What this calculator does
The specific heat capacity of water is 4186 joules per kilogram per degree Celsius: the energy needed to raise one kilogram of water by one degree. It is one of the highest specific heat values of any common substance, which is why water is slow to heat up and slow to cool down compared with metal, air or oil, and why it is used so widely for storage heaters, radiators and cooling systems.
Because that figure is fixed, the only real work in this calculation is multiplying it out: mass, times the temperature change, times 4186. This calculator does that directly for water, rather than asking you to look the value up and enter it yourself, and adds a time estimate if you also know the power of the heater doing the work.
The formula
Heat energy Q equals mass times specific heat capacity times temperature change: Q = mcΔT. With c fixed at 4186 J/(kg·°C) for water, entering a mass and a start and target temperature gives Q directly. Dividing that energy by a heater's power in watts gives the time needed, assuming all of that power actually goes into the water with no losses.
| Term | Meaning |
|---|---|
| Q | Heat energy: the energy needed for the temperature change, in joules. |
| m | Mass of water being heated or cooled, in kilograms. |
| c | Specific heat capacity: 4186 J/(kg·°C) for water, a fixed physical property. |
| ΔT | Temperature change: target temperature minus starting temperature. |
The inputs explained
| Field | What to enter |
|---|---|
| Mass of water (kg) | The mass of water in kilograms. One litre of water has a mass of almost exactly one kilogram. |
| Starting temperature (°C) | The starting temperature of the water, in degrees Celsius. |
| Target temperature (°C) | The temperature you want the water to reach. This can be lower than the starting temperature, for a cooling calculation. |
| Heater power (0 to skip) (W) | The power of the heater or element doing the work, in watts. Leave at zero to skip the time estimate. |
When to use it
Sizing a kettle or water heater
Working out the energy needed to bring a known volume of water to temperature, and the time a given element takes to deliver it, is the standard sizing calculation behind kettles, urns and hot water systems.
Estimating a heating bill
Converting the energy required into kilowatt-hours gives a rough running cost once multiplied by the price per kWh, for anything from a hot water cylinder to a swimming pool heater.
Comparing water to another fluid
Water's specific heat capacity of 4186 J/(kg·°C) is roughly double that of most oils and around four times that of most metals, which is why the same heater warms a litre of oil noticeably faster than a litre of water.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much energy does it take to heat 2 kg of water to different temperatures?
The same 2 kg starting at 20°C, heated to a range of target temperatures.
| Target temperature | Heat energy required | Time to heat at 2,000 W |
|---|---|---|
| 40°C | 167.44 kJ | 1:24 (mm:ss) |
| 60°C | 334.88 kJ | 2:47 (mm:ss) |
| 80°C | 502.32 kJ | 4:11 (mm:ss) |
| 100°C | 669.76 kJ | 5:35 (mm:ss) |
How does heater power change the time to boil 5 kg of water?
The same 5 kg, heated across an 80°C rise, on a range of heater powers.
| Heater power | Heat energy required | Time to heat |
|---|---|---|
| 1000 W | 1,674.40 kJ | 27:54 (mm:ss) |
| 1500 W | 1,674.40 kJ | 18:36 (mm:ss) |
| 2000 W | 1,674.40 kJ | 13:57 (mm:ss) |
| 3000 W | 1,674.40 kJ | 9:18 (mm:ss) |
Questions
Why is water's specific heat capacity so high compared with other materials?
It comes down to the hydrogen bonds between water molecules, which absorb a large amount of energy before the temperature actually starts to rise. This is also why large bodies of water moderate the climate of nearby land, warming and cooling far more slowly than the ground or air around them.
Does this calculator account for heat lost to the surroundings?
No. It assumes all of the heater's power goes directly into the water, which is optimistic for anything not well insulated. A real kettle or tank will usually take somewhat longer than this figure suggests, because some heat escapes before the water reaches temperature.
What is the difference between specific heat capacity and heat capacity?
Specific heat capacity is a property of the substance itself, per kilogram; heat capacity is that value multiplied by the actual mass involved, which is what this calculator computes as Q. Two different masses of water share the same specific heat capacity but need different amounts of energy to heat.
Can I use this for ice or steam instead of liquid water?
No. Ice and steam have their own, different specific heat capacities, and this calculator applies only to liquid water. Changing state, from ice to liquid or liquid to steam, also needs a separate latent heat energy on top of any temperature change, which this tool does not include.
For heating or cooling a substance other than water, or where you need to enter your own specific heat value, see the general heat energy calculator. For the physical constant water evaporates against rather than heats to, see the evaporation calculator.