Gas Line Sizing Calculator

You enter the appliance demand in BTU per hour, the developed length of pipe from the gas meter (or second-stage LP regulator) to the appliance, and whether you have natural gas or propane. The calculator reads the NFPA 54 longest-length capacity table for that pipe length and returns the smallest Schedule 40 pipe size whose rated capacity meets or exceeds your load. It also converts your BTU demand into cubic feet per hour so you can compare it against the table directly.

Developed length is the total measured run of pipe from the point of delivery to the most remote appliance outlet, following the actual route. The longer the run, the more pressure is lost to friction, so a longer pipe carries less gas at the same allowable pressure drop. The calculator rounds your length up to the next table column (for example a 47 ft run uses the 50 ft column), which is the conservative approach the longest-length method requires.

Divide the BTU per hour load by the heating value of the gas in BTU per cubic foot. Natural gas is taken at 1,000 BTU per cubic foot, so 150,000 BTU/hr is 150 cubic feet per hour. Propane is about 2,516 BTU per cubic foot, so 100,000 BTU/hr is roughly 39.7 cubic feet per hour. Natural gas tables are published in cubic feet per hour, while the propane table is published in thousands of BTU per hour.

The two fuels use separate NFPA 54 tables with different assumptions. Natural gas is sized at a 0.5 inch water column pressure drop with a 0.60 specific gravity, while propane uses a 0.5 inch water column drop at 11 inch water column delivery with a 1.50 specific gravity. Propane is denser but delivered at higher pressure, so a given pipe size carries a different volume. Always size with the table that matches your actual fuel and delivery pressure.

It assumes standard Schedule 40 metallic pipe (black iron or steel) sized for a 0.5 inch water column pressure drop, which is the most common low-pressure residential setup downstream of the meter or LP regulator. If you use CSST, polyethylene, or a 2 psi system with a separate appliance regulator, the capacity tables are different and you should size from the table that matches that material and pressure.

Use it as a planning estimate, not as a substitute for a licensed professional or your local code authority. Gas piping must be designed and installed per NFPA 54 (the National Fuel Gas Code) and any local amendments, and final sizing should account for total connected load, branch layout, fittings, and inspection requirements. Have a qualified plumber or gas fitter verify the design and pull the proper permit.