Drawing No. EH–FM–009 // Fluid Mechanics & Piping
Gas Pipe Flow Calculator
Reviewed August 2026
Estimate low-pressure gas flow capacity with the historical Spitzglass equation as an educational screening calculation; final piping must be sized by the locally adopted code method.
What problem does this solve?
Undersized gas piping starves appliances of the fuel they need under load, causing poor combustion, nuisance shutdowns, or in severe cases unsafe operation — but the relationship between pipe size, length and capacity isn't obvious without doing the calculation. This tool applies the classical low-pressure gas sizing formula so you can check a specific run's capacity, or work backward from a required BTU/hr load to see what pipe size is needed.
Inputs
Results
Background
Q = 3550·√[ΔH·d⁵/(SG·L·(1+3.6/d+0.03d))], where Q is flow in cubic feet per hour (at standard conditions), ΔH is allowable pressure drop in inches of water column, d is pipe internal diameter in inches, SG is the gas's specific gravity relative to air, and L is total developed pipe length in feet. This is a historical low-pressure fuel-gas screening correlation. Current IFGC/NFPA 54 methods use prescribed equations and tables with defined conditions, so this result can differ materially from current code capacities and is not a compliance calculation.
The length used in gas pipe sizing isn't just the straight-line pipe run — it includes an allowance for the extra resistance of every elbow, tee and valve along the path, often approximated by adding an equivalent length for each fitting (or by simply using a standard multiplier on the longest straight run for quick sizing). Using only the physical straight-line distance without any fitting allowance will under-size the calculated pressure drop and lead to an undersized pipe.
Propane (SG≈1.52) is denser than natural gas (SG≈0.60) relative to air, and the Spitzglass equation shows capacity scales with 1/√SG — so for the same pipe, length and allowable pressure drop, a propane system carries meaningfully less volumetric flow (cubic feet per hour) than a natural gas system would. Propane's much higher heating value per cubic foot (roughly 2,500 BTU/ft³ versus roughly 1,000 BTU/ft³ for natural gas) partly offsets this in terms of delivered heating capacity, which is why propane and natural gas systems use different sizing tables.
The standard approach for a branched gas piping system sizes each pipe segment based on the load it carries and the total developed length from the meter to the farthest appliance the segment serves — even segments serving a closer appliance are sized using this longest overall distance, which keeps the method conservative and simple rather than requiring a full pressure-drop calculation for every branch individually.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
Because capacity scales with d^2.5 in the Spitzglass equation (d⁵ inside a square root) — going from 1/2 in to 1 in pipe (roughly doubling the diameter) increases capacity by a factor of about 4.5, far more than a naive area-based (d²) scaling would suggest, since larger pipes also have proportionally less friction loss relative to their flow area.
For typical residential low-pressure systems (supplied at around 7 in. w.c.), 0.5 in. w.c. total allowable drop is a common design value found in NFPA 54 tables, though some jurisdictions or specific appliance requirements call for a more conservative 0.3 in. w.c. — check the applicable code and the minimum inlet pressure required by the appliance nameplate, since undersized piping can starve appliances of gas under full load even if it passes at idle.
No. Current fuel-gas codes provide prescribed sizing tables and/or equations with defined conditions. Spitzglass is retained here as a historical screening correlation and can differ materially from current code-table capacities. Use the adopted code method for design and compliance.
No — this is specifically the low-pressure formula, valid for gauge pressures below roughly 1.5 psi. Higher-pressure gas transmission and distribution piping (the kind used by utilities between a regulator station and a neighborhood, for example) uses entirely different equations (such as Weymouth or Panhandle) that account for the gas's compressibility at higher pressure, which the low-pressure Spitzglass formula does not.