Pool Heat-Up Time Calculator
How long a pool or hot tub heater takes to raise the water to the temperature you want, with the gas or electricity it uses and the cost.
Result
Enter thermal efficiency to see the result.
https://calcgallery.com/home-systems/pool-heat-up-time-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Pool Heat-Up Time Calculator
- Enter the volume of water, for a pool or a hot tub.
- Enter the temperature now and the temperature you want.
- Choose the heater and copy its rating, with the efficiency for gas or the COP for a heat pump.
- Add your gas or electricity price under More options to see the cost.
How it is calculated
heat needed = water volume × density × specific heat × temperature riseheat per hour = gas input × efficiency, or the heat pump's output, or the element's ratingtime = heat needed ÷ heat per hourenergy bought = heat needed ÷ efficiency (gas) or ÷ COP (heat pump); an element turns all of it into heatOne BTU raises a pound of water by one degree Fahrenheit, and a US gallon of water weighs about 8.345 pounds, so each gallon needs about 8.345 BTU per degree. In metric, a litre needs 4.1868 kJ per degree Celsius.
Gas, heat pump or element
A gas heater’s rating plate gives its input: the gas it burns in an hour. Only the efficiency’s share of that heats the water, so the page multiplies the two. A heat pump is rated by the heat it puts out and by its COP, the heat out for each unit of electricity in, and both fall as the air cools. An electric element, as in most hot tubs, turns all the electricity it uses into heat in the water.
Assumptions
The water is heated as a single body with no heat lost to the air, the ground or evaporation, and the heater runs steadily at its rating until the target is reached. The answer is therefore the shortest the heating can take. Setting a safe hot tub temperature, venting and gas or electrical work are outside this calculation and belong to the heater maker’s instructions and local codes.
With your numbers
- Heat = 15,000 gal of water × 17 °F = 2,128,078 Btu
- Time = 2,128,078 Btu ÷ 336,000 BTU/h delivered = 6.334 h
Quick reference
A minimum: evaporation and heat lost through the surface and walls make it slower, much more so without a cover.
| Job | Time to heat |
|---|---|
| 15,000 gal, 65 to 82 °F, 400,000 BTU gas | 6.3 h |
| 20,000 gal, 70 to 84 °F, heat pump COP 5 | 21.2 h |
| 10,000 gal, 60 to 80 °F, 250,000 BTU gas | 8.1 h |
| Hot tub 400 gal, 60 to 102 °F, 5.5 kW | 7.5 h |
| Hot tub 300 gal, 90 to 104 °F, 4 kW | 2.6 h |
| 50,000 L, 18 to 28 °C, 30 kW heat pump | 19.4 h |
| Hot tub 1,500 L, 15 to 38 °C, 3 kW | 13.4 h |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
2 sources
NIST Chemistry WebBook, water at 4 °C and 0.101325 MPa: 999.9749 kg/m³ (IAPWS-95), https://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185, rounded to the conventional 1000 kg/m³ of NIST SP 811 (2008) Appendix B, centimeter of water, conventional = 98.0665 Pa.
Used for
- The conventional density of water, 1000 kg/m³. Pure water at 4 °C, its densest, is 999.97 kg/m³.
NIST SP 811 (2008), Appendix B.9, p. 61: Btu_IT/(lb·°F) = 4.1868 E+03 J/(kg·K), in bold, so exact. https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication811e2008.pdf
Used for
- Specific heat of water: exactly 1 BTU per pound per degree Fahrenheit under the International Table BTU.
Common questions about the Pool Heat-Up Time Calculator
- How long does it take to heat a pool?
- At least the heat the water needs divided by the heat the heater delivers. Every gallon of water takes about 8.3 BTU to warm by one degree Fahrenheit, so a large pool needs millions of BTU for a rise of 15 to 20 degrees, and even a big gas heater takes hours. The real time is longer, because the pool loses heat while it warms.
- How long does a hot tub take to heat up?
- A hot tub holds far less water than a pool but has a far smaller heater, often a few kilowatts, so filling one with cold water and heating it to a soaking temperature commonly takes several hours. Enter the capacity from the manual and the element rating to work it out for yours, and keep the cover on while it heats.
- Why is my pool heating slower than this?
- Because the page ignores heat loss. Evaporation from the surface is usually the largest loss, then wind, cool nights and heat through the walls. A cover cuts evaporation. A heat pump also delivers less heat in cool air than its rating at warm test conditions, so use the output and COP its data sheet gives for your air temperature.
- What size heater do I need?
- This page does not size heaters. Sizing has to account for the heat a pool loses in your climate, wind and season, which a heat-up calculation leaves out. Use it to see how long a heater you have, or one you are considering, takes to warm the water.