Quick answer
Propane tank size for a pool heater comes down to the heater’s BTU input, not the pool’s gallon count: a 250-gallon tank (about 200 usable gallons) comfortably covers a smaller 100,000-150,000 BTU heater used occasionally, while a 300,000-400,000 BTU heater — the size most 15,000-gallon-plus pools need per our sizing chart — burns roughly 3-4 gallons an hour and usually calls for a 500-gallon tank if it runs daily. A 20-pound grill-style cylinder is never the right size; pool heaters are built around 250-gallon-and-up stationary tanks.
The burn-rate math, by heater size
Propane heaters burn roughly 1 gallon of fuel for every 100,000 BTU of input, per hour at full fire — a rule of thumb that comes from propane’s energy content of about 91,500 BTU per gallon. That means a 150,000 BTU heater draws about 1.5 gallons an hour, and a 400,000 BTU heater — the top of this site’s gas-heater range, covered on our best gas pool heater guide — draws roughly 4 gallons an hour running flat out.
| Heater input (BTU/hr) | Approx. burn rate | Usable hours on a 250-gal tank (~200 gal usable) | Usable hours on a 500-gal tank (~400 gal usable) |
|---|---|---|---|
| 100,000-120,000 | ~1-1.2 gal/hr | ~165-200 hrs | ~330-400 hrs |
| 150,000 | ~1.5 gal/hr | ~133 hrs | ~266 hrs |
| 250,000 | ~2.5 gal/hr | ~80 hrs | ~160 hrs |
| 400,000 | ~4 gal/hr | ~50 hrs | ~100 hrs |
Those hours assume continuous full-fire operation, which almost never happens in practice — a thermostat-controlled heater cycles on and off once the water reaches temperature, so real-world tank life typically runs longer than this table’s worst-case numbers. Tanks are also only ever filled to about 80% of rated capacity (not 100%) to leave room for propane’s natural expansion, which is why a “250-gallon” tank only holds roughly 200 usable gallons — the figure this chart is built on.
Why tanks are only filled to 80%
Propane expands as it warms, and a sealed tank filled all the way to the brim has nowhere for that expansion to go — the 20% headspace is a safety requirement, not a supplier shorting you on fuel. Every stationary propane tank size follows the same rule: a 500-gallon tank’s usable capacity is about 400 gallons, a 1,000-gallon tank’s is about 800. Confirm the usable number with your supplier rather than assuming the tank’s rated size is what you actually get delivered.
Flame King 100 lb. Steel Propane Tank
- Sized for occasional use on a smaller spa or above-ground pool heater, not a daily-fired 300,000+ BTU in-ground unit — see the chart above for when you need to step up to a 250- or 500-gallon stationary tank instead.
- Pair with our above-ground pool heater picks if you're sizing both the heater and the tank at once.
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Renting vs. owning the tank
This is the honesty-hook most sizing guides skip entirely: renting a tank almost always ties you to that supplier’s propane price, not just a rental fee. Forum-reported annual rental charges run roughly $50-$100 a year — cheap compared to the $650-$1,000+ it costs to buy a 250-to-500-gallon tank outright. But renters report paying noticeably more per gallon than owners who can shop between suppliers for the best rate; one commonly cited example put a renter’s effective cost around $5.35/gallon against a $3.675/gallon posted rate once fees were folded in. Run the math on your own expected usage: light, occasional heating most years favors renting (the fee alone can stay cheaper than buying for well over a decade), while a heater that fires most of the swim season every year usually loses more to the fuel-price lock-in than it saves on the rental fee.
| Rented tank | Owned tank | |
|---|---|---|
| Upfront cost | $0 (supplier-installed) | $650-$1,000+ (250-500 gal) |
| Ongoing fee | ~$50-$100/year | None |
| Fuel supplier | Locked to tank owner | Shop any local supplier |
| Best for | Light/occasional heating | Daily in-season firing |
Tank placement and clearance
A propane tank for a pool heater can’t just sit wherever there’s room — distance rules under NFPA 58 (the Liquefied Propane Gas Code, as locally adopted) scale with tank size: smaller above-ground tanks up to 250 gallons typically need at least 10 feet of clearance from buildings and property lines, while larger 500-gallon-and-up tanks step up to roughly 25 feet. Local fire marshals and AHJs apply their own adopted version of these tables, and how they treat a pool heater, pump, or electrical sub-panel sitting near the tank varies by jurisdiction — confirm placement with your installer or local building department before the tank is set, not after, since moving a buried or set stationary tank later is far more disruptive than confirming clearance up front. This is a separate question from the heater’s own gas line sizing, which covers the pipe run from tank to heater rather than the tank’s placement itself.
Raypak Digital Propane Pool Heater (266,000 BTU)
- Falls between the 250,000 and 400,000 BTU rows in the chart above — budget tank size closer to the 400,000 BTU line if you expect to run it most days through a cool season.
- See our full propane pool heater picks for the complete BTU range and matching tank guidance.
The bottom line
Size the propane tank off the heater’s BTU input, not the pool’s gallon count: a 250-gallon tank suits a smaller heater used occasionally, a 500-gallon tank suits a 300,000-400,000 BTU heater fired daily through the season, and a 20-pound grill cylinder is never the right size for either. The bigger decision most buyers skip is rent vs. own — a rented tank’s low upfront cost comes with a real, often-overlooked fuel-price lock-in to a single supplier, while owning costs more upfront but lets you shop for the best per-gallon rate every season.
Sources: TroubleFreePool — propane tank size forum thread, Propane 101 — propane tank sizes and usable capacity, Ferrellgas — propane pool heaters, Propane 101 — tank distance requirements (NFPA 58)