Gas pipe sizing calculator

How the pipe size is found

The longest length method in IFGC Appendix A sizes every part of a low-pressure system from one row of the capacity table. You convert each appliance input to the unit of the table, pick the row for the longest run, and read across for the first size that carries the load. The form returns the main from the meter and the branch to each appliance. For a section in between, enter the total input it feeds as one appliance with the same longest run. The main line in that result is your section.

Gas demand

cfh = input BTU/h / heating value BTU per cu ftNatural gas tables are in cubic feet per hour. At 1,000 BTU per cu ft, a 100,000 BTU/h furnace is 100 cfh. The propane table is in thousand BTU/h, so a propane load goes in as input / 1,000 with no heating value.

Load of a section

load = sum of the inputs downstream of that sectionIFGC 402.2 sizes for every appliance at full input at the same time. The main from the meter carries all of them, a branch carries only its own appliance. A section in between carries the appliances past it. Enter their total as one appliance with the same longest run, and the size the result shows for the main line is the size of that section.

Row

row = the longest run, or the next longer rowMeasure from the point of delivery to the farthest outlet and add the fitting allowance if you use one. A 52 ft run is read on the 60 ft row. That one row sizes the whole system, branches included.

Size

size = smallest pipe with capacity >= load on that rowNo interpolation. If the load sits between two capacities, the larger pipe wins. A load equal to a printed capacity fits that pipe.

Reference formula

Q = 2313 x D^2.623 x (dH / (Cr x L))^0.541IFGC Appendix A104 low-pressure formula. D is inside diameter in inches, dH the 0.5 in. w.c. drop, L the length in feet, Cr 0.6094 for natural gas and 1.2462 for propane (IFGC Table 402.4). The result shows it next to the table answer for natural gas. The table row is what sizes the pipe.

The code's example house, and single appliances

IFGC Appendix A106.1 sizes a house with a furnace, a water heater, a range and a dryer, 60 ft from the point of delivery to the farthest outlet. Every line goes through the same function as the form. The form shows the outlets and Section 3, the main. Sections 1 and 2 come out the same way if you enter their total input as one appliance, and the main line of that result is the section.

Section 2 carries 135 cfh and 3/4 inch carries 137 cfh on the 60 ft row, so it stays at 3/4 inch with 2 cfh to spare. Add one more small appliance to that section and it goes to 1 inch.

One appliance on its own line, natural gas at 1,000 BTU per cu ft. The furnace and tank inputs are examples. The two DOE rows sit at the limits in 10 CFR 430.2. A gas storage water heater is rated at 75,000 BTU/h or less and a gas tankless water heater under 200,000 BTU/h.

On a 50 ft run the tankless heater needs 1 inch where the 40,000 BTU/h tank gets by on 1/2 inch. On propane the same tankless takes 3/4 inch at 50 ft, because each cubic foot of propane carries more heat. When a tank is swapped for a tankless, check the old branch and the main against the new total.

IFGC Table 402.4(2), natural gas, cfh, 1/2 to 2 inch. The full table runs to 12 inch and 2,000 ft.

IFGC Table 402.4(28), propane after the regulator at 11.0 in. w.c., thousand BTU/h, 1/2 to 2 inch.

Codes and data sources

• 2021 International Fuel Gas Code, Section 402 and Tables 402.4(2) and 402.4(28) - the maximum demand rule (402.2), the longest length method (402.4.1), the Schedule 40 capacity tables for natural gas at a 0.5 in. w.c. drop and for undiluted propane at 11.0 in. w.c. inlet and 0.5 in. w.c. drop, and Cr from Table 402.4. Sizes 1/2 to 4 inch, rows 10 to 2,000 ft.
• 2021 IFGC Appendix A, Sizing and Capacities of Gas Piping (informative) - the steps of the longest length method (A103.1), the low-pressure formula (A104) and Example 1 (A106.1), which this calculator reproduces line for line as its default.
• 2021 Seattle Fuel Gas Code, Chapter 4 and Appendix A - a second published copy of the same IFGC tables. Every cell this page uses was matched against it.
• 10 CFR 430.2, definitions for consumer water heaters (DOE) - the input limits in the water heater examples. A gas-fired storage water heater has a nameplate input of 75,000 BTU/h or less, a gas-fired instantaneous water heater less than 200,000 BTU/h.
• Appliance rating plate and installation instructions - the input rating to enter and the minimum inlet pressure the appliance needs.
• Generac Guardian 20, 22 and 24 kW spec sheet, Part No. A0000937814 Rev. E - a generator listed in cfh: 306 ft3/hr of natural gas and 142 ft3/hr of propane at full load for the 24 kW, and the note to multiply ft3/hr by 1,000 (NG) or 2,500 (LP) for BTU content. With a local heating value, the page says to enter such an appliance as cfh x 1,000, or cfh x the local value when it is above 1,000, so the pipe never carries less than the listed cfh.

Limitations of this calculator

• A preliminary size. The adopted code, the gas supplier, the appliance installation instructions and the local inspector govern the installed system. A licensed gas fitter or plumber pulls the permit, installs the pipe and runs the pressure test.
• Schedule 40 steel only, at an inlet pressure under 2 psi with a 0.5 in. w.c. drop. CSST, copper, polyethylene, 2 psi systems and other pressure drops have their own tables and are not sized here.
• Natural gas is read from Table 402.4(2), built on a specific gravity of 0.60. IFGC Appendix A102.4 lets gas of 0.70 or less use the tables directly. Heavier gas needs the gravity multiplier, which this page does not apply.
• Propane is read from Table 402.4(28), for pipe between a single or second-stage regulator at 11.0 in. w.c. and the appliance. Pipe between the first and second stage regulators uses a different table.
• Every section uses the longest run. The branch length method (IFGC 402.4.2) can allow smaller branches and is not used here.
• All appliances are counted at full input at once, as IFGC 402.2 sets out. Diversity is allowed only where it can be established, and this page does not apply it.
• IFGC 402.2 requires the flow to be adjusted for altitude above 2,000 ft. This page does not make that adjustment.
• Sizes stop at 4 inch and rows at 2,000 ft. Fitting losses count only if you enter an allowance. Meter and regulator capacity are not checked.
• Written to the 2021 IFGC. Some jurisdictions adopt a different edition, local amendments, NFPA 54 or the Uniform Plumbing Code for fuel gas, so check what your inspector enforces.

Common gas pipe sizing mistakes

1. Sizing a branch by its own short length. With the longest length method the farthest outlet sets one row for the whole system. A water heater 10 ft from the meter is still sized on the row for the longest run in the house.
2. Entering the output rating. The tables size for gas burned, so the number is the input rating on the plate. An 80,000 BTU/h input furnace at 96 percent AFUE still burns 80,000 BTU/h of gas.
3. Adding a tankless heater to the old water heater line. The old line was sized for the tank. A 199,000 BTU/h tankless burns about 5 times the gas of a 40,000 BTU/h tank, and on a 50 ft run it needs a 1 inch branch where the tank needed 1/2 inch. Rerun the main from the meter with the new total too.
4. Reading the propane table in cubic feet. The propane table lists thousands of BTU per hour, not cfh. Look up a 199,000 BTU/h input as 199 in that table. A generator listed in cfh of propane is cfh x 2,500 BTU/h, so Generac's 142 cfh for the 24 kW at full load is 355,000 BTU/h, 355 in that table.
5. Using steel capacities for CSST or copper. Nominal sizes do not match inside diameters across materials. Each material has its own table, and CSST goes by the EHD number in the manufacturer's tables.
6. Leaving out appliances planned for later. IFGC 401.8 asks for pipe sized to supply the full number of outlets for the intended purpose. If a range or generator is planned, put it in the sum now.

Common questions about gas pipe size

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