Water Heating Time Calculator

Calculator that tells you how long it takes to heat water from start to end temperature with a given heating power.

Other water heating calculators are: power and energy.

The calculator supports Celsius/Centigrade, Fahrenheit, Watts (w), Kilowatts (Kw), Btuh, Joule, British thermal unit (Btu), liter, gallon, kg, lb, cubic inch, cubic foot etc.

The calculator supports decimal numbers, use dot or comma.

Amount of water
Start temperature
End temperature
Heating power

If you are using the calculator for swimming pool heating, please add a sufficiently large margin of safety to compensate for all the heat losses.


The formula to calculate the heating time of water is as follows:

(amount of water in kg) • (end temperature in °C – start temperature in °C) • (4186 joules/kg/°C)
(heating power in watts)
= heating time in seconds

To use the formula:
1) calculate the weight of water in kg
2) multiply that with the temperature difference in Celsius
3) multiply that with the constant of 4186
4) divide that by your heating power in watts
and the result is the heating time in seconds.

4186 is the constant for the heat capacity of water and means that it takes 4186 joules of energy to increase the temperature of 1 kilogram of water by 1 degree Celsius.

The heating power is in watts in the formula. A watt is defined as joules per second, e.g. 1W = 1 j/s = 1 joule per second.

Don Pedro - July 21, 2022
Been using this formula to size direct heating of Hot water systems from solar panels
Works extremly well thank you
gtcfoadm - June 22, 2021
Marcel wrote:
the constant for the heat capacity of water is 4186.8, not 4168.

Hi Marcel

Thank you for your comment.
The tool uses 4186, but it was misspelled in the text. It's fixed now.

But you will find other values as well in the Internet. This "constant" is different at different temperatures. So, it is not really a constant. It is only a constant if you can live with some inaccuracy. I had a look at some tables and I have seen values of 4213 (1°C, 34°F) then going down to 4178 (35°C, 95°F) and up again to 4210 (95°C, 203°F). For most practical purposes, the tool should be accurate enough.
Bryce - April 21, 2021
Great estimator tool. It obviously varies on ambient, pool thermal insulation, covering, etc.
Jawad - January 5, 2021
Thanks for the calculator! very helpful
Mike - September 3, 2020
A continuous flow calculator for instant water heaters would be useful, e.g. DHW gas boilers, electric showers, so that temperature rise for a given flow rate could be calculated, or input power needed to achieve a target Δ° at a particular flow rate, etc.
Marcel - July 15, 2020
the constant for the heat capacity of water is 4186.8, not 4168.

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