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Plumbing

Gas Pipe Size Calculator

Size residential natural gas and propane pipe by the longest-length method. Enter the load and the run, get the Schedule 40 size and the capacity math — with a tankless retrofit check up front.

Gas
Delivery pressure
Mode
ft

Measure meter → farthest appliance, then add ~20–30% for fittings.

in WC

Standard is 0.5" WC; tighten to 0.3" WC on long, fully loaded runs.

BTU/hr

Use the appliance's own rating plate when you have it.

Pick a size to check a retrofit — the verdict says whether it holds.

Advanced assumptions
BTU/ft³

Default 1000 BTU/ft³ for natural gas. Override for local gas quality.

ft

Above 2,000 ft, appliance input is derated — reduce the connected load per NFPA 54.

Gas pipe · single line
Run 1" pipe
1"Sch 40
Demand
199cfh
Capacity
320cfh
Used
62%
Capacity used62%
ƒShow the math
  1. 1.Developed (longest) lengthper IFGC 402.4.2 / NFPA 54 6.2

    Every branch and the trunk are sized at the longest developed length to the most remote appliance — the defining rule of the method. Include an equivalent-length allowance for fittings.

    L = max(longest run, 10 ft)
    max(40, 10)
    = 40 ft
  2. 2.Gas demandper IFGC 402.4.2
    appliance input ÷ heating value
    199,000 BTU/hr ÷ 1000 BTU/ft³
    = 199 cfh
  3. 3.Capacity of 1" at 40 ftper IFGC App. A / NFPA 54 Annex B
    Q = 2294.131 · D^2.623 · (ΔH/(Cr·L))^0.541, ΔH 0.5" WC, Cr 0.6
    D = 1.049" (Sch 40 ID)
    = 320 cfh ≥ 199 cfh demand
  4. 4.Why not 3/4"

    3/4" carries only 170 cfh at 40 ft — short of the 199 cfh demand, so the next size up is required.

    = 3/4" → 170 cfh (insufficient)
Before you rely on this
  • Developed length must include fittings. Add an equivalent-length allowance (roughly 20–30% over the straight-pipe run, or your fitting-loss table) before you trust the size.
  • Confirm the gas meter and service regulator can supply the total connected load — pipe sizing does not check meter capacity.
  • This tool sizes Schedule 40 black-iron pipe only. CSST is sized by the manufacturer's EHD tables (not this equation); copper and semi-rigid have their own capacity tables.
  • Gas work is dangerous and typically requires a licensed installer and a permit. Verify against the code edition your AHJ adopts and pressure-test the piping before use.

How the sizing works

The tool sizes pipe by the longest-length method (IFGC 2021 · NFPA 54 (2021), IFGC 402.4.2 / NFPA 54 6.2). Every section — the trunk and each branch — is sized at the developed length to the most remote appliance. That is what makes the method simple and conservative: one length column does the whole system.

Capacity is computed from the published low-pressure friction equation, not read from a reprinted code table:

Q = C · D2.623 · ( ΔH / (Cr · L) )0.541
  • Q is capacity in cfh, D the Schedule 40 inside diameter, ΔH the allowable pressure drop (in WC), L the developed length, and Cr the gas density factor (0.60 natural gas, 1.52 propane).
  • The constant C is calibrated against the published IFGC 402.4(2) table; the regenerated capacities match it to under 1%.
  • Demand per section is the connected input divided by heating value — 1,000 BTU/ft³ for natural gas, 2,516 BTU/ft³ for propane — so propane needs fewer cfh for the same BTU.

The result is an estimate for planning. It sizes rigid Schedule 40 black-iron pipe only; CSST is sized by manufacturer EHD tables. Always include fitting equivalent length, confirm meter capacity, and verify against the edition your jurisdiction adopts.

Frequently asked

Do I need a bigger gas line for a tankless water heater?
Usually yes. A 199,000 BTU/hr tankless draws about 199 cfh of natural gas. At a 40 ft developed length, 3/4" Schedule 40 carries only ~170 cfh, so it falls short — you need 1". A 1/2" line (~81 cfh at 40 ft) is far too small. Enter your run length and the unit's input rating to see the exact size and how much a smaller line would actually carry.
What is the longest-length method?
You size every section of pipe — the trunk and each branch — using the developed length to the most remote appliance, not the length of each individual branch. It is the simplest sizing method in IFGC 402.4.2 / NFPA 54 6.2 and gives a conservative result. Measure from the meter to the farthest appliance and add an allowance for fittings before you read the size.
How do I measure developed length?
Measure the actual pipe run from the meter (or second-stage regulator) to the most distant appliance, then add equivalent length for fittings — every elbow, tee, and valve adds resistance. A common allowance is 20–30% over the straight-pipe run, or use a fitting-loss table if you have one. Undersizing the length is the most common way installers end up with a starved appliance.
Does pressure drop change the pipe size?
Yes. Capacity scales with the allowable pressure drop to the 0.541 power. The standard low-pressure (under 2 psi, 7" WC) tables use a 0.5" WC drop; tightening to 0.3" WC lowers capacity by about 24% and can push you up a size. A 2 psi system with an appliance regulator carries far more through the same pipe, which is why it is common for long tankless runs.
Does this work for CSST or corrugated tubing?
No. This tool sizes rigid Schedule 40 black-iron pipe only. CSST (corrugated stainless steel tubing) is sized by each manufacturer's EHD (Equivalent Hydraulic Diameter) tables, which are specific to that product and not interchangeable with pipe. Copper and semi-rigid tubing have their own capacity tables as well.
Is a permit required for gas work?
Almost always. Gas piping work typically requires a permit and, in most jurisdictions, a licensed gas installer, plus a pressure test before the system is put in service. Use this calculator to plan and check sizes, then verify against the code edition your Authority Having Jurisdiction adopts.

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Sources & references

Gas work is dangerous and typically requires a licensed installer and a permit. Verify against the code edition your AHJ adopts and pressure-test the piping before it is put in service.

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a licensed gas installer. See our methodology, sources, and code editions.