Calculate the recommended Schedule 40 black iron gas pipe size for natural gas or propane based on BTU load and pipe run length in feet or meters.
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Gas Piping Size Formula
The calculator sizes Schedule 40 black iron gas pipe by converting the appliance load from BTU/hr to gas flow in cubic feet per hour, then finding the smallest standard pipe size with enough capacity for the selected run length.
CFH = BTU/hr / HV
Q = 3550 * sqrt(D^5 / (1 + 3.6/D + 0.03*D)) * sqrt(ΔP / (L * SG))
Recommended Size = smallest standard pipe size where Q >= CFH
- CFH = required gas flow in cubic feet per hour
- BTU/hr = total connected appliance load
- HV = heat value of the gas, 1,000 BTU/ft³ for natural gas and 2,516 BTU/ft³ for propane
- Q = pipe capacity in cubic feet per hour
- D = inside diameter of the pipe in inches
- ΔP = allowable pressure drop, using 0.5 inches water column
- L = pipe run length in feet
- SG = specific gravity of the gas, 0.60 for natural gas and 1.52 for propane
For natural gas, the calculator uses a lower heat value per cubic foot, so the same BTU/hr load requires more cubic feet per hour. For propane, the higher heat value means the same BTU/hr load needs fewer cubic feet per hour, but the gas also has a higher specific gravity. If you enter the run length in meters, it is converted to feet before sizing. Runs shorter than 10 feet are treated as 10 feet because common sizing tables normally start there.
Typical Appliance Loads and Gas Sizing Inputs
Use the appliance load as a starting point only. The nameplate rating on the actual appliance is the value to use when it is available.
| Appliance | Typical Load | Sizing Note |
|---|---|---|
| Gas fireplace | 30,000 BTU/hr | Often a small branch load |
| Clothes dryer | 35,000 BTU/hr | Use the dryer nameplate if different |
| Storage water heater | 40,000 BTU/hr | Common residential input |
| Cooking range | 65,000 BTU/hr | Total input can vary by model |
| Mid-size furnace | 80,000 BTU/hr | Use input rating, not output rating |
| Tankless water heater | 199,000 BTU/hr | Often requires a larger gas line |
| Gas Type | Heat Value Used | Specific Gravity Used |
|---|---|---|
| Natural gas | 1,000 BTU/ft³ | 0.60 |
| Propane, LP | 2,516 BTU/ft³ | 1.52 |
Example Gas Pipe Size Calculations
Example 1: Natural gas furnace
You have an 80,000 BTU/hr natural gas furnace and a 40 ft longest run.
CFH = 80000 / 1000 = 80 CFH
The calculator checks each Schedule 40 pipe size at 40 ft using natural gas specific gravity. The recommended size is the smallest pipe with capacity at or above 80 CFH.
Example 2: Propane tankless water heater
You have a 199,000 BTU/hr propane tankless water heater and a 60 ft longest run.
CFH = 199000 / 2516 = 79.1 CFH
The calculator checks the propane capacity for each standard pipe size at 60 ft and selects the first size that meets or exceeds 79.1 CFH.
FAQ
Should I enter the total appliance load or only one appliance?
Enter the load that the pipe section must carry. For a branch serving one appliance, enter that appliance’s BTU/hr input. For a main section feeding several appliances, add the BTU/hr inputs of the appliances downstream of that section.
What pipe run length should I use?
Use the longest practical run from the gas source to the appliance or to the farthest outlet being sized. Include horizontal and vertical pipe lengths. Fittings, valves, and local sizing methods can add equivalent length, so a field design may require more length than the straight-line measurement.
Can this replace a code-approved gas piping design?
No. Gas piping must be installed and sized according to the applicable fuel gas code, utility requirements, appliance instructions, pressure system, material, and fittings. Use the result as a sizing estimate, then confirm it with a licensed gas fitter or the authority having jurisdiction.
