Can I size a fuel-gas system from this result alone?

No. Verify the locally adopted IFGC/NFPA 54 or other applicable method, the serving utility data and the appliance minimum inlet pressure.

Why does diameter have such a large effect?

In the historical correlation the diameter appears to approximately the fifth power before inversion, so modest bore changes can cause very large pressure-drop changes.

What pressure regime is this intended for?

Low-pressure appliance piping only. Do not use this simplified correlation for high-pressure transmission/distribution piping or compressible-flow engineering.

Drawing No. EH–FM–010 // Fluid Mechanics & Piping

Gas Pipe Pressure Drop Calculator

Reviewed August 2026

Estimate the pressure drop of a low-pressure gas pipe from flow, actual internal diameter, developed length and gas specific gravity. This is a screening calculation; final piping must be checked with the locally adopted fuel-gas code method.

Scope: Educational low-pressure smooth-pipe screening using the inverse of the historical Spitzglass capacity correlation. It does not replace the current IFGC/NFPA 54 sizing equations or tables. Verify adopted code, supply pressure, appliance minimum inlet pressure, fittings/equivalent length and gas-supplier properties.

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.

1. Select gas typeNatural gas or propane, which sets typical specific gravity.

2. Enter pipe size and lengthNominal pipe internal diameter and total developed length (including fitting allowance).

3. Enter allowable pressure dropThe maximum pressure drop the system can tolerate, typically 0.3–0.5 in. w.c. for residential low-pressure systems.

4. Read capacityMaximum flow rate in cubic feet per hour and equivalent heating capacity in BTU/hr.

Inputs

Gas type

Nominal Schedule 40 steel size

Total developed length ft

Include fittings using equivalent length or another adopted sizing allowance.

Gas demand input as

Gas flow ft³/h at standard conditions

Upstream gauge pressure in. w.c.

Enter positive diameter, length, gas demand and specific gravity.

Results

Estimated pressure drop

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Drop per 100 ft

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Estimated outlet pressure

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Gas flow used

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Background

Frequently asked questions

Practical questions about interpreting the result.