Gas Pipe Sizing Calculator

This calculator sizes natural gas or propane branch piping with the longest-length method and a simplified Weymouth formula, the same logic behind IFGC Table 402.4 and its companion standard, NFPA 54 (the National Fuel Gas Code, jointly designated ANSI Z223.1). Total BTU Load is converted to a cubic-feet-per-hour flow rate, and the calculator solves for the smallest standard trade pipe size (1/2" up to 4") that keeps the theoretical diameter at or above the calculated requirement. Inlet Pressure's relationship to Pipe Capacity is not a clean, one-directional one: it feeds both the pressure-differential term that Pipe Capacity scales with AND the theoretical diameter, and raising Inlet Pressure can actually shrink that calculated diameter enough to drop the selected standard trade size down a bracket -- which cuts Pipe Capacity even though the underlying pressure term rose.

Total BTU Load, not Inlet Pressure, is the real dominant driver of Pressure Drop: a proportional change in Total BTU Load moves Pressure Drop several times further than the same proportional change in Inlet Pressure, whose effect on Pressure Drop is close to flat by comparison. Longest Pipe Run and Gas Specific Gravity both push Pressure Drop up as well, each by roughly half of what an equivalent change in Total BTU Load produces: a longer run spreads the same pressure differential over more distance, and a denser gas resists flow more per foot -- propane's specific gravity near 1.52 is roughly two and a half times natural gas's 0.60. This tool does not add fitting or valve equivalent lengths, branch takeoffs, or code-required derating for corrugated stainless tubing; treat the result as a sizing starting point to check against a full IFGC longest-length table, not a substitute for a stamped gas load calculation.