Gas Pipe Sizing Calculator

*Results vary with temperature, SG, roughness, and fittings assumptions. Always validate with your governing fuel gas code and appliance requirements.

This calculator estimates diameter by limiting pressure drop using Darcy–Weisbach:

• Pressure drop: ΔP = f · (L/D) · (ρ·V²/2)
• Velocity: V = Q/A, where A = πD²/4
• Reynolds number: Re = ρVD/μ
• Friction factor: Swamee–Jain for turbulent flow; f = 64/Re for laminar flow.

A low-pressure compressible approximation converts standard flow to line flow using average absolute pressure and temperature. The solver iterates to find the minimum internal diameter that meets the allowable drop.

1. Choose the gas type and enter your expected flow.
2. Enter inlet pressure, allowable drop, and pipe length.
3. Add extra equivalent length for fittings and valves.
4. Select a pipe material catalog for standard internal diameters.
5. Adjust advanced inputs if you have project-specific values.
6. Click Calculate Pipe Size to view results above.
7. Download the output as CSV or PDF for records.

If predicted drop is close to the allowable limit, consider selecting the next larger size, shortening runs, reducing fittings, or increasing supply pressure (where permitted).

Design inputs that drive sizing

Pipe size is governed by required flow, equivalent length, and allowable pressure loss. Enter straight length and add equivalent length for elbows, tees, valves, regulators, and risers. When fittings are unknown, a common allowance is 25 to 50 percent added length. A smaller allowable drop forces a larger internal diameter so friction remains low and appliance inlet pressure stays stable at peak demand.

Interpreting flow in practical units

Equipment schedules may list loads in BTU per hour, while field work uses SCFH or metric volume rates. The calculator converts inputs to a standard flow basis so mixed schedules stay consistent. A planning assumption is about 1,000 BTU per standard cubic foot for natural gas and about 2,516 for propane vapor. Confirm local heating value for long runs or tight margins.

Pressure, elevation, and gas properties

Low pressure distribution is sensitive to small changes in available pressure. The tool estimates absolute inlet pressure from elevation and gauge pressure, then evaluates losses against the allowable drop in inches of water column. Specific gravity and viscosity adjust density and Reynolds number, influencing friction factor and pressure loss. Typical specific gravity is about 0.60 for natural gas and about 1.52 for propane vapor. Use project data when properties differ.

How the model estimates pressure drop

The solver applies Darcy–Weisbach and iterates on diameter until the predicted pressure drop meets the selected limit. Standard flow is converted to line flow using average absolute pressure and temperature, giving a compressibility approximation for low pressure systems. Friction factor is computed using Swamee–Jain for turbulent flow and the laminar relationship for low Reynolds numbers. Material roughness is included, and you may override it for special pipe.

Using results for construction decisions

Use the recommended nominal size for preliminary layouts, procurement, and coordination with sleeves, trenches, and penetrations. Review velocity as a reasonableness check; high velocity can increase noise and makes fitting losses more important. If predicted drop is close to the limit, select the next larger size, reduce fittings, shorten the route, or allow higher supply pressure where permitted. Validate final design with fuel gas codes and utility guidance.

Is this calculator code compliant?

This tool supports planning and education. Final sizing must follow your governing fuel gas code, local jurisdiction requirements, and utility standards, with approval by a qualified professional where required.

What should I use for allowable pressure drop?

Use the design criterion for your system, often set by appliance inlet requirements and regulator capacity. For low pressure distribution, many projects target 0.3 to 1.0 inches of water column, but confirm project specs.

How do I estimate equivalent length for fittings?

Add published equivalent lengths from fitting tables, or apply a conservative allowance during early design. When layouts are uncertain, adding 25 to 50 percent to straight length is a common planning approach.

Why does elevation matter?

Atmospheric pressure decreases with elevation. For the same gauge inlet pressure, the absolute pressure is lower at higher elevations, which changes density and can increase velocity and pressure loss for a given standard flow.

Can I enter BTU per hour instead of flow?

Yes. The tool converts BTU per hour to standard flow using typical heating values. For the best accuracy, replace assumptions with your local utility heating value and verify the gas type selected.

What if the predicted drop is near the limit?

Consider selecting the next larger nominal size, reducing fittings, shortening the run, or allowing higher supply pressure where permitted. Then recheck the calculation and confirm using code tables before construction.