Natural Gas Pipe Quick Estimator

Inputs

Gas Load (MBH) 1 MBH = 1,000 BTU/hr; converted at 1,020 BTU/ft³ (natural gas) or 2,516 (propane)

Gas Type

Pipe Length (ft) Developed length — add 50% for fittings if no fitting count

Inlet Pressure

Allowable Pressure Drop (in. w.c.) Low pressure: commonly 0.3–0.5 in. w.c. (the IFGC table drops)

Results

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Recommended Size

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Gas Load (CFH)

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Gas Velocity (FPS)

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Actual ΔP (in.wc)

All Pipe Sizes

Pipe Size	ID (in)	Capacity (CFH)	Velocity (FPS)	ΔP (in.wc)	Status

Gas Velocity — Guidance, Not Code

Source	Velocity	Notes
IFGC / NFPA 54	No limit	Pipe is sized by allowable pressure drop (IFGC §402.4 tables and equations)
Rule of thumb	~60 FPS	Above this, noise and dust carry-over become concerns; flagged in the table
Utility service piping	—	Per the serving utility’s requirements

Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This natural gas pipe quick estimator sizes a single gas run in Schedule 40 black iron pipe, at low pressure or on a 2 or 5 psig system. Enter the gas load in MBH, the gas type (natural gas or propane), the inlet pressure, the developed length, and the allowable pressure drop; the tool recommends a pipe size and reports the gas demand in CFH, the gas velocity, and the actual pressure drop, plus a full capacity table for every size. It gives the same size as the Natural Gas Pipe Sizer for the same inputs.

It is meant for a fast first pass on a single run. Final design should follow the longest-length or branch-length tables in NFPA 54 / IFGC, confirm utility pressure, and account for fittings and local code amendments.

Formula & Method

Low pressure (IFGC Eq. 4-1): Q = 2313 × D^2.623 × (ΔH ÷ (C_r · L))^0.541
2 and 5 psig (IFGC Eq. 4-2): Q = 2237 × D^2.623 × ((P₁² – P₂²) · Y ÷ (C_r · L))^0.541
Velocity: v (fps) = Q ÷ 3,600 ÷ A × 14.7 ÷ P_avg where A = π · (ID ÷ 24)²

These are IFGC 2021 Equations 4-1 and 4-2 (the same equations appear in NFPA 54) written for capacity: Q in CFH, D the Schedule 40 inside diameter (in), L the developed length (ft), ΔH the allowable drop (in. w.c.), and P₁, P₂ the absolute inlet and outlet pressures (psia, on a 14.7 psia atmosphere). C_r and Y are the IFGC Table 402.4 gas factors: natural gas 0.6094 and 0.9992, undiluted propane 1.2462 and 0.9910. The low-pressure form reproduces IFGC Tables 402.4(1) and 402.4(2) to within rounding — for example, 1 in. pipe at 100 ft and 0.5 in. w.c. carries 195 CFH. Loads convert at 1,020 BTU/ft³ for natural gas and 2,516 for propane. Both equations assume sea-level conditions (14.7 psia); at 5,000 ft the 2 and 5 psig capacities read about 7–9% high on that basis, so at high elevation use your utility’s local heating value and confirm with the utility. The codes set no gas velocity limit; the tool reports velocity at the average line pressure and flags values above about 60 fps as a noise rule of thumb.

Frequently Asked Questions

How do I quickly size a low-pressure gas pipe?

Enter the gas load in MBH, the gas type, the inlet pressure, the developed pipe length, and the allowable pressure drop. The tool converts the load to cubic feet per hour, applies the IFGC / NFPA 54 sizing equation (Equation 4-1 below 1.5 psi, Equation 4-2 for the 2 and 5 psig options) to find each Schedule 40 black iron size's capacity, and recommends the smallest pipe that carries the load within the allowable drop. Its capacities match the IFGC Schedule 40 tables.

How is MBH converted to CFH for gas?

Natural gas carries about 1,020 BTU per cubic foot, so one MBH (1,000 BTU/hr) is just under one cubic foot per hour: CFH = MBH × 1,000 ÷ 1,020. Propane carries about 2,516 BTU per cubic foot, so the same MBH needs about 60 percent fewer CFH; the tool applies that conversion when propane is selected. Use your utility's heating value for final design, especially at high elevation.

What gas velocity is acceptable in the pipe?

Neither the IFGC nor NFPA 54 sets a gas velocity limit: building gas piping is sized by allowable pressure drop, and at typical low-pressure drops the velocity stays modest. The tool reports the velocity at each size (corrected to line pressure for the 2 and 5 psig options) and flags anything above about 60 fps, a common rule of thumb for noise and dust carry-over rather than a code requirement.

Should I add length for fittings?

Yes. Use the developed length to the most remote outlet, and if you are not counting fittings individually, a common rule of thumb is to add roughly 50 percent to the straight run as an equivalent-length allowance for elbows, tees, and valves. Underestimating length understates the pressure drop and can lead to an undersized pipe and starved appliances.

Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.