Enter your pool's or hot tub's gallons, how many degrees warmer you want it and how long you will wait. You get the BTU that takes, the smallest Pentair gas heater or heat pump that does it in time, or how long a hot tub's own heater takes, by Pentair's own heater sizing formula.
Your heater
A MasterTemp 200 heats 20,000 gallons 20°F warmer in 19.9 hoursit takes 165,000 BTU input to do it in 24 hours · 3,336,000 BTU into the water, losses aside
Quick answer: heating a pool takes its gallons × 8.34 × the degrees, in BTU. Pentair divides that by the heater’s input × its efficiency to get the hours. 20,000 gallons, 20°F warmer, takes 3,336,000 BTU: a 200,000 BTU heater does it in about 20 hours, a 400,000 BTU heater in about 10. To heat in 24 hours, a gas heater needs about 165,000 BTU input. A 400-gallon hot tub, 30°F warmer, takes about 7.3 hours on a 4 kW electric heater, or 36 minutes on a 200,000 BTU gas heater.
How to use the pool heater size calculator
- Enter the gallons. Not sure? The pool volume calculator works them out.
- Enter how many degrees warmer, from the water’s temperature now to the one you want, and how many hours you will wait.
- Pick a gas heater or a heat pump. For a heat pump, pick how warm the air is: it takes its heat from the air.
- Heating a hot tub? Tap “A hot tub”, then a gas heater and the minutes you will wait, or the tub’s own electric heater and its kW.
You get the smallest heater in Pentair’s range that does it in time, the hours each size takes and the BTU it all comes to. The button under the answer takes the same gallons to the pool pump and filter size calculator.
The formula, and where it comes from
Pentair’s heater sizing (its 2025–2026 pool product catalog, Engineering Data): “TIME IN HOURS = Vol. in Gal. x 8.34 lb./gal. x temprise ÷ Heater BTUH input x efficiency of heater”. One BTU warms a pound of water by one degree, and a gallon weighs 8.34 lb.
Efficiency. Pentair rates its MasterTemp 200, 250, 300 and 400 heat exchangers at 84%, and the MasterTemp 125 at 82%. A 200,000 BTU heater puts about 168,000 BTU an hour into the water.
Heat pumps are rated by the heat they deliver (AHRI 1160). Pentair’s UltraTemp 110 gives 101,000 BTU an hour with 80°F air and 70,000 with 50°F air, the 120Q 120,000 and 80,000, and the 140 135,000 and 90,000.
Losses. Pentair’s times are “discounting losses”. It notes that in cooler periods a pool “will usually lose 2–4°F. per day”, so a real heat-up takes longer.
Hours to heat a pool 20°F with a gas heater
| Pool | 125,000 BTU | 200,000 | 250,000 | 300,000 | 400,000 |
|---|---|---|---|---|---|
| 10,000 gal | 16.3 | 9.9 | 7.9 | 6.6 | 5 |
| 15,000 gal | 24.4 | 14.9 | 11.9 | 9.9 | 7.4 |
| 20,000 gal | 32.5 | 19.9 | 15.9 | 13.2 | 9.9 |
| 25,000 gal | 40.7 | 24.8 | 19.9 | 16.5 | 12.4 |
| 30,000 gal | 48.8 | 29.8 | 23.8 | 19.9 | 14.9 |
| 40,000 gal | 65.1 | 39.7 | 31.8 | 26.5 | 19.9 |
Pentair’s MasterTemp sizes, at 82% (125) and 84% (the others), losses aside. For 10°F, halve the hours.
How long does it take to heat a hot tub?
The same formula, in minutes. Pentair’s spa sizing: “TIME IN MINUTES = Vol. in Gal. x 8.34 lb./gal. x temprise x 60 min ÷ Heater BTUH input x efficiency of heater”. Its own table heats 200 to 1,000 gallons 30°F: the MasterTemp 200 row gives 35 minutes for 400 gallons, the 400 row 18.
A tub’s own electric heater is rated in kW. A kilowatt is 3,412 BTU an hour, and an electric element puts all of it into the water, so a 4 kW heater adds about 13,650 BTU an hour: about 4°F an hour in 400 gallons. Jacuzzi says “most hot tubs heat up at a rate of 3 to 6 degrees per hour”. Intex says its PureSpa heater raises the water “by 2°F per hour PROVIDED the spa cover is properly in place”.
Hours to heat a hot tub 30°F with its electric heater
| Hot tub | 1 kW | 2 kW | 4 kW | 5.5 kW |
|---|---|---|---|---|
| 250 gal | 18.3 | 9.2 | 4.6 | 3.3 |
| 300 gal | 22 | 11 | 5.5 | 4 |
| 400 gal | 29.3 | 14.7 | 7.3 | 5.3 |
| 500 gal | 36.7 | 18.3 | 9.2 | 6.7 |
| 600 gal | 44 | 22 | 11 | 8 |
Minutes to heat a hot tub 30°F with a gas heater
| Hot tub | 125,000 BTU | 200,000 | 250,000 | 300,000 | 400,000 |
|---|---|---|---|---|---|
| 200 gal | 29.3 | 17.9 | 14.3 | 11.9 | 8.9 |
| 400 gal | 58.6 | 35.7 | 28.6 | 23.8 | 17.9 |
| 600 gal | 87.9 | 53.6 | 42.9 | 35.7 | 26.8 |
| 800 gal | 117.2 | 71.5 | 57.2 | 47.7 | 35.7 |
| 1,000 gal | 146.5 | 89.4 | 71.5 | 59.6 | 44.7 |
Losses aside. Filled from the hose, a tub often needs more than 30°F: double the time for 60°F. Keep the cover on while it heats.
Worked examples
A MasterTemp 200 heats 20,000 gallons 20°F warmer in 19.9 hours: 20,000 × 8.34 × 20 = 3,336,000 BTU, ÷ (200,000 × 0.84).
A MasterTemp 125 heats 13,594 gallons 10°F warmer in 11.1 hours: a 24 ft round pool with 4 ft of water, from 70°F to 80°F.
An UltraTemp 110 heats 20,000 gallons 20°F warmer in 33 hours: a heat pump at 80°F air, given two days.
Even the MasterTemp 400 heats 33,000 gallons 15°F warmer in 12.3 hours: Pentair’s 40 × 20 ft example pool, asked for in 12 hours. Give it 13, or heat it over two evenings.
A 4 kW heater warms a 400-gallon hot tub 30°F in 7.3 hours: about 4.1°F an hour, inside Jacuzzi’s 3 to 6 degrees.
A MasterTemp 125 heats a 400-gallon hot tub 30°F warmer in 58.6 minutes: the smallest gas heater does it within the hour.
Common mistakes
- Using the heater’s input as its output. A 400,000 BTU gas heater at 84% puts 336,000 BTU an hour into the water.
- Rating a heat pump in warm air and using it in cool. Pentair’s UltraTemp 140 gives 135,000 BTU an hour at 80°F air and 90,000 at 50°F: half again as long to heat.
- Forgetting the losses. The formula leaves them out. A pool losing 2–4°F a day is still losing it while it heats.
- Guessing the gallons. The hours scale with them: 10% too many gallons is 10% too long.
More on pools at Dreamy Home Style
Choosing the tub itself? The hot tub size calculator sizes it for your space. To plan the yard around the pool, use the free 2D floor planner.
Pool heater questions
What size pool heater do I need?
Gallons × 8.34 × the degrees, divided by the hours you will wait and the heater’s efficiency. For 20,000 gallons 20°F warmer in 24 hours, that is about 165,000 BTU input at 84%: Pentair’s MasterTemp 200 does it in about 20 hours.
How do I use a pool heater size calculator?
Enter the gallons, the degrees and the hours. It applies Pentair’s formula to each heater size and names the smallest that does it in time.
How long does it take to heat a pool?
Gallons × 8.34 × degrees ÷ (BTU input × efficiency). A 200,000 BTU heater warms 20,000 gallons 20°F in about 20 hours, a 400,000 BTU one in about 10, before losses.
What size heat pump do I need for my pool?
Use its output at your air temperature. Pentair’s UltraTemp 140 gives 135,000 BTU an hour at 80°F air: 20,000 gallons 20°F warmer in about 25 hours, about 37 at 50°F air.
How many BTUs does it take to heat a pool?
8.34 BTU a gallon for each degree. 20,000 gallons × 8.34 × 20°F = 3,336,000 BTU into the water, before losses.
How long does it take to heat a hot tub?
Gallons × 8.34 × the degrees ÷ the heater’s BTU an hour. A 400-gallon tub 30°F warmer takes about 7.3 hours on a 4 kW electric heater; Jacuzzi says most hot tubs heat 3 to 6 degrees an hour.
How long does a gas heater take to heat a hot tub?
Minutes, not hours: Pentair’s MasterTemp 200 heats 400 gallons 30°F in about 36 minutes and the 400 in about 18, losses aside.
Is a bigger pool heater better?
Pentair: a larger heater heats faster and can be turned off between uses; a smaller one run during the daily filter cycle of 4–6 hours holds the temperature if you swim every day.