Water Heating Calculator for Time, Energy, and Power

The calculators on this page compute how long it takes to heat water, how much energy is consumed, and how much heating power is required.

Only input whole numbers, do not use a comma or point.

The calculators support Celcius/Centigrade, Fahrenheit, Watts (w), Kilowatts (Kw), Btuh, Joule, British termal unit (Btu), liter, gallon, kg, lb, cubic inch, cubic foot etc.

The calculators assume 100% efficiency and no loss of energy during the heating process.

Water Heating Time Calculator

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

Amount of water
Start temperature
End temperature
Heating power

Water Heating Energy Calculator

This calculator tells how much energy will be consumed to heat the water from the start to end temperature.

Amount of Water
Start temperature
End temperature

Water Heating Power Calculator

This calulator tells you how much minimum heating power is required to heat the water within a specified amount of time.

Amount of water
Start temperature
End temperature
Time period available to heat the water (minutes)

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  1. Dave -

    is there a formula to calculate the temperature required for 5 gallons of water that will increase the temperature of a stainless steel 15.5 gallon tank from 46F to 161F. The volume in gallons (5), ambient temperature (46F) and target temperature (161) are variables that will change.

  2. Brian -

    Very useful! Thanks:-)

  3. rox -

    great calculators 🙂

  4. Frank -

    Thanks! This was super helpful in calculating the power needed to produce my hot water via solar photovoltaics. For less than the price of a new heater tank, I now have free hot water. Sweet!

  5. Sven -

    Very nice and handy, Thanks !

  6. New pool, cold water ! -

    How do I calculate btu size for heating a swimming pool?

    I’m looking at a 50,000 btu boiler with a heat exchanger to heat a 33,000 gallon pool to 90 degrees f.

    I’ve run then calculation through the formula above, and it’s coming up with a 600,000 btu boiler to raise the water temp (68f) in 5 hours to 90f, which seems to be a really high number?

  7. antonio -

    Hi “New pool, cold water !”
    I have done the calculations in the calculator above for heating 33’000 US gallons from 68F to 90F in 5h (300min) and got a result of approximately 1’200’000 btuh.
    I have done the calculation again manually in an Excel spreadsheet and got the same result.

  8. David -

    What formula did you use to calculate the water heating power?

  9. Willaim -

    I am looking for how much energy is need to maintain water temperature. This is for a hot tub, I do not need to increase or decrease the temperature. I just want to maintain the temperature. 250 us gallons at 105f with moderate insulation… would 100 watts DC heater maintain this temperature?

  10. antonio -

    Hi Willaim,

    that is impossible to calculate with the information given. It very much depends on the surrounding temperature, the heat conductivity of your hot tub material, and the shape of the tub.

    But I have an idea. If you can test how fast the water cools down in your tub, then you can calculate how much heating power is required to prevent that from happening. Let’s make a calculation example for a tub with 250 us gallons that cools down from 105f to 103f in 2 hours. Using the “Water Heating Power Calculator” above (250 us gallons, start temperature 103f, end temperature 105f, 120min) tells us that a heating power of 611 watts is required. If you use a 611 watts heater in this example, then it will be about enough to reverse this cool-down from 105f to 103f by heating the water back up from 103f to 105f during the same period (realistically, of course, the water does not cool down, but stays at the same temperature).

    That’s just an example, I have no idea how fast a tub cools down. Also, you should have some extra power available, just to be sure. And for testing/sampling the cool-down, measure a larger drop in temperature, such as 10f, else the result will be unreliable (reading 1f or 2f changes off a small thermometer may be very inaccurate). And as always, double check with some other source and my help is without any guarantee or similar.

  11. Anonymous -

    Ace. Thanks 🙂

  12. Jonas Ntiako -

    Thanks. Very useful 🙂

  13. Allen -

    Thanks!!! I am tasked to maintain a 110L aquarium at 22-24C outdoors in Texas. Your calculators are a big help in figuring the size of the heat sump.

  14. Haroma -

    Great help to calculate water flow and energy consumption!

  15. John McGrath -

    Problems with limit calculations: It complains if the end temp is 212°F or even 211°F, saying it must be less than boiling. It complains when volume is 0.25 US gallon, saying volume must be greater than zero.

  16. antonio -

    Hi John,
    it takes the same amount of energy to heat water from 48 degrees to 52 degrees as it takes to heat water from 58 degrees to 62. But when the state of water changes from solid to fluid (e.g. -2°C to + 2°C) or from fluid to gas (e.g. 98°C to 102°C) this does not hold true any more. It would be more complicated to build calculators that can handle that and I have not done so. This is why the calculators complain in these situations.
    The calculators cannot handle points or commans, only whole numbers. So they may complain when you input commas or points.

  17. Larry Moore -

    To determine BTU lost can I use the “Water Heating Energy Calculator” only backwards? Start temp as final and end temp as start.

  18. antonio -

    Hi Larry, the energy that is used to heat water from 40 degrees to 60 is the same amount that is lost when the water cools down from 60 to 40 (only changed sign).

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