Gas Line Size Calculator
Determine the required pipe diameter for natural gas or propane distribution lines using the Spitzglass formula (low-pressure) and the Weymouth formula (high-pressure).
Formulas Used
Spitzglass Low-Pressure (< 1 PSI):
Q = 3550 × √[ (ΔH × D&sup5;) / (Sg × L × (1 + 3.6/D + 0.03D)) ]
Spitzglass High-Pressure (≥ 1 PSI):
Q = 3550 × √[ (P&sub1;² − P&sub2;²) × D&sup5; / (Sg × L × (1 + 3.6/D + 0.03D)) ]
Weymouth (High-Pressure Transmission):
Q = 433.5 × (Tb/Pb) × √[ (P&sub1;² − P&sub2;²) / (Sg × T × L × Z) ] × D8/3
Where: Q = flow rate (CFH), D = inside diameter (in), L = pipe length (ft), Sg = specific gravity of gas (air=1.0), ΔH = pressure drop (in. W.C.), P&sub1; = inlet pressure (PSIA), P&sub2; = outlet pressure (PSIA), Tb = base temperature (520 °R), Pb = base pressure (14.73 PSIA), T = flowing temperature (520 °R), Z = compressibility factor (1.0 assumed)
Assumptions & References
- Spitzglass Low-Pressure formula applies to systems operating below 1 PSI (typically residential distribution at ½ in. W.C. to 2 in. W.C. pressure drop).
- Spitzglass High-Pressure formula applies to systems operating at or above 1 PSI.
- Weymouth formula is used for high-pressure gas transmission pipelines; assumes standard conditions: Tb = 520 °R, Pb = 14.73 PSIA, flowing temperature T = 520 °R, compressibility Z = 1.0.
- Natural gas specific gravity is typically 0.55–0.65 (commonly 0.60); propane is approximately 1.52.
- Pipe inside diameters are based on Schedule 40 steel pipe (ASME B36.10). Actual IDs vary by material and schedule.
- Always select the next standard nominal pipe size larger than the calculated minimum diameter.
- Results must be verified against applicable codes: NFPA 54 (National Fuel Gas Code), IFGC (International Fuel Gas Code), ASME B31.8 (Gas Transmission Pipelines), and local authority having jurisdiction (AHJ).
- This calculator does not account for fittings, valves, or elevation changes. Apply appropriate equivalent length additions for fittings (typically 10–20% of pipe length for simple systems).
- For systems with significant elevation changes, pressure corrections for gas column weight should be applied.