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 IFGC / NFPA 54 sizing equations — the low-pressure equation for the in. w.c. options and the high-pressure equation for the 2 and 5 psig options — so its capacities agree with the code’s Schedule 40 tables, for natural gas or 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 Schedule 40 inside diameter (in), L the developed length including the fittings allowance (ft), ΔH the allowable pressure drop (in. w.c.), and P1, P2 the absolute inlet and outlet pressures (psia, on a 14.7 psia sea-level atmosphere). Cr and Y are the IFGC Table 402.4 gas factors: natural gas Cr = 0.6094, Y = 0.9992; undiluted propane Cr = 1.2462, Y = 0.9910. These are IFGC 2021 Equations 4-1 and 4-2 (the same equations appear in NFPA 54) written for capacity; the low-pressure form reproduces IFGC Tables 402.4(1) and 402.4(2) to within rounding — for example, 1 in. pipe at 100 ft and a 0.5 in. w.c. drop carries 195 CFH. Size each segment for the total developed length to the most remote appliance (longest-length method); the tool computes one size at the entered length. Natural gas is taken at 1,020 BTU/ft³ and propane at 2,516. Both equations assume sea-level conditions (14.7 psia); at 5,000 ft the 2 and 5 psig capacities read about 7–9% high on that basis, so at high elevation use your utility’s local heating value and confirm with the utility.
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?
Both are classic engineering flow equations: Spitzglass for low-pressure gas, with the drop in inches of water column, and Weymouth for higher pressures, where capacity depends on the difference of the squared absolute pressures. The fuel gas codes use their own pair of sizing equations, IFGC Equation 4-1 below 1.5 psi and Equation 4-2 at 1.5 psi and above (the same equations appear in NFPA 54), and the code capacity tables are computed from them. This tool uses the code equations, so its capacities match the IFGC Schedule 40 tables. At 100 ft and a 0.5 in. w.c. drop, Spitzglass runs about 23 percent below those tables in 1/2 in. pipe and 9 to 13 percent above them in 3 in. and larger, and Weymouth differs by about 5 percent at 100 ft but 15 percent or more on very short or long runs; the gaps shift with length because both use length to the 0.5 power.
Does pipe length affect the required gas pipe size?
Yes. Capacity falls roughly as the square root of length (the code equation uses length to the 0.541 power), 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 different gas factors and a different heating value; selecting Propane switches both (IFGC Cr = 1.2462, Y = 0.9910, and 2,516 BTU per cubic foot).
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