch.

Worked example. A 100,000 Btu/h furnace and a 40,000 Btu/h water heater load the first section with 140 CFH (at an assumed 1,000 Btu per cubic foot). With a 50-ft longest run on Schedule 40 steel, low pressure, 0.3-inch w.c. drop, the 50-ft row lists 114 CFH for 3/4-inch — not enough — and 215 CFH for 1-inch, so the first section is 1-inch.

Western jurisdictions on the Uniform Plumbing Code (IAPMO) regulate fuel gas piping in the UPC’s fuel-gas chapter, with tables derived from NFPA 54. Pipe dimensions: steel pipe schedule chart, copper tube sizes, and NPT thread chart. Field PM keeps permits, pressure-test records, and inspection sign-offs with the job.

FAQ

How do I size a natural gas line?+

Total the appliance inputs, convert Btu/h to cubic feet per hour with your supplier’s heating value, pick the IFGC 402.4 table that matches your gas, pipe material, inlet pressure, pressure drop, and specific gravity, find the row for your length, and read across to the smallest pipe that carries the load. Read the tables free at codes.iccsafe.org — and have a licensed gas fitter do the work.

What is the longest-length method?+

Under IFGC 402.4.1, every section of pipe is sized using the single longest run from the point of delivery to the most remote outlet, together with the load that section carries. It is simple and conservative.

What is the branch-length method?+

Under IFGC 402.4.2, the main run is sized with the longest length, but each branch not already sized uses the length from the point of delivery to the most remote outlet on that branch. It can allow smaller branch piping than the longest-length method.

Why do gas tables list pressure drop and specific gravity?+

Each table is built for one set of conditions: how much pressure the pipe may lose and how dense the gas is. Change either and the capacity changes, which is why you must match every item in the header before reading a number.

Does the UPC size gas pipe the same way?+

Jurisdictions on the Uniform Plumbing Code (IAPMO, common in western states) cover fuel gas piping in the UPC’s fuel-gas chapter, which uses similar sizing methods and tables derived from NFPA 54. Use the code your jurisdiction adopted and consult it directly.

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