Which formula does this calculator use?

It uses the low-pressure Spitzglass formula referenced by NFPA 54 and IPC Appendix A for fuel gas under 1.5 psi. The equation relates flow, inside diameter, length, and specific gravity, and is solved here for the pressure drop in inches of water column.

What flow value should I enter in SCFH?

Enter the connected appliance demand carried by that segment. If you know the load in BTU per hour, divide by the gas heating value — roughly 1030 BTU per cubic foot for natural gas and 2516 for propane — to get SCFH. The tool also shows the heat input back-calculated from your flow.

Why is 0.3 in WC the usual target?

Most residential and light-commercial low-pressure systems are designed so the total drop from the meter or regulator to the farthest appliance does not exceed 0.3 in WC, preserving appliance manifold pressure. A 0.5 in WC allowance is sometimes used where the supply pressure has margin.

Does this include fittings and valves?

No — it sizes one straight segment. Add the equivalent length of elbows, tees, and valves to the measured pipe length before entering it, or check each segment and sum the drops along the longest run to the most remote outlet.

Can I use this for medium or high-pressure gas?

No. The low-pressure Spitzglass form assumes the pressure drop is small relative to atmospheric. Systems above about 1.5 psi require the high-pressure sizing equations and a different compressibility treatment, so use the appropriate NFPA 54 method for those.

Fuel Gas Pressure Drop Calculator (NFPA 54)

Keeping gas pressure where it belongs

A fuel gas pipe must deliver enough gas to every appliance without dropping the pressure below what the burners need. NFPA 54 and the IPC let designers size low-pressure systems so the total pressure loss stays within a small allowance — commonly 0.3 inches of water column. This calculator computes the drop through a pipe segment so you can confirm a run is large enough before it goes in the wall.

How it works

For low-pressure gas the code references the Spitzglass formula. Its low-pressure form relates flow to the square root of diameter to the fifth power times pressure drop, divided by length and specific gravity, with a diameter-dependent friction factor Cr = 1 / sqrt(1 + 3.6/D + 0.03·D). Solving for the pressure drop gives:

H = ( Q / (3550 · Cr) )^2 · ( Cg · L ) / D^5

where H is the drop in inches of water column, Q is flow in SCFH, D is the actual inside diameter in inches, Cg is gas specific gravity (0.60 for natural gas, 1.52 for propane), and L is length in feet. The tool uses Schedule 40 steel inside diameters and back-calculates the heat input from your flow using the gas heating value.

Example and notes

A 150 SCFH natural-gas load through 40 ft of 3/4″ Schedule 40 pipe drops well under 0.3 in WC, so the run passes. Increase the flow or length and the drop climbs with the square of flow and linearly with length, so doubling the run doubles the loss. Always add equivalent lengths for fittings, check the longest path to the most remote appliance, and confirm the delivered manifold pressure — about 3.5 in WC for natural gas and 11 in WC for propane — is still met.