What size pool heater do I need?
Sizing rules of thumb hide the one thing you need to know: gas heaters and heat pumps do not measure their ratings the same way.
Quick picks
Two heaters that illustrate the rating difference, and the cover that changes the answer.
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | No photo Size on delivered output Hayward HeatPro Pool Heat Pump A heat pump's rating is what it puts into the water. What you see is what you get. | Lowest running cost per degree when you want real temperature control | |
| 2 | No photo Size on input, then multiply Raypak Digital Natural Gas Pool Heater A gas heater's rating is fuel burned. Multiply by efficiency to get delivered heat. | Heating fast and occasionally, rather than holding a temperature | |
| 3 | No photo Buy this first and size smaller Sun2Solar 1200 Series Crystal Clear Solar Cover Reduce the loss and the heater you need shrinks with it. | Most pools, most budgets — the default recommendation |
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Do not start with a rule of thumb
Sizing guidance in this category tends to be a table of pool volumes against BTU ratings, which hides two things that matter: how fast you actually want the heat, and whether the rating quoted is input or output. Both change the answer materially.
Start from the energy instead. It is one multiplication.
| gallons x 8.33 x the degrees you want | |
| 20,000 x 8.33 x 10°F | = 1,666,000 BTU |
| Required delivered output = BTU ÷ hours you will accept | |
| 1,666,000 ÷ 12 hours (overnight) | = 138,833 BTU/hr |
| 1,666,000 ÷ 6 hours (an afternoon) | = 277,667 BTU/hr |
| 1,666,000 ÷ 3 hours (impatient) | = 555,333 BTU/hr |
The same pool needs a 139,000 or a 555,000 BTU/hr heater depending only on how long you will wait
This is why a single sizing table cannot be right. Speed is a purchase decision, not a property of your pool.
Step 3 — the rating trap, and it is the important one
The heating time calculator handles this switch for you, so you can compare two real heaters honestly.
Step 4 — the maintenance load nobody adds
Everything above sizes for a temperature rise. It assumes no heat is escaping while you heat, which is false — and how false depends on your climate, your wind exposure and whether the pool is covered.
There is no honest way for us to give you a single number for that: it depends on air temperature, humidity, wind and sky conditions, and we have no fetchable source that quantifies it for a residential pool. What we can tell you is the direction and the lever.
- Add headroom in a cool or windy climate. Sizing exactly to the rise arithmetic will feel slow in practice.
- Or reduce the load instead. DOE puts about 70% of pool heat loss down to evaporation and a 7 mph wind at a 300% increase in energy consumption. A cover and a windbreak reduce the maintenance load enough that you can size the heater closer to the rise arithmetic — which is a smaller, cheaper heater.
The practical shortcut
- Work out your gallons.
- Decide your realistic temperature rise. Most people want 8 to 12 degrees over ambient pool temperature.
- Decide how long you will wait. Overnight is the sensible default and it halves the heater you need against an afternoon.
- Divide to get required delivered BTU/hr.
- For gas, divide again by 0.82 to get the input rating to shop for. For a heat pump, shop for that number directly.
- Add 20-30% headroom in a cool or exposed location — or buy a cover and skip the headroom.
Is bigger always better?
Not quite. An oversized heater costs more to buy, and on gas it cycles more, which is harder on the equipment. It does not cost more to run per delivered BTU — a heater’s efficiency does not change with how big it is — so the running-cost argument against oversizing is weaker than people assume.
The real argument for restraint is that the money saved by buying one size down and spending a hundred dollars on a cover buys you a better outcome than the larger heater does. Reducing the load beats increasing the supply, every time.
Every pick, in detail
What each one publishes, what is good and bad about it, and — the section this category usually omits — what we could not judge without owning one.
Hayward HeatPro Pool Heat Pump
If you are heating a pool with electricity, this is the shape of machine to do it with. A heat pump moves heat instead of making it, and at the assumptions we publish it delivers a million BTU into the water for about a fifth of what an electric resistance heater costs to deliver the same. It is slower than gas and it fades as the air gets colder, which is exactly when you want it — that trade is the whole decision.
Best for Lowest running cost per degree when you want real temperature control
| Type | Air-source heat pumpThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Rating convention | Rated by BTU/hr OUTPUTThis is the trap in the category. Gas heaters are rated by fuel INPUT, heat pumps by heat OUTPUT. A 140,000 BTU heat pump and a 140,000 BTU gas heater do not deliver the same heat. | Published standard |
| Cost per million BTU delivered | About $10.75 at our published assumptionsUS average residential electricity at 18.34 cents/kWh (EIA, June 2026) and an assumed COP of 5.0. Both are substitutable on our running-cost calculator. | Our arithmetic |
- Cheapest running cost per delivered BTU of any controllable heater
- No gas line, no combustion, no flue
- Holds a set temperature rather than depending on the weather
- Slow — expect most of a day to move a full-size pool ten degrees
- Efficiency falls with air temperature, so it is weakest in the shoulder season
- High purchase price against a gas heater of comparable output
- The COP this specific unit achieves at your conditions
- Compressor and control-board longevity
- Noise at the distance your deck actually sits from it
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Raypak Digital Natural Gas Pool Heater
The right answer when speed matters more than running cost — a weekend pool you want warm on Friday afternoon rather than one you hold at temperature all season. At the assumptions we publish, gas costs roughly two and a half times what a heat pump costs per delivered BTU, and it will get there in a fifth of the time. Buy the speed if you need the speed.
Best for Heating fast and occasionally, rather than holding a temperature
| Type | Natural gas or propane fired, digital controlThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Rating convention | Rated by BTU/hr INPUTA 400,000 BTU gas heater burns 400,000 BTU an hour and delivers roughly 82% of it. Do not compare that number directly against a heat pump's output rating. | Published standard |
| Cost per million BTU delivered | About $28.36 at our published assumptionsUS average residential natural gas at $24.09/Mcf (EIA, June 2026) and an assumed 82% thermal efficiency. | Our arithmetic |
- Fast — hours rather than most of a day on a full-size pool
- Works the same in cold air, unlike a heat pump
- Lower purchase price than a heat pump of equivalent real output
- Roughly two and a half times a heat pump's cost per delivered BTU
- Needs a gas line sized for it, which is often the real cost
- Propane instead of natural gas raises the running cost by about a quarter again
- The efficiency this specific unit achieves in service
- Heat exchanger life against your water chemistry
- Whether your existing gas supply can feed it without an upgrade
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Sun2Solar 1200 Series Crystal Clear Solar Cover
The value pick, and the one whose logic actually holds up. A clear cover transmits more sunlight into the body of the pool instead of absorbing it at the surface, and 12 mil is the point where lifespan stops being the limiting factor. Bob Vila measured this and the 16-mil blue at the same 4 to 5 degrees in an hour, so you are paying less for the same measured result and giving up some years of service life.
Best for Most pools, most budgets — the default recommendation
| Thickness | 12 milThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Color | Clear | Retail listing |
| Stated lifespan | 3 to 4 years for the 12-mil clear tierSwim University's published figure for 12-mil clear covers generally. | Retail listing |
| Bubble side | Bubbles face the water | Published standard |
4 to 5 degrees F in one hour
Claimed by Bob Vila, measured across all covers in their group. Conditions: One hour under 2pm sun on 89 to 92 degree F days. Source
- Clear transmits more light into the water instead of stopping it at the surface
- 12 mil is the sweet spot: real service life without 16-mil bulk
- Cheaper than the 16-mil tier with no measured heating penalty
- Shorter published lifespan tier than 16 mil
- A clear cover shows algae and debris more obviously, which some people dislike
- Whether clear genuinely outlasts or under-lasts blue in the same thickness
- Real-world tear resistance at a pool with rough edges
- How much the clear material clouds over a season of UV
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Questions people actually ask
What size pool heater do I need?
Is a 400,000 BTU gas heater the same as a 400,000 BTU heat pump?
Can a pool heater be too big?
- Average retail price of electricity to US residential customers: 18.34 cents/kWh, June 2026U.S. Energy Information Administration · read 2026-09-02
- US residential natural gas price: $24.09 per thousand cubic feet, June 2026U.S. Energy Information Administration · read 2026-09-02
- US residential propane price: $2.674 per gallon, week ending 30 March 2026U.S. Energy Information Administration · read 2026-09-02
- Energy content conversion factors: 3,412 BTU per kWh, 1,036 BTU per cubic foot of natural gas, 91,452 BTU per gallon of propaneU.S. Energy Information Administration · read 2026-09-02
- U.S. Department of Energy pool-energy figures (evaporation share, BTU per pound evaporated, cover savings, recommended temperatures, wind effect)Recreonics, reproducing and attributing the U.S. Department of Energy · read 2026-09-02
- Pool heater roundup: the 8.33 BTU per gallon per °F figure, attributed on the page to Alicia Toedter of Leslie'sBob Vila · read 2026-09-02
Every URL above was fetched and returned content on the date shown. Where a figure is ours rather than somebody else’s, the arithmetic is on the page and on the methodology page. If you find an error in it, we would genuinely like to know.
Read next
How long does it take to heat a pool?
The heating-time formula with worked examples across pool sizes and heater types, and the input-versus-output rating difference that makes listings misleading.
How to heat a pool fast
What actually governs heating speed, the input-versus-output rating trap that makes gas heaters look faster than they are, and the tricks that do and do not help.
Heat pump vs gas pool heater
The two options priced per delivered BTU and timed on the same pool, with the one question that decides which trade suits you.
Pool heating time calculator
Enter your gallons, the rise you want and your heater's rating. Handles the gas-versus-heat-pump rating trap and prints the whole calculation.
What a pool heater really costs
Purchase, install and running cost taken together, and why the cheapest heater to buy is regularly the most expensive one to own.