Pool heater BTU calculator
How many BTU per hour you actually need — given in fuel input for gas listings or heat output for heat pump listings, so the answer matches the number on the page you are reading.
Quick picks
What the answer usually points at, by size. Prices come from the live retail listing.
| # | Product | Best for | Price |
|---|---|---|---|
| 1 |  If it says about 140,000 output Hayward HeatPro Pool Heat Pump The full-size heat pump benchmark. Titanium exchanger, stated saltwater compatibility. | Lowest running cost per degree when you want real temperature control | |
| 2 |  If it says 300,000+ input Raypak Digital Natural Gas Pool Heater A big gas heater is the only thing that gets a large pool warm in an afternoon. | Heating fast and occasionally, rather than holding a temperature | |
| 3 |  If it says about 55,000 output FibroPool FH255 55,000 BTU Pool Heat Pump The small-pool heat pump, with a published COP of 5.92. | A smaller pool where a full-size heat pump is more machine than the water needs | |
| 4 |  If it says under 20,000 output TURBRO Manatee M18 18,000 BTU Above-Ground Pool Heat Pump Plug-in 18,000 BTU on a normal outlet. Days, not hours, and the maker says so. | An above-ground pool up to about 6,000 gallons, with no 240V circuit available | |
| 5 |  Do this and the number gets smaller Sun2Solar 1600 Series Ultimate Blue Solar Cover A cover cuts the loss allowance, which cuts the heater you need to buy. | An inground pool you cover most nights, every season |
#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 October 17, 2026.
What size pool heater do you need?
Rectangular: length x width x average depth x 7.48
From where the pool sits now to where you want it.
12 is an overnight heat-up. 4 is a gas heater. 48 is a small heat pump.
The answer below is given in the same units the listings use, so you can compare it directly.
Our default is 20%. Raise it for a windy site or an uncovered pool; drop it toward 5% if the cover stays on.
167,000 BTU per hour
- Energy needed for the rise
- 1,666,000 BTU
- Delivered per hour, before losses
- 138,833 BTU/hr
- With a 20% allowance for heat lost while heating
- 166,600 BTU/hr delivered
- Listing rating to look for (heat output)
- 167,000 BTU/hr
| 20,000 gal x 8.33 x 10°F | energy needed for the rise |
| = 1,666,000 BTU | |
| 1,666,000 ÷ 12 hours | delivered per hour |
| = 138,833 BTU/hr | |
| x 1.20 | allowance for loss while heating |
| = 166,600 BTU/hr delivered | |
| Heat pump listings already quote output | no efficiency division needed |
Round up to the next size the manufacturer sells, and do not agonize over it — an oversized heater reaches temperature sooner and then idles, which costs almost nothing extra because it burns fuel only while it runs. An undersized one runs continuously and may never arrive on a cold week. The 8.33 BTU per gallon per degree and every other constant used here are listed with their sources on our methodology page. The loss allowance is our assumption, not a published figure, which is why it is an editable field.
The formula it uses
| Energy needed = gallons x 8.33 x ΔT | BTU |
| Delivered per hour = energy needed ÷ hours you will wait | BTU/hr |
| Add an allowance for heat lost while heating | default 20% |
| If shopping for gas: ÷ thermal efficiency, because listings quote INPUT |
The 8.33 is BTU per gallon per degree Fahrenheit, from water's density and the definition of a BTU. Every constant on this site is listed with its source on the methodology page.
Why the last line is the important one
This is the same trap the heating time calculator exists to defuse, coming at you from the other direction.
Gas and propane heaters are sold by fuel input. Heat pumps are sold by heat output. A 400,000 BTU gas heater delivers about 328,000 BTU into the water at 82% efficiency; a 140,000 BTU heat pump delivers 140,000. If the calculator told you that you need 150,000 BTU/hr delivered and you went shopping for a “150,000 BTU” gas heater, you would buy one that delivers about 123,000 and wonder why it takes longer than you were told.
So the selector changes the units of the answer rather than just the working. Pick the kind of heater you are actually shopping for and the number you get is the number to type into a search box.
Bigger is not wrong
A common worry is that an oversized heater wastes money. It mostly does not. A heater burns fuel while it runs and stops when the thermostat is satisfied, so a larger unit reaches temperature sooner and then idles — the total energy delivered is about the same either way. What you pay for is the purchase price and, on a gas heater, a larger gas line.
Undersizing is the expensive mistake. A heater that cannot outpace the pool’s losses on a cold week runs continuously and never arrives, which is the worst of both bills. Our sizing guide works through the reasoning at length, including the rules of thumb we think are unhelpful.
What the answer usually looks like
| Pool | In 12 hours | In 24 hours | In 4 hours |
|---|---|---|---|
| 6,000 gal above-ground | 50,000 BTU/hr out | 25,000 BTU/hr out | 150,000 BTU/hr out |
| 10,000 gal | 83,000 BTU/hr out | 42,000 BTU/hr out | 250,000 BTU/hr out |
| 15,000 gal | 125,000 BTU/hr out | 63,000 BTU/hr out | 375,000 BTU/hr out |
| 20,000 gal | 167,000 BTU/hr out | 83,000 BTU/hr out | 500,000 BTU/hr out |
| 30,000 gal | 250,000 BTU/hr out | 125,000 BTU/hr out | 750,000 BTU/hr out |
Delivered BTU per hour, which is the heat pump convention. For a gas heater divide by 0.82 to get the input rating the listing will quote — so the 20,000 gallon twelve-hour figure becomes about 204,000 BTU/hr input.
Look down the four-hour column and you can see why gas exists. Nothing electric delivers 500,000 BTU/hr on a domestic supply, and nothing solar does it at all. Look down the 24-hour column and you can see why heat pumps do: 83,000 BTU/hr is an ordinary machine on an ordinary circuit, running overnight for a quarter of the fuel cost.
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 October 17, 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 October 17, 2026.
FibroPool FH255 55,000 BTU Pool Heat Pump
The heat pump worth looking at when a full-size one is more machine than the pool needs. FibroPool publishes 55,000 BTU/hr of output and a rated COP of 5.92, which is above the 5.0 we assume for heat pumps generally — so on the pool sizes the manufacturer actually names, the running cost lands below the category figure. The catch is the other side of the same number: 55,000 BTU/hr is not much heat, and on a pool larger than FibroPool lists it will run all day and never catch up.
Best for A smaller pool where a full-size heat pump is more machine than the water needs
| Rated output | 55,000 BTU/hrPublished as OUTPUT, which is the heat-pump convention. It is not comparable with a gas heater's input rating. | Manufacturer |
|---|---|---|
| Pool sizes the manufacturer names | Above-ground up to 21 ft round or 15 x 30 ft oval; inground up to 10,000 gallons | Manufacturer |
| Supply | 240 V | Manufacturer |
| Rated COP | 5.92FibroPool words this as 5.92 times the heat for every dollar of electricity. The conditions it was rated under are not published, and a heat pump's COP falls as the air cools, so read it as a best case rather than a season average. | Manufacturer |
| Cost per million BTU delivered | About $9.08 at the published COPUS average residential electricity at 18.34 cents/kWh (EIA, June 2026) divided by the published COP of 5.92. At the more conservative COP of 5.0 we use across the site it comes out at about $10.75. | Our arithmetic |
- A published COP of 5.92, above the 5.0 we assume for the category
- All electric — no gas line, no flue, no combustion air to arrange
- Sized and priced for pools that a full-size heat pump is wasted on
- 55,000 BTU/hr is modest: about 30 hours to lift 20,000 gallons ten degrees
- Wants a dedicated 240 V circuit, which is an electrician's visit if you do not have one
- Like every heat pump, it fades as the air cools — the shoulder season you bought it for
- The COP this unit reaches at your air temperature and humidity
- Compressor and control-board life, which decides whether a cheaper heat pump is a bargain
- How loud it is 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 October 17, 2026.
TURBRO Manatee M18 18,000 BTU Above-Ground Pool Heat Pump
The plug-in heat pump, and the honest version of that category. It runs on a normal 110-120V GFCI outlet, carries a titanium exchanger, and TURBRO states a COP of 5.5 — in the same range the established brands publish rather than the impossible numbers import listings often print. It is also the only listing we read that tells you the truth about speed: TURBRO's own note says reaching your target temperature 'may take a few days rather than immediate results'. Believe that note.
Best for An above-ground pool up to about 6,000 gallons, with no 240V circuit available
| Rated output | 18,000 BTU/hrHeat pumps rate by output. 18,000 BTU/hr is about an eighth of a full-size unit. | Manufacturer |
|---|---|---|
| Stated COP | 5.5No test condition is published with it. Read it as a warm-air best case, as with every COP in this category. | Manufacturer |
| Supply | 110-120V, GFCI plug, IPX4The whole point of the unit. No electrician, no dedicated 240V circuit. | Manufacturer |
| Heat exchanger | Titanium | Manufacturer |
| Pool size the maker names | Up to 6,000 gallons — round pools to 16 ft, rectangular to 12 x 20 ft | Manufacturer |
| Operating range | Heats to 59-104°F in ambient air from 41-109.4°F; also cools | Manufacturer |
| Warranty | 2-year full machine | Manufacturer |
| Time to lift 6,000 gallons ten degrees | About 28 hours of run time6,000 x 8.33 x 10 = 499,800 BTU, divided by 18,000 BTU/hr. Ignores the heat the pool loses while heating, so the real figure is longer — which is what TURBRO's 'a few days' note is telling you. | Our arithmetic |
- Plugs into a standard outlet — no electrician, which can be the larger cost on a small pool
- Titanium exchanger at this price, so a salt pool is not ruled out
- The listing states the slow heat-up rather than hiding it
- 18,000 BTU/hr means days, not hours, to reach temperature from cold
- Almost useless without a cover — it cannot outrun overnight evaporation loss
- TURBRO is an import brand with no long service record we could check
- Compressor life, which is the whole question on a budget heat pump
- The COP it actually reaches at 60°F air
- Parts and warranty support in three years' time
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 October 17, 2026.
Sun2Solar 1600 Series Ultimate Blue Solar Cover
The cover to buy if you intend to keep using it in five years. 16 mil is the thickest tier the category sells, and thickness on a solar cover buys durability rather than heat — Bob Vila measured every cover in its hands-on group at the same 4 to 5 degrees, from 12-mil clear to 16-mil blue. Pay the extra for the one that survives being dragged over the coping twice a day, not for a temperature difference that did not show up in the only published one-hour test in this category.
Best for An inground pool you cover most nights, every season
| Price shown for | 24 ft roundThis product is sold in several sizes on one listing. The live price and photograph on this page are the ones Amazon returns for this variant; other sizes are priced separately. | Retail listing |
|---|---|---|
| Thickness | 16 milThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
| Color | Blue | Retail listing |
| Stated lifespan | 5 to 10 years for the 16-mil tierSwim University's published figure for 16-mil covers generally, not a Sun2Solar warranty statement. | Retail listing |
| Bubble side | Bubbles face the waterCategory-wide instruction, not brand-specific. The bubbles are the insulating layer and they have to touch 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
Their figure was the same for every cover they put on the water. Read our reconciliation of what a one-hour reading actually measures before you plan around this number.
- Thickest tier sold, which is where the durability difference actually is
- Blue absorbs more of the incoming energy at the surface than clear does
- Cuts oversize to your pool shape with household scissors
- Heavy and awkward at 16 mil — a large inground cover is genuinely a two-person job without a reel
- Costs meaningfully more than the 12-mil version for no measured heating advantage
- Whether the seams hold up over five seasons of daily handling
- How the bubble layer survives repeated dragging over rough coping
- Whether the stated lifespan tier holds for this specific product line
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 October 17, 2026.
Questions people actually ask
What size heater do I need for a 20,000 gallon pool?
How many BTU do I need per gallon of pool water?
Is it bad to oversize a pool heater?
Why does the calculator ask if I am shopping for gas or a heat pump?
- 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
- 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
- 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
What size pool heater do I need?
How to size a heater from your pool's actual energy requirement, the input-versus-output rating trap, and why the answer gets smaller once you own a cover.
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.
Which pool heater to buy
Every type of pool heater ranked on what it costs to deliver a million BTU into the water, how many hours it needs to do it, and what the install actually involves.
Pool heater running cost calculator
Prices the same heating job across solar, heat pump, gas, propane and electric resistance simultaneously, with every energy price and efficiency editable.
Pool heat pumps worth buying
Full-size and small heat pumps ranked on published output, exchanger material and what the manufacturer will actually say about efficiency. With the sizing math for your pool.