Pool heater size & heat-up cost calculator

Enter your pool's volume, surface area, temperatures and heater, and get the heater size for the time you can wait, how long yours takes and what it costs, with a curve of the water warming up.

Inputs

Not sure? Work it out with the pool volume calculator.

Length × width for a rectangle. The pool volume calculator shows the area of any shape.

78–82 °F is the usual range for a pool (Pool & Spa News, American Red Cross).

The average for the coldest month you will swim in, day and night together.

A cover cuts heating cost by 50–70% (DOE); we take off 60% of the heat loss.

Gas, propane and electric: the input rating on the label. Heat pump: the heating output (check the rating conditions).

Federal minimum for gas heaters is 82%; most are 82–95%.

How long you are willing to wait. Many sizing charts assume 24 hours.

Check your bill. The EIA’s US residential average was $14.95 per thousand cubic feet in February 2026, about $1.44 per therm.

Heater size to heat it in 24 hours

140,000BTU/h

Input rating, for 15,000 gal going from 65 °F to 82 °F. Holding 82 °F needs at least 69,000 BTU/h.

It gets there in 11 hours, inside your 24 hours.

Your heater heats it in
11 hours
Cost to heat up
$42.88
Cost per day to hold it
$24.89
Minimum to hold the temperature
69,000 BTU/h
With no heat loss
10 hours
Your pool warming up
60708090100051015Time (hours)Water temperature (°F)Now 65 °F82 °F in 11 hours
  • Water with your heater
  • Your target
  • Air

Heating 15,000 gal from 65 °F to 82 °F in 24 hours takes a heater of about 140,000 BTU/h (input), allowing for heat lost to 70 °F air. Your 250,000 BTU/h natural gas heater heats it in 11 hours for about $42.88. The curve plots the water temperature over time against the target and the air temperature.

Safety and equipment

Sizing a gas or electric heater involves fuel and mains power. The calculator estimates; your heater’s rating plate and installation manual decide what is safe.

  • Have gas and electrical work done by a qualified installer. Gas lines, venting, clearances and electrical supply all have codes and size limits.
  • Check the manufacturer’s sizing guidance and ratings for your model; ratings vary by maker and by operating conditions.
  • Do not exceed the water temperature your heater and local rules allow, and keep spa water at or below 104 °F.

How it works

The pool is a tank of water that the heater warms and the air cools. The calculator solves the balance exactly instead of dividing one number by another:

C · dT/dt = P − G · (T − T_air)
time to reach T₁   = −(C/G) · ln((T_air + P/G − T₁) ÷ (T_air + P/G − T₀))
heater for H hours = G (T₁ − T_air − (T₀ − T_air) e^(−GH/C)) ÷ (1 − e^(−GH/C))
to hold the target = G (T₁ − T_air)
  • C is the heat in the water per degree: gallons × 8.32 BTU per °F.
  • P is the heater’s output. Gas and propane output is input × efficiency (82% by default, DOE’s federal minimum). Electric resistance is 99%. A heat pump is rated by its output, and its COP sets the electricity it uses.
  • G is the heat lost through the surface per degree of difference: surface area × 10.5 BTU/h per sq ft per °F, the factor the maintenance sizing charts use for a pool sheltered from wind. It is multiplied by 1.25 for a breezy site and 2.0 for an exposed one, and by 0.4 with a cover on.
  • With no loss the same equations reduce to the textbook gallons × 8.33 × degrees ÷ output, which the chart’s “with no heat loss” chip shows.
  • Cost is the heat delivered ÷ efficiency (or ÷ COP) converted to therms, propane gallons or kWh, at the price you enter.

Worked example

A 15,000 gallon pool, 450 sq ft of surface, warmed from 65 to 82 °F with 70 °F air, sheltered, with a 250,000 BTU/h gas heater:

  • Heat capacity: 15,000 × 8.32 = 124,800 BTU per °F. Loss: 450 × 10.5 = 4,725 BTU/h per °F.
  • Holding 82 °F: 4,725 × 12 = 56,700 BTU/h of output, or 69,000 of input.
  • The heater: 250,000 × 0.82 = 205,000 BTU/h of output, which would settle at 70 + 205,000 ÷ 4,725 = 113 °F, so it gets there: −(124,800 ÷ 4,725) × ln((113.4 − 82) ÷ (113.4 − 65)) = 11 hours.
  • Cost: 205,000 × 11.4 h = 2.34 million BTU of heat, ÷ 0.82 = 28.5 therms, at $1.50 = about $43.
  • To do it in 24 hours takes about 111,000 BTU/h of output, which is 135,000 BTU/h of gas input; the calculator rounds that up to the next 5,000, 140,000 BTU/h.

A 100,000 BTU/h heat pump needs 28 hours for the same job, but at a COP of 5 and 17¢ a kWh costs about $28.

Tips from the pool pad

  • Use the coldest month you will swim in for the air temperature, day and night together, as the sizing guides advise.
  • Size for the wait you can live with. A heater that only just beats the heat loss takes days; doubling its size more than halves the time.
  • Keep the target at the low end. The DOE notes that 78–82 °F is the usual range and lower is cheaper.
  • Altitude and shade. The Bryan chart adds 4% for every 1,000 ft above sea level, and Pool & Spa News suggests a little extra for a shaded pool. Neither is in the calculator.

FAQ

What size heater do I need for my pool?

It depends on how fast you want it warm and how much heat it loses. The calculator sizes it for the hours you choose. The usual maintenance rule is surface area × 10.5 BTU/h × the degrees your water is above the air, which for a 450 sq ft pool 12° above the air is about 57,000 BTU/h of output just to hold the temperature. To heat faster you need more on top.

How long will it take to heat my pool?

With no heat loss it is gallons × 8.33 × degrees ÷ the heater's output; a 15,000 gal pool warmed 17° by 205,000 BTU/h takes about 10 hours. Real pools lose heat to the air as they warm, which adds an hour or more, and a heater too small to beat the loss never gets there at all. The curve shows which one you have.

How much does it cost to heat a pool?

The energy to raise the water's temperature, divided by the heater's efficiency, at your fuel price. One therm of gas is 100,000 BTU and a gallon of propane 91,452. A kWh is 3,412 BTU, and a heat pump with a COP of 5 turns it into five times that. Enter your own prices; the defaults are placeholders.

Is a heat pump cheaper than a gas heater?

To run, usually, because it moves heat instead of burning fuel. DOE's figures put heat pump COPs at 3.6–6.1 against 82% for gas. Its output and COP both fall in cool weather. The calculator shows the running cost of both for your pool.

Why does a cover matter so much?

Most of a pool's heat goes out through its surface, mainly by evaporation. The DOE says covers save 50–70% of pool heating costs, so the calculator cuts the heat loss by 60% when you pick one. It shortens the heat-up and cuts the daily cost of holding the temperature.

Sources

  1. Heater Sizing Techniques (Ultimate Tech Manual), Pool & Spa News (APSP). Maintenance method chart (surface area × degrees above the air × 10.5 BTU/h, wind 3½ mph; ×1.25 at 5 mph, ×2.0 at 10 mph), the time-rise formula gallons × 8.33 × degrees rise, and a spa heat-up chart that assumes 80% efficiency. 78–82 °F for pools. Its 300 sq ft at 20° cell reads 73,000, which should be 63,000.
  2. Pool Heater Sizing Chart, Form 1078, Bryan Boilers. Surface-area method on a 3½ mph wind with the same 1.25 and 2.0 wind multipliers, 4% more per 1,000 ft of altitude, and a time-rise method based on 1 °F an hour.
  3. Energy Conservation Standards for Consumer Pool Heaters, notice of proposed rulemaking (87 FR 22640), U.S. Department of Energy. Gas-fired pool heaters have had to reach 82% thermal efficiency since April 2013; electric resistance heaters are 99%; heat pump efficiency levels run from a COP of 3.6 to 6.1; residential pool heaters typically range from 50,000 to 400,000 Btu/h.
  4. British thermal units (Btu), U.S. Energy Information Administration. 1 kWh = 3,412 Btu; 1 therm = 100,000 Btu; 1 gallon of propane = 91,452 Btu.
  5. Raypak Avia pool and spa heater installation and operation manual, Raypak. Its under-sized heater check divides heater output (BTUH) by pool gallonage × 8.33, which it says does not take into account heat loss due to weather.
  6. Pool Efficiency fact sheet, U.S. Department of Energy, Better Buildings. Pool covers save 50–70% of pool heating costs; keeping the pool at the low end of 78–82 °F saves energy.

Formula and sources last checked September 30, 2026. How we test formulas.

  • Pool volume calculatorHow many gallons is your pool? Rectangle, round, oval, oblong or kidney, with a flat or sloped floor, drawn from above and from the side.
  • Pool chemical calculatorHow much chlorine, baking soda, acid, calcium chloride and stabilizer to balance your pool, in order, with before and after gauges for FC, pH, TA, CH and CYA.
  • Hot tub volume calculatorEstimate how many gallons your hot tub or spa holds from its size, for portable acrylic spas, wooden and in-ground tubs, and inflatables.
  • Hot tub chemical calculatorHow much dichlor or chlorine, baking soda, pH down and calcium to balance your hot tub by its size, in order, with before and after meters for spa ranges.
  • Pond liner size & volume calculatorWhat size pond liner you need and how many gallons your pond holds, for rectangular, round, oval and free-form ponds, drawn on the liner.
  • Pool salt calculatorHow many pounds of salt and how many bags take your saltwater pool from its current ppm to the level your chlorine generator wants, with a ppm meter and bags.

Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.