Quick answer
Gas line size depends on two numbers: the heater’s BTU input rating and the pipe’s run length, not the heater’s price or physical size. On a 50-foot run of Schedule-40 black-iron pipe, a 150,000 BTU heater needs 3/4-inch pipe, a 250,000 BTU heater needs 1-inch pipe, and a 400,000 BTU heater needs 1-1/4-inch pipe — each size steps up again once the run stretches to 100 feet. Undersized gas line is a real, common failure mode: per pool-service sources, it’s behind roughly 80% of rich-burning, sooting, or ignition-lockout calls on gas pool heaters, not a defective heater.
The BTU-to-pipe-diameter chart
This is Schedule-40 black-iron pipe sized for natural gas at standard residential pressure — the same chart a licensed gas fitter works from (NFPA 54 Table 402.4). Always size off the heater’s input BTU rating on its nameplate, not the “output” number some marketing copy leads with — they’re not the same figure, and sizing off output is one of the two most common gas-line mistakes installers see.
| Heater input (BTU/hr) | Pipe size @ 50 ft. run | Pipe size @ 100 ft. run |
|---|---|---|
| 150,000 | 3/4 inch | 1 inch |
| 250,000 | 1 inch | 1-1/4 inch |
| 400,000 | 1-1/4 inch | 1-1/2 inch |
Two things to double-check before you hand this chart to a contractor: “run length” is the developed length, not the straight-line distance — add roughly 50% to your measured pipe length to account for elbows, tees, and unions, then size off that larger number. And if your meter or branch line already feeds a furnace, water heater, range, or dryer, the gas line has to be sized for all of those appliances firing at once, not just the pool heater in isolation — see our installation cost guide for what a dedicated new line costs if your existing supply can’t cover the combined load.
Black iron vs. CSST — which pipe type
Schedule-40 black-iron pipe is the traditional choice and the one most BTU/diameter charts (including the one above) are built around — rigid, threaded, and relatively cheap in material cost, though it takes more labor to thread and run than flexible tubing. CSST (corrugated stainless steel tubing) installs faster through walls and tight equipment-pad runs since it bends instead of requiring elbow fittings, but it runs on its own separate sizing tables under NFPA 54/58 rather than the black-iron chart above — a gas fitter sizing a CSST run has to pull the CSST-specific table, not reuse the black-iron numbers. Polyethylene (PE) pipe is common for the buried underground portion of a run but has to transition to black iron or CSST above grade; it is not rated for exposed outdoor use. Ask your installer which type they’re quoting and confirm they’re sizing from the matching table for that material — the diameter that’s correct for black iron is not automatically correct for CSST at the same BTU and length.
Propane changes the math
Propane’s higher energy density (~2,500 BTU per cubic foot vs. natural gas’s ~1,000) means a propane-fed heater generally needs a smaller-diameter line to deliver the same BTU input — don’t assume a propane conversion needs the same pipe as the natural-gas chart above suggests. The catch is regulation, not diameter: propane runs longer than about 100 feet need two-stage regulation — a roughly 10 psi first-stage regulator at the tank, stepped back down to about 11 inches water column at the heater itself — which a natural-gas single-regulator setup doesn’t require. If you’re converting an existing natural-gas heater to propane (or buying a propane-specific model — see our propane pool heater picks), confirm your gas fitter is pricing the regulation stage, not just the pipe.
How to tell if your existing line is already too small
The only test that actually proves a gas line is sized correctly is a manometer reading taken while the heater is firing at full output — a static reading with the heater off will look fine even on an undersized line, because the pressure drop only shows up under real gas flow. Per pool-service sources, inlet pressure that falls below 5 inches water column once the heater is running is the threshold that signals an undersized supply. If your heater is sooting, rich-burning, or locking out intermittently and the manufacturer’s troubleshooting steps (see our pool heater not heating guide) don’t turn up an obvious cause, ask your service tech for a load-test manometer reading before replacing parts — misdiagnosing a gas-supply problem as a bad igniter or control board is a common way to spend money without fixing anything.
Reotemp Natural Gas Pressure Test Gauge Kit (manometer)
- Lets a gas fitter (or you, to confirm before/after a tech visit) take the full-fire manometer reading described above — a static off-heater reading won't catch an undersized line.
- Pair with our installation cost breakdown if the reading confirms you need a new or upsized run.
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What a correctly-sized line costs
Running new gas line costs $15-$50+ per linear foot installed (materials, labor, and permits), per HomeGuide’s 2026 figures — already the canon this site uses on the installation cost breakdown. Material alone splits roughly $0.50-$1.50 per linear foot for black-iron pipe and $2-$10 per linear foot for CSST; CSST’s higher material cost is usually offset by faster, lower-labor installation on runs with multiple bends. None of that is the diameter question this guide answers — a correctly-sized 1-inch line and an incorrectly undersized 3/4-inch line cost close to the same per foot to install, which is exactly why it’s worth confirming the diameter with your gas fitter before the trench is dug rather than after a lockout call.
Raypak Digital Natural Gas Pool Heater (206,000 BTU)
- A 206,000 BTU input sits just under the 250,000 BTU row in the chart — size to the next row up (1 inch at 50 ft.) rather than rounding down to the 150,000 BTU line.
- See our full Raypak lineup for the full BTU range and matching line sizes.
The bottom line
Gas line diameter is a function of BTU input and developed run length, not heater price or cabinet size — a 250,000 BTU heater needs 1-inch pipe at 50 feet and 1-1/4 inch at 100 feet on Schedule-40 black iron, with CSST and propane each needing their own separate sizing math. The two mistakes that cause most service calls are sizing off the wrong BTU number (output instead of input) and ignoring other appliances sharing the same branch line. If a heater is already installed and acting up, a full-fire manometer reading — not a static pressure check — is the only test that actually confirms whether the line itself is the problem.
Sources: Florida’s Best Pools — Gas Heater Line Sizing, HomeGuide — Gas Line Installation Cost, up.codes — Tables for Sizing Gas Piping Systems (NFPA 54)