Almighty Calculator

Gas Pipe Sizing Calculator

Size the pipe so its capacity at your longest length and allowed pressure drop covers the total BTU load, using IFGC Equation 4-1 for pressures under 1.5 psi.

Estimate only. Results are estimates. Verify against the applicable code and manufacturer specifications before relying on them for safety-related work.

Gas
BTU/hr
Add up the input rating of every appliance on the pipe.
BTU/ft³
About 1,000 for natural gas and about 2,500 for propane. Your utility or supplier can tell you.
ft
ft
Optional. Add the code's equivalent length for elbows, tees, and valves.
in. w.c.
0.5 in. w.c. is common for natural gas at 7 in. w.c. inlet; check your utility's and appliance requirements.
Minimum Schedule 40 pipe size
1in
Gas flow required200 cfh
Capacity of that pipe at this length and drop (with 2% margin)254 cfh
Same capacity in BTU/hr253,711 BTU/hr

Verify with the appliance manufacturers' requirements, your local code edition, and the gas utility. Have a licensed gas fitter install and pressure-test the piping.

Show the math

Flow = 200,000 BTU/hr ÷ 1,000 BTU/ft³ = 200 cfh
Capacity Q = [19.17 × D × (ΔH ÷ (C_r × L))^0.206]^(1 ÷ 0.381) with ΔH = 0.5 in. w.c., C_r = 0.6094, L = 60 ft
1 in pipe (inside diameter 1.049 in) carries 254 cfh with the 2% margin, against 200 cfh needed
Minimum Schedule 40 pipe = 1 in

Rounded the same way as the result above.

How it works

The International Fuel Gas Code gives an equation for low-pressure gas piping. It ties together the inside diameter of the pipe, the flow, the length, and the pressure drop. This calculator solves it for capacity and picks the smallest standard pipe whose capacity covers your load at your longest run.

The load is the sum of all appliances that could run at once, converted from BTU per hour to cubic feet per hour using the heating value of the gas. Sizing is based on the longest length from the source to the farthest appliance (the "longest length" method), applied to every section of pipe.

Low-pressure equation (IFGC 4-1): D = Q^0.381 ÷ [19.17 × (ΔH ÷ (C_r × L))^0.206] Solved for flow: Q = [19.17 × D × (ΔH ÷ (C_r × L))^0.206]^(1 ÷ 0.381) Q = cubic feet per hour, D = inside diameter (in), L = equivalent length (ft), ΔH = pressure drop (in. w.c.) C_r = 0.6094 natural gas, 1.2462 undiluted propane

Worked example

A 200,000 BTU/hr natural gas load (about 200 cfh) with a 60 ft longest run and a 0.5 in. w.c. allowable drop:

  1. Flow = 200,000 BTU/hr ÷ 1,000 BTU/ft³ = 200 cfh
  2. Capacity Q = [19.17 × D × (ΔH ÷ (C_r × L))^0.206]^(1 ÷ 0.381) with ΔH = 0.5 in. w.c., C_r = 0.6094, L = 60 ft
  3. 1 in pipe (inside diameter 1.049 in) carries 254 cfh with the 2% margin, against 200 cfh needed
  4. Minimum Schedule 40 pipe = 1 in
InputValue
GasNatural gas
Total connected load200000 BTU/hr
Heating value of the gas1000 BTU/ft³
Longest run from the meter or regulator to the farthest appliance60 ft
Extra equivalent length for fittings0 ft
Allowable pressure drop0.5 in. w.c.
ResultValue
Minimum Schedule 40 pipe size1 in
Gas flow required200 cfh
Capacity of that pipe at this length and drop (with 2% margin)254 cfh
Same capacity in BTU/hr253,711 BTU/hr

Schedule 40 pipe inside diameters

Nominal size (in)Inside diameter (in)
1/20.622
3/40.824
11.049
1-1/41.38
1-1/21.61
22.067
2-1/22.469
33.068
44.026
55.047
66.065
87.981
1010.02
1211.938
Show the schedule 40 pipe inside diameters table
Nominal size (in)Inside diameter (in)
1/20.622
3/40.824
11.049
1-1/41.38
1-1/21.61
22.067
2-1/22.469
33.068
44.026
55.047
66.065
87.981
1010.02
1211.938

Inside diameters as listed in the International Fuel Gas Code sizing tables.

Assumptions and limits

  • Low-pressure equation only (under 1.5 psi). Two-psi and higher systems, corrugated stainless tubing, copper, and polyethylene have their own tables.
  • Schedule 40 steel pipe with the inside diameters printed in the code tables. The equation matches the published capacity tables to within about 1–2%, slightly high, so this tool applies a 2% margin and never shows more capacity than the printed tables.
  • Natural gas specific gravity 0.60, propane 1.50, at 60°F. Adjust the flow for altitude above 2,000 ft as the code requires.
  • Fittings are added as equivalent length. Some designers add a percentage instead.
  • This is a sizing estimate. Code, appliance requirements and utility rules control.

Common questions

What size gas pipe do I need for a 100,000 BTU furnace?

At 30 ft on natural gas with a 0.5 in. w.c. drop, 3/4 inch pipe carries about 199 cfh. That covers 100,000 BTU/hr (100 cfh) with room to spare. Enter your own length above.

How do I convert BTU to cubic feet of gas?

Divide BTU per hour by the heating value: about 1,000 BTU per cubic foot for natural gas and about 2,500 for propane vapor.

Why does the longest run matter?

Pressure drop builds along the whole path to the farthest appliance, so the code sizes every section for that longest length, which keeps the farthest appliance supplied.

Can I use the same table for propane?

The equation covers it with a different C_r, and propane carries far more BTU per cubic foot, so pipes run smaller. Propane piping also needs a properly sized regulator setup.

Sources

  • International Fuel Gas Code, Section 402.4: Equation 4-1 and Table 402.4 (C_r = 0.6094 natural gas, 1.2462 undiluted propane), as reproduced in the 2021 Seattle Fuel Gas Code Chapter 4.
  • Schedule 40 inside diameters from the sizing tables (Tables 402.4(1) and 402.4(2) headers; IFGC Appendix A Table A105.1). Checked against Tables 402.4(1), 402.4(2) and 402.4(28).
  • Read them: 2021 Seattle Fuel Gas Code, Chapter 4 (IFGC Section 402 and tables)

Updated 2026-09-30