Hot tub heat-up time calculator
Heating water is simple physics: it takes a fixed amount of energy to warm a known mass of water, and your heater delivers energy at a fixed rate. Enter your gallons, the temperatures, and the heater output for a time estimate and your degrees-per-hour.
A typical plug-in tub is 1 to 1.5 kW; a wired tub is 4 to 6 kW.
Enter your gallons and temperatures for a time estimate.
The math
One BTU raises one pound of water one degree Fahrenheit, and a US gallon of water weighs 8.34 lb, so the energy needed is gallons times 8.34 times the temperature rise. An electric heater delivers its kilowatts as 3,412 BTU per hour each; a gas heater delivers its rated input times its efficiency. Real heat-ups run a little longer than the ideal because the tub keeps losing heat to the air while it warms, more so on a cold, windy day with a poor cover.
Common questions
How many degrees per hour should I expect?
A wired 5.5 kW heater warms a typical 350 to 400 gallon tub by roughly 5 to 6 degrees an hour. A plug-in 1.5 kW tub manages closer to 1.5 degrees an hour, which is why filling a plug-in tub can take most of a day to come up to temperature.
Should I leave the cover on while it heats?
Yes, always. The cover cuts the heat loss fighting your heater, so the tub reaches temperature faster and cheaper. Heating with the cover off wastes a large share of the heater output.
Why did my real heat-up take longer than this?
This is the ideal time with no losses. On a cold day a real tub is losing heat the whole time it warms, so subtract a little from the effective heater output. A poor or wet cover, wind, and a very low starting temperature all stretch it out.