Latent Heat Calculator (melting & boiling)

Understanding the latent heat calculator — concept and principle

Latent heat is the energy a substance absorbs or releases while it changes phase, such as ice melting or water boiling, without any change in temperature. This calculator multiplies the mass by the specific latent heat to give the energy in kilojoules and kilowatt-hours. Use it to estimate the energy to boil water away or the cooling a load of ice provides.

Phase change soaks up heat without raising the temperature: 2,257 kJ to boil away 1 kg of water.

How it is calculated

Energy: Q = m × L, with m in kg and L in kJ/kg, giving kJ.
kWh: Q (kJ) ÷ 3600, since 1 kWh = 3600 kJ.

You can choose water's latent heat of fusion (melting, 334 kJ/kg) or of vaporisation at 100 °C (2257 kJ/kg), or type a custom value for another substance; a custom value above zero overrides the list.

The principle

In a solid, molecules are held in a fixed arrangement; in a liquid they move past one another; in a gas they are almost free. Changing phase means breaking or loosening these attractions, and that takes energy. While a phase change is under way, the added heat goes into separating molecules rather than making them move faster, so the temperature stays constant: ice water stays at 0 °C until the ice is gone, and boiling water stays at about 100 °C at standard pressure. The same energy is released in reverse when steam condenses or water freezes.

Worked example

Inputs: m = 1 kg, phase change = boiling water at 100 °C (L = 2257 kJ/kg), custom value 0 (use the list).
Q = 1 × 2257 = 2,257 kJ.
In kWh: 2257 ÷ 3600 = 0.6269444444 kWh.

Each point is the calculator re-run with only “Or custom latent heat (kJ/kg, 0 = use list)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Or custom latent heat (kJ/kg, 0 = use list)	Energy (kJ)
0	2,257
1	1
2	2
3	3
4	4
5	5
6	6

How to read the result

Compare with sensible heat: warming 1 kg of water from 0 °C to 100 °C takes about 4.19 kJ/K × 100 K ≈ 419 kJ, so turning that water into steam takes more than five times as much energy as heating it. Melting 1 kg of ice (334 kJ) takes as much energy as warming the same water by about 80 °C. That is why ice is such an effective coolant and why boiling a pot dry consumes far more energy than bringing it to the boil.

Common mistakes

Forgetting the sensible heat before or after the phase change; heating ice at −10 °C to steam needs ice warming, melting, water warming and boiling added together.
Mixing J/kg and kJ/kg; 2257 kJ/kg is 2.257 × 10⁶ J/kg.
Using 2257 kJ/kg at other pressures; latent heat of vaporisation changes with the boiling temperature (it is higher at lower temperatures).

Keep in mind: Values are for pure water at standard atmospheric pressure. Dissolved substances, altitude and heater losses change real energy use, so treat the result as a theoretical minimum.

Frequently asked questions

Why does sweating cool the body?

Evaporating water absorbs latent heat from the skin. Near skin temperature the latent heat of vaporisation is even higher than at 100 °C, around 2400 kJ/kg.

Is latent heat of melting the same as of freezing?

Yes, the magnitude is the same; melting absorbs 334 kJ/kg and freezing releases it.

How much ice does it take to cool a drink?

Each kilogram of melting ice absorbs about 334 kJ, plus a little more as the meltwater warms; divide the heat to be removed by that figure.

Sources: D. Halliday, R. Resnick and J. Walker, Fundamentals of Physics, Wiley, chapter on temperature, heat and the first law · IAPWS, Revised Release on the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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