Pool heating time calculator
How long from cold to swimmable, for your pool and your heater — with the arithmetic printed beside the answer.
Quick picks
What to buy once the calculator has told you what your current setup does.
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | No photo Fastest option Raypak Digital Natural Gas Pool Heater If the calculator says twelve hours and you want five, this is the answer. | Heating fast and occasionally, rather than holding a temperature | |
| 2 | No photo Slower, much cheaper to run Hayward HeatPro Pool Heat Pump The extra hours cost you nothing if you can start the night before. | Lowest running cost per degree when you want real temperature control | |
| 3 | No photo Shortens every heat-up Sun2Solar 1200 Series Crystal Clear Solar Cover Your heater is racing the heat loss. This slows the loss down. | 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.
How long will it take to heat your pool?
Rectangular: length x width x average depth x 7.48
The number on the box or the listing.
This matters. Gas heaters are rated by what they burn; heat pumps by what they deliver.
Our assumption is 82%. Substitute your heater's rated figure from its data plate.
5 hours 5 minutes
- Energy needed
- 1,666,000 BTU
- Delivered by your heater
- 328,000 BTU per hour
| 20,000 gal x 8.33 x 10°F | energy needed |
| = 1,666,000 BTU | |
| 400,000 BTU/hr input x 82% | what actually reaches the water |
| = 328,000 BTU/hr delivered | |
| 1,666,000 ÷ 328,000 | hours |
| = 5.08 hours |
This assumes nothing escapes while you heat, which is not quite true — your pool is losing heat the whole time, and DOE attributes about 70% of that loss to evaporation. Real heat-ups take somewhat longer, and the gap is larger in cold or windy weather. Covering the pool while it heats closes most of it. The 8.33 BTU per gallon per degree comes from our methodology page, which lists every constant with its source.
The formula it uses
| Energy needed = gallons x 8.33 x ΔT | |
| Hours = energy needed ÷ delivered BTU per hour | |
| Delivered = rating x efficiency, if the rating is fuel INPUT |
The 8.33 is BTU per gallon per degree Fahrenheit, derived from water's density and the definition of a BTU. Every constant on this site is listed with its source on the methodology page.
The correction that matters
Why your real heat-up will be slower
The arithmetic assumes a perfectly insulated pool. Yours is not: while the heater runs, the pool is losing heat to evaporation, to the air and to the sky, and DOE attributes about 70% of that loss to evaporation alone. Your heater is filling a bucket with a hole in it.
Two things close most of the gap, and both are free:
- Cover the pool while it heats. It matters most in cool air and wind — DOE puts a 7 mph breeze at a 300% increase in energy consumption.
- Heat during the day. Warmer air, a contribution from the sun, and no radiative loss to a clear night sky.
Typical answers
For most residential pools with a sensibly sized heater, expect overnight for a heat pump and an afternoon for a gas heater. If you can plan a day ahead, the heat pump’s extra hours cost you nothing but foresight — and save you the running cost multiple every time you use it.
To find out what the heat-up will cost rather than how long it takes, the running cost calculator prices the same job across all five heating options.
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.
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.
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.
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
How accurate is this pool heating calculator?
Do I enter my heater's input or output rating?
Does this work for above-ground pools?
- 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
- Water density: 62.424 pounds per cubic foot at 39.2°FU.S. Geological Survey, Water Science School · 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
- 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
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.
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.
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.
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.
Solar cover savings calculator
Turns your pool's overnight temperature drop into a saving per night, per season, and a payback period — the calculation nobody in this category publishes.