This natural gas pipe sizer turns a schedule of gas appliance loads into a recommended Schedule 40 steel pipe size. Enter each appliance input in BTU/h with a quantity and demand factor, set the gas type, system pressure, heating value, and the developed pipe length, and the tool computes the design demand in cubic feet per hour (CFH) and the smallest pipe that carries it within the allowable pressure drop.
It uses the Spitzglass formula for low-pressure systems and the Weymouth formula for medium-pressure systems, and supports both natural gas and propane. Results are preliminary: final design should follow the longest-length or branch-length method in NFPA 54 / IFGC, with utility pressure and fitting allowances confirmed for the project.
D is the pipe inside diameter (in), L the developed length (ft), Sg the gas specific gravity (≈0.60 natural gas, 1.52 propane), and ΔP the pressure drop — inches of water column for Spitzglass, psia for Weymouth, where P1 and P2 are absolute inlet and outlet pressures. These are recognized engineering forms behind the capacity tables in NFPA 54 (National Fuel Gas Code) and the IFGC, which size pipe by the longest-length or branch-length method to the most remote appliance.
Frequently Asked Questions
How do I size a natural gas pipe from BTU/hr?
First convert the connected appliance load in BTU/h to a volumetric demand in cubic feet per hour (CFH) by dividing by the gas heating value, about 1,020 BTU/SCF for natural gas. Then pick the smallest pipe whose capacity at the design length and allowable pressure drop meets or exceeds that CFH demand. This tool runs that comparison across Schedule 40 sizes and flags the first that fits.
What is the difference between the Spitzglass and Weymouth formulas?
The Spitzglass formula is used for low-pressure gas systems below 1 psig where the drop is measured in inches of water column. The Weymouth formula is used for medium-pressure systems where the drop is a meaningful fraction of the absolute pressure and capacity depends on the difference of the squared pressures. This tool automatically applies Spitzglass for low pressure and Weymouth for medium pressure selections.
Does pipe length affect the required gas pipe size?
Yes. Capacity falls as the square root of length, so doubling the run reduces capacity by about 30 percent and often pushes you up a pipe size. Codes use the longest-length or branch-length method, sizing every segment for the total developed length to the most remote outlet. Add an equivalent-length allowance for fittings, since elbows and tees behave like extra straight pipe.
Is this calculator code-compliant for permit drawings?
Use it for preliminary sizing only. Final gas piping design must follow the longest-length or branch-length tables in NFPA 54 (the National Fuel Gas Code) or the IFGC, confirm utility delivery pressure, apply fitting allowances, and meet local amendments. Propane (LP) sizing uses a different specific gravity and heating value, both of which this tool accounts for.
This calculator handles one step. AIM Works runs the complete MEP design workflow — thermal load calculations, duct & pipe networks, equipment selection, code compliance, and an AI design assistant — in one tool.