Gas Pipe Size Calculator (BTU & Longest Run)
Find the gas pipe size for your total load and longest run, using a labeled NFPA-54-style longest-length band.
Calculator
A 200,000 BTU/hr load (200.0 CFH) over a longest run of 40 ft needs about a 1 in Schedule-40 line on a labeled NFPA-54-style longest-length band. ⚠️ Gas work must be done by a licensed professional with a permit and inspection; your local code governs.
Gas pipe is not sized by the load alone — it is sized by the load and the distance together. The farther gas has to travel, the more pressure it loses to friction, so a longer run needs a bigger pipe to carry the same BTU. The standard approach in the codes is the longest-length method: you take the total connected load and the developed length to the farthest appliance, then read the smallest pipe whose capacity covers the load at that length.
This tool converts your load to CFH for the fuel you pick, then looks up a labeled NFPA-54-style capacity band (Schedule-40 metallic pipe at a typical 0.5 in WC drop) scaled for your run length. It returns a starting pipe size and the capacity at that size, so you can see the headroom.
Formula
First convert the load to cubic feet per hour:
CFH = total BTU/hr ÷ heating value (natural gas 1000, propane 2516)
Then pick the smallest nominal size whose tabled capacity at your run length covers that CFH. Capacity scales from the 40 ft reference run as:
capacity(length) ≈ capacity(40 ft) × √(40 ÷ length)
The suggested size is the first one where capacity ≥ CFH demand.
Worked example
A 200,000 BTU/hr load on natural gas is 200,000 ÷ 1000 = 200 CFH. Over a 40 ft longest run, a 1 in Schedule-40 line carries about 205 CFH at the reference band — just over the 200 CFH needed — so the tool suggests 1 in. A 3/4 in line (≈ 190 CFH at 40 ft) would fall short, and a longer run would push you to 1-1/4 in. This is the classic "1 in for 200,000 BTU at 40 ft" result.
Developed length, fittings and pipe type
"Developed length" means the measured pipe length to the farthest appliance plus an allowance for fittings — every elbow and tee adds equivalent length. When in doubt, measure long and add margin. Also remember the longest-length method sizes the whole system off that single farthest run for simplicity; a branch-length design can sometimes use smaller branches, but that is a licensed designer's call.
The capacity band here is for standard black iron / Schedule-40 pipe on a low-pressure (about 0.5 in WC drop) natural-gas system. CSST (flexible corrugated tubing), semi-rigid copper and 2-psi propane systems use different tables entirely, and every manufacturer publishes its own CSST sizing chart. Treat this result as a planning starting point, never as a final design.
Reference table
Schedule-40 metallic pipe, natural gas (~0.60 sp. gr.) at a typical 0.5 in WC drop, tabled at the 40 ft reference run. Capacity falls on longer runs (≈ 1 ÷ √(length ÷ 40)). Labeled NFPA-54-style planning band — your gas fitter and local code set the real sizing.
| Nominal size | Capacity (CFH) | ≈ Natural-gas load (BTU/hr) |
|---|---|---|
| 1/2 in | 95 | 95,000 |
| 3/4 in | 190 | 190,000 |
| 1 in | 205 | 205,000 |
| 1-1/4 in | 435 | 435,000 |
| 1-1/2 in | 650 | 650,000 |
| 2 in | 1,250 | 1,250,000 |
Frequently asked questions
What size gas pipe do I need for 200,000 BTU?
On natural gas at a 40 ft longest run, 200,000 BTU/hr (200 CFH) lands on a 1 in Schedule-40 line in the labeled band. A longer run or propane changes the answer — enter your own numbers above and confirm with a licensed gas fitter.
Why does the pipe length matter so much?
Gas is delivered at very low pressure, so friction over distance eats into the little pressure available. The longer the run, the less gas a given pipe can deliver — so long runs need larger pipe for the same load.
What is the longest-length method?
It is the code method that sizes the system from the total connected load and the developed length to the farthest appliance. You read the smallest pipe whose capacity at that length covers the load — which is exactly what this tool does.
Does this work for CSST or propane?
The band shown is for Schedule-40 metallic pipe on low-pressure natural gas. CSST, copper and 2-psi propane systems use different tables — pick propane above to convert the load, but always size CSST from the manufacturer's own chart with a licensed installer.
Should I add fittings to the run length?
Yes. Elbows, tees and valves add "equivalent length." Measure the actual pipe run to the farthest appliance and round up generously to account for fittings, then confirm the final design with a professional.