Plumbing & gas · Calculator

Gas pipe sizing calculator.

Reviewed by the unitcheatsheet.com editorial team · Last updated November 2026

Gas Total loadAdd up the input ratings of all appliances on the line Load unit Longest run length (ft)Meter or regulator to the farthest appliance Pressure drop ΔH (in. w.c.)0.5 is common for 7 in. w.c. natural gas systems Heating value (BTU/ft³)Ask your gas supplier for the exact value

Use the length of the longest run and the total load. At a 0.5 in. w.c. pressure drop, 1/2 in Schedule 40 pipe carries about 117 CFH of natural gas at 20 ft and 3/4 in carries about 244 CFH. One CFH of natural gas is about 1,000 BTU/h. Confirm sizes with your local code and gas supplier.

Schedule 40 steel pipe, low-pressure systems (under 1.5 psi). Calculated with IFGC Equation 4-1 — the same equation behind the code's sizing tables.

Next step Pipe size chart (NPS) — Schedule 40 dimensions

How the calculation works

IFGC Equation 4-1 (low pressure, under 1.5 psi):
D = Q^0.381 ÷ [19.17 × (ΔH ÷ (Cr × L))^0.206]

D  = inside diameter (in)       Q  = flow (ft³/h)
ΔH = pressure drop (in. w.c.)   L  = equivalent length (ft)
Cr = 0.6094 natural gas, 1.2462 undiluted propane

The calculator solves the equation for Q at each Schedule 40 inside diameter and picks the smallest pipe that carries the load. The printed IFGC capacity tables come out slightly below the equation: up to 1.2% lower for natural gas and about 1.8% lower for propane (checked against the 2006 IFGC tables). So every capacity on this page is the equation's value reduced by 2%, which keeps it at or below the code tables. The code tables your jurisdiction has adopted still govern.

Natural gas pipe capacity chart (Schedule 40, ΔH 0.5 in. w.c.)

Capacity in cubic feet per hour (CFH) of 0.60 specific gravity natural gas, calculated with Equation 4-1 and reduced by 2% to stay at or below the code tables. Multiply by about 1,000 for BTU/h.

Length	1/2 in	3/4 in	1 in	1-1/4 in	1-1/2 in	2 in	2-1/2 in	3 in	4 in
10 ft	170	355	669	1,374	2,059	3,966	6,323	11,183	22,820
20 ft	117	244	460	944	1,415	2,727	4,347	7,688	15,687
30 ft	94	196	369	758	1,137	2,190	3,491	6,174	12,599
40 ft	80	168	316	649	973	1,874	2,988	5,285	10,784
50 ft	71	149	280	575	862	1,661	2,649	4,684	9,559
60 ft	64	135	254	521	781	1,505	2,400	4,244	8,661
80 ft	55	115	217	446	669	1,289	2,054	3,633	7,414
100 ft	49	102	193	396	593	1,142	1,821	3,220	6,571
125 ft	43	91	171	351	525	1,012	1,614	2,854	5,824
150 ft	39	82	155	318	476	917	1,462	2,586	5,277
200 ft	34	70	132	272	407	785	1,252	2,214	4,517

How to size a gas line (longest-length method)

Add up the input ratings (BTU/h) of every appliance. Divide by the heating value to get CFH.
Measure the length from the meter or regulator to the farthest appliance. Use this one length for every section.
For each section, add the loads of the appliances it feeds, and find the smallest pipe that carries that load at the longest length.

This is the longest-length method described in IFGC Section 402.4. Fittings are covered by the tables' allowance in most residential work; check your local code.

Common questions

How do I size a natural gas pipe?

Add the BTU/h of all appliances, divide by about 1,000 to get CFH, measure the longest run, and pick the smallest pipe whose capacity at that length is at least the load. At 0.5 in. w.c. drop and 40 ft, 3/4 in Schedule 40 carries about 168 CFH.

How many BTU can a 1/2 inch gas line carry?

It depends on length and pressure drop. With natural gas at 0.5 in. w.c. drop, 1/2 in Schedule 40 carries about 170 CFH (about 170,000 BTU/h) at 10 ft, and about 71 CFH at 50 ft.

What is CFH?

Cubic feet per hour of gas at standard conditions (60 °F and 30 in Hg). Natural gas has about 1,000 BTU per cubic foot, so 1 CFH is about 1,000 BTU/h.

Does this work for propane?

Yes, for undiluted propane at low pressure: choose Propane, which uses Cr = 1.2462 and a default heating value of 2,516 BTU/ft³. Propane pipe carries less volume than natural gas, but each cubic foot has about 2.5 times the energy.

Does this work for CSST or copper?

No. This calculator is for Schedule 40 steel pipe. Corrugated stainless steel tubing (CSST) must be sized with the manufacturer's tables, and copper tubing has its own tables in the code.

Is this calculation enough for a permit?

No. Gas work usually needs a permit and a licensed installer. Use the tables in the code your jurisdiction has adopted, and follow the appliance and gas supplier requirements.

Sources

Sizing equation and Cr values: International Fuel Gas Code (IFGC), Section 402.4, Equation 4-1 and Table 402.4 — International Code Council. The same equation appears in NFPA 54, National Fuel Gas Code.
Pipe inside diameters: ASME B36.10M, Schedule 40 (as listed in the IFGC sizing tables).

Checked against the 2006 International Fuel Gas Code (ICC): 7 November 2026. Cr values, pipe inside diameters and the capacity tables (results are kept at or below Table 402.4(2)). The current IFGC edition could not be read.

Last content change: 7 November 2026 · Changelog

Disclaimer. Gas piping is safety-critical. This calculator is for planning only. Installation must follow the fuel gas code adopted in your area and be done or inspected by a qualified person. Where results differ from the adopted code tables, the code tables govern.