Gas Line Sizing Calculator
To size a gas line, add up the total BTU/hr demand of every appliance on the run, measure the longest length from the meter to the farthest appliance, then read the required pipe diameter from the NFPA 54 table for your pipe material and pressure. Convert total load to cubic feet per hour (divide BTU/hr by ~1,000 for natural gas), size on the longest run — never the sum of lengths — and confirm the pressure drop still serves the farthest appliance. The free PlumbCalc Pro gas line calculator runs the NFPA 54 table for you and returns the size, load in CFH, and pressure check with the safe defaults dialed in.
The three numbers that size every gas line
Every gas sizing job comes down to three inputs — miss one and the line is wrong:
- Load — total connected demand in BTU/hr (converted to CFH).
- Length — the longest run from the meter or second-stage regulator to the farthest appliance.
- Pressure & material — the delivery pressure and whether you are running black iron, CSST, or copper, which selects the code table.
Step 1: Total your appliance load (BTU/hr)
Add the input rating of every appliance the line feeds. Use the nameplate value on each unit — these typical figures are only anchors:
| Appliance | Typical input (BTU/hr) |
|---|---|
| Tank water heater | ~40,000 |
| Tankless water heater | ~150,000–199,000 |
| Furnace | ~80,000–100,000 |
| Range / cooktop | ~65,000 |
| Clothes dryer | ~30,000 |
| Gas fireplace | ~30,000–40,000 |
For natural gas (~1,000 BTU per cubic foot), divide total BTU/hr by 1,000 to get CFH. A house at 300,000 BTU/hr is a 300 CFH load. Propane carries ~2,500 BTU/ft³, so divide by 2,500 instead — and use the propane table.
Step 2: Measure the longest run
Gas sizing uses the longest length method: find the distance from the meter to the farthest appliance, and size the entire system to that run. You do not add up branch lengths. Capacity falls as length grows because pressure loss rises with distance, so the farthest appliance is always the worst case.
Step 3: Pick the material and read the table
The pipe material decides which NFPA 54 table you read:
- Black iron (Schedule 40 steel): sized by nominal diameter — the traditional, most-documented choice.
- CSST: sized by the manufacturer's EHD rating, not nominal size, and it must be bonded per code.
- Copper: allowed for gas in many jurisdictions with its own capacity table; confirm local code.
Exact pipe-size-versus-length capacities come straight from the NFPA 54 / IFGC tables — the numbers depend on your load, run, pressure and material together, so we do not print a one-size chart here. Enter your load and longest run in the calculator and it returns the correct size from the live table.
"Size the gas line to the farthest appliance and the hardest material table — a line that starves the back of the house passed nobody's inspection."
Size your gas line free →
Enter total BTU load and longest run; PlumbCalc Pro returns the pipe size, CFH, and pressure check from the NFPA 54 tables. No sign-up, built by 30+ years in the trades.
Frequently asked questions
How do you size a gas line?
Total the BTU/hr demand of every appliance on the run, measure the longest length from the meter (or regulator) to the farthest appliance, then read the required pipe diameter from the NFPA 54 sizing table for your material and delivery pressure. Size on the longest run, not the sum of all lengths.
What size gas line do I need for my appliances?
It depends on total connected load and distance. A 40,000 BTU water heater, an 80,000–100,000 BTU furnace, a 65,000 BTU range and a 30,000 BTU dryer add up quickly. Convert total BTU/hr to CFH (divide by ~1,000 for natural gas), find your longest run, and read the size off the code table — the calculator does it instantly.
Does natural gas sizing differ from propane?
Yes. Propane carries about 2,500 BTU per cubic foot versus roughly 1,000 for natural gas, and it is delivered at a different pressure, so the two use separate NFPA 54 tables. A line sized for natural gas is not automatically correct for propane.
CSST vs black iron — which should I use?
Black iron (Schedule 40 steel) is sized by nominal diameter and is the traditional choice. CSST installs faster but is sized by its EHD rating using the manufacturer's tables, not nominal size, and requires proper bonding. Both are valid — use the table that matches what you install.
How long can a gas line run before you upsize?
Capacity drops as length grows because pressure loss rises with distance. The same pipe that carries a load at 20 feet may be undersized at 80 feet. That is why the longest run governs the whole line — size to the farthest appliance, then verify the pressure drop still leaves each appliance within spec.
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