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.

Improved calculators that support comma, dot, and efficiency, are available here: time (including formula), energy, and power.

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).

  19. Stan -

    Thank you! I’ve been looking for a simple way to ballpark the size of a solar thermal hot water system – and this makes it dead simple!

    This is the best set of calculators on the internet 🙂

    Thanks again!

  20. Giulio -

    hello, thanks for this,

    is this calculation valid only when submerging the heating device in the water, or also when the water is circulating through a heating pipe using a pump?

    Wouldn’t it be faster in the latter case, because the water flow would exchange heat more efficiently than just by convection?

    I’m trying to figure out what would it take to heat my pool using a heating pipe after the filtration pump.


  21. antonio -

    Hi Giulio

    I do not think it makes much of a difference in practice.

    As for efficiency, an electrical water heater can convert electricity with almost 100% efficiency to heat that can be transferred to the water. The insulation of where the heating device sits and whether any heat can escape into something other than the water (like a wall, concrete etc.) is more important for efficiency.

    As for effectiveness, a large enough pipe with a pump can probably in theory transfer more heat per second (meaning power, e.g. watts) to water. When using a simple coil submerged in water, then it will start to heat the water just around it. Since water gets lighter when heated it will start to ascend around the coil and cold water will flow to the coil from below. This way a circulation develops in the water container given the shape of the water container permits it. I think of it this way: the power limit of heating coil simply submerged in water is reached when the water starts to boil around the coil while the water in other places is still much colder. Is that realistic?

    Please remember, that the calculators above assume 100% conversion efficiency. For an electrical heater that can be a good assumption, but not for a gas heater.

    Also, the calculator is only correct in the theoretical case that while the water is being heated, no heat escapes from the water to the environment (air or walls etc.). For that reason, these values are the minimum and some margin should be added to them. How much depends on the shape of your pool, the quality of the insulation of the pool, the difference of the desired water temperature to the environment and how long you are willing to wait until it is heated.

  22. Andrew -

    Nice calculator buttt…… you’ve got the abbreviations for the metric units incorrect. A Watt is named after James Watt hence the abbreviation is a capital W.

    A thousand of something is a “kilo” for example a kilogram. In turn the abbreviation, for one thousand, is k.

    Bringing these two together, a kilowatt is a “kW”, not as you have, “Kw”. Over an hour it would be “kWh”.

  23. Rafek Tamer -

    Hello Antonio,

    Appreciate the calculators but i was wondering if you could tell me the exact equations used for the “Water Heating Energy Calculator” or give some sort of excel sheet to double check it. I am guessing its q=m*cp*dt but when I do the calcs by hand, i get something off so i would like to know my mistake

  24. Ishfaq Ahmad -

    How much heat energy in joules is necessary to raise the temperature Of 7000 kg (7 M3) of water from 20 °C to 80 °C?
    1 litre of water is 1kg
    1 meter cube = 1000 Ltrs
    M = 7 meter cube = 7000 Ltrs or 7000 Kg
    The heat capacity Cp of water is 4.186kJ/kg-C
    ΔT = 80-20 = 60 C
    So, the energy required to raise the temperature of 7000 kg of water from 20C to 80C is:
    Energy E = m•Cp•ΔT = 7000 x 4.186 x 60 = 1758120 kJ
    or = 488 .36 KW
    Power = Energy / time
    And this is 488.36 Kilo Watts of power (since 1J/s = 1W)
    1758120 kilojoule/hour = 488.36666667 kilowatt hour
    488 kwh for 1 hour
    For 4 hours 488/4 = 122kw
    For 8 hours 488/8 = 61kw

  25. John -

    This calculator is great. I’m curious, though. I just recently learned about hybrid heat pump water heaters for the home that have much higher heating efficiency than standard electric. They are saying the modern heat pumps can give you 3.8 watts worth of hot water (or something close to that) for each watt that the heat pump uses. It would be really cool to see a calculator that takes this new technology into account.

  26. Anna -

    This is great. I am assuming calculations take into account the specific heat capacity of water? I am wondering though, doesn’t heat capacity increase with temperature, potentially throwing off the calculation?

  27. antonio -

    The calculators use the specific heat capacity of water of 4186 J/kg/°C (Joules per kilogram per degree Celsius). The heat capacity is largely constant in the temperature range that the calculators work (34-210°F or 1-99°C). It is the way it is taught at school. For practical purposes, it should be precise enough. For rocket science one might want to calcualte it more accurately.

  28. Heat field -

    Great calculator. Straight to the point. Thank you

  29. Coliny -

    I entered 688 litres start temperature 29 C end temp 37 C with 1600 watts of power. The answer was 4 hours.
    If i change the power to 1.6kw . The answer is 6 hours 24 minutes.
    An I missing something here?

  30. antonio -

    The calculators cannot deal with dots or comma. It makes 1 out of 1.6

  31. Ralph -

    excellent piece of kit. I have saved the page to my home screen. 🙂

    As another calculator for us math dummies could be total kw x cost of electrical supply

    eg 250kw x .26c = $65

    At any rate a really good calculator q

  32. David Snitt -

    Thanks for this calculator. I use it all the time to figure things out for various DIY heating ideas. It is very usefull and nicely straightforward. My only wish is that there was a way to say the calculations in metric instead of always defaulting to imperial.

    Other than that a very handy tool and thank you for creating and hosting it.

  33. BugReporter -

    I used 33°F as the start temp, and I get an alert box saying starting temp must be above freezing. Last I checked it was 32°F?

  34. Emil -

    If i need to calculate for coold water i receveid errors.

  35. antonio -

    The calculators use a simple constant for how much energy is needed to heat water by 1 degree (4168 joules/kg/°C). In reality, the amount of energy is slightly different at different starting temperatures, but the constant is accurate enough for practical purposes. The problem is that this constant does not work well around freezing. The result would be inaccurate.

  36. Dan M. -

    Very helpful calculator – It has been my reliable ‘go to’ water heating calculator for some time. Thank you very much!

  37. SpaFreaking -

    GREAT calculator! After freaking out that my new hot tub was not hearing properly (8hrs of heating). The calculator helped me relax given it should that 19hr to hear 576 gal/50deg to100deg with 4kW heater. I wonder how much heating I could get running pumps full blast. As in the tiny amount of her transfer due to circulation..LOL

  38. Imran -

    Very easy way to calculate energy vs time.

  39. David Smith -

    Thanks for these extremely useful tools . I refer to them all the time for different calculations for various heating ideas. They let me know straight away wether they are viabe or not.
    Much appreciation for your efforts in helping others.

  40. Clark -


    I left school in 1986, and even though physics was one of my top subjects – after 35 years of not doing anything like this – I am a bit rusty 🙂

    This is exactly what I was looking for, cheers!

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