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Gas Pipe Size Calculator

List your gas appliances and the longest run from the meter, and get the pipe size for every branch and the main line.

Units
Gas
Btu/cu ft

1,000 is the code example's value; some utilities deliver about 1,100. Ask yours.

Longest run
ft
in

Meter or regulator to the farthest appliance, along the pipe. Add fittings as extra length if there are four or more.

Appliances
  • Btu/h
  • Btu/h
  • Btu/h
  • Btu/h

Preset inputs are typical values; use the Btu/h on your appliance's rating plate.

Main line from the meter, Schedule 40 steel

1inch pipe

235,000 Btu/h in total, 235 CFH, sized from the 60 ft row of IFGC Table 402.4(2).

Total input
235kBtu/h
Gas flow
235CFH
Table row
60ft
metermain 1 in¾ inFurnace½ inWater heater½ inRange½ inDryer

  • Furnace 100,000 Btu/h, 100 CFH

    ¾ in

  • Water heater, 30 to 40 gal 35,000 Btu/h, 35 CFH

    ½ in

  • Range, freestanding 65,000 Btu/h, 65 CFH

    ½ in

  • Clothes dryer 35,000 Btu/h, 35 CFH

    ½ in

The math

  1. Total load

    100,000 + 35,000 + 65,000 + 35,000 Btu/h = 235,000 Btu/h

  2. Gas flow

    235,000 ÷ 1,000 Btu/cu ft = 235 CFH

  3. Table row

    longest run 60 ft = 60 ft, next row up = 60 ft

  4. Main line

    first size carrying 235 CFH in the 60 ft row = 1 in

Longest length method (IFGC 402.4.1): every section is sized from the same row, the one for the longest run.

IFGC Table 402.4(2), natural gas, 0.5 in w.c. drop, cubic feet per hour

Length½ in¾ in1 in1¼ in1½ in2 in
10 ft1723606781,3902,0904,020
20 ft1182474669571,4302,760
30 ft951993747681,1502,220
40 ft811703206579851,900
50 ft721512845838731,680
60 ft651372575287911,520
70 ft601262374867281,400
80 ft561172204526771,300
90 ft521102074246351,220
100 ft501041954006001,160
125 ft44921733555321,020
150 ft4083157322482928
175 ft3777144296443854
200 ft3471134275412794
Gas piping needs a permit and, in most places, a licensed gas fitter. The inspector checks the sizing, then the system is pressure tested before the gas is turned on. Use this to plan and to check a bid, not in place of either.

A furnace, water heater, range and dryer add up to 235,000 Btu/h, which is 235 cubic feet of natural gas per hour. With the farthest appliance 60 ft of pipe from the meter, the main line needs 1 in Schedule 40 steel pipe; the furnace branch needs ¾ in and the other three ½ in. Those sizes come from the International Fuel Gas Code's capacity table for low-pressure natural gas with a 0.5 in water column pressure drop.

How gas pipe is sized

The IFGC sizes pipe from two numbers: the gas load a section carries and the length of the longest run in the system.

load (cubic feet per hour) = total Btu/h ÷ heating value (Btu per cubic foot)

pipe size = the smallest size whose capacity, in the row for the longest run, is at least the load

Section 402.2 of the code sets the load as the sum of the maximum input of every appliance the pipe serves, as if all run at once. IFGC Appendix A says to divide the Btu/h by the heating value your gas supplier delivers; its worked examples use 1,000 Btu per cubic foot, and the heating value field defaults to that. Some utilities deliver more: one Bay Area handout uses 1,100, which lowers the flow for the same appliances.

The longest length method (IFGC 402.4.1) picks one row of the table, the length from the meter to the farthest appliance, rounded up to the next row, and sizes every section of the system from it. That is conservative: the branch to an appliance 10 ft from the meter is sized as if it were 60 ft away.

Worked example

SectionLoad60 ft row capacitySize
Main (all four)235 CFH¾ in 137, 1 in 2571 in
Furnace, 100,000 Btu/h100 CFH½ in 65, ¾ in 137¾ in
Water heater, 35,000 Btu/h35 CFH½ in 65½ in
Range, 65,000 Btu/h65 CFH½ in 65½ in
Dryer, 35,000 Btu/h35 CFH½ in 65½ in

The range lands exactly on the ½ in capacity of 65 CFH, which the code accepts: the table figure must be at least the load. Add a 40,000 Btu/h fireplace and the main carries 275 CFH, more than 1 in pipe's 257, so it moves up to 1¼ in.

The City of Pleasanton's gas sizing handout works the same kind of example: furnace, water heater, range and fireplace, 240 CFH, a 69 ft run read in the 70 ft row, and a main of 1¼ in because 1 in carries only 237 CFH at 70 ft.

Natural gas capacity table

IFGC Table 402.4(2), Schedule 40 metallic pipe, inlet pressure under 2 psi, 0.5 in w.c. pressure drop, specific gravity 0.60, in cubic feet per hour:

Length½ in¾ in1 in1¼ in1½ in2 in
10 ft1723606781,3902,0904,020
20 ft1182474669571,4302,760
40 ft811703206579851,900
60 ft651372575287911,520
80 ft561172204526771,300
100 ft501041954006001,160
150 ft4083157322482928
200 ft3471134275412794

The calculator shows every row from 10 to 200 ft and uses the full table to 500 ft.

Propane pipe sizing

Propane is sized from IFGC Table 402.4(28), for Schedule 40 pipe between the second-stage regulator (11 in w.c.) and the appliances, and that table is already in thousands of Btu per hour, so there is no heating value to convert. Propane carries more energy per cubic foot, so the same pipe feeds more appliance: at 60 ft, ½ in carries 110,000 Btu/h of propane against 65,000 of natural gas. The four appliances in the example need a 1 in main (235 against ¾ in's 231) and ½ in to every appliance, the furnace included.

How far can you run ½ inch gas pipe?

It depends on the load. At 50 ft, ½ in carries 72 CFH of natural gas; at 100 ft, 50 CFH. A 40,000 Btu/h fireplace (40 CFH) can be fed by ½ in pipe up to a 150 ft longest run, where the table gives exactly 40. A 100,000 Btu/h furnace (100 CFH) can use ½ in only up to 20 ft, where the table gives 118; from 30 ft on it needs ¾ in.

Typical appliance inputs

When the appliance is not bought yet, the presets use the typical inputs from the California Plumbing Code table in the Pleasanton handout: furnace 100,000 Btu/h, 30 to 40 gal water heater 35,000, 50 gal water heater 50,000, freestanding range 65,000, clothes dryer 35,000, direct-vent fireplace 40,000, gas logs 80,000, barbecue 40,000. An instantaneous (tankless) water heater is far larger: the same table lists 142,800 Btu/h for 2 gallons a minute and 285,000 for 4 gallons a minute, so read its rating plate. It is the appliance that most often forces a bigger main.

Fittings, altitude and CSST

The tables assume a straight run. IFGC Appendix A adds equivalent length for runs with four or more fittings, so add a few feet per elbow and tee to your longest run. Above 2,000 ft of elevation the code requires the flow to be adjusted for altitude. Corrugated stainless steel tubing (CSST) is sized from the manufacturer's own tables, which vary by brand, so it is not included here.

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