What this calculator does
Changing state takes energy without changing temperature. Ice at 0°C absorbs a great deal of heat becoming water at 0°C, and the thermometer does not move at all while it happens.
The quantities are large. Boiling a kilogram of water takes 2,260 kJ, which is nearly seven times the 334 kJ needed to melt the same kilogram of ice, and more than five times what it takes to heat that water from freezing to boiling in the first place.
The formula
Multiply the mass by the specific latent heat of the substance for the relevant transition. The result is the total energy absorbed on melting or boiling, and released on freezing or condensing.
| Term | Meaning |
|---|---|
| Latent heat of fusion | Energy to melt a substance without temperature change. Water is 334 kJ/kg. |
| Latent heat of vaporisation | Energy to boil it. Water is about 2,260 kJ/kg, far larger than fusion. |
| Sensible heat | By contrast, heat that does change the temperature, which is the specific heat calculation. |
The inputs explained
| Field | What to enter |
|---|---|
| Mass (kg) | The mass changing state, in kilograms. |
| Specific latent heat (L) (kJ/kg) | The specific latent heat in kilojoules per kilogram. Ice melting is 334; water boiling about 2,260. |
When to use it
Sizing a freezer or chiller
Freezing a product requires removing its latent heat as well as cooling it, and the latent part usually dominates.
Understanding why steam burns are severe
Condensing steam releases 2,260 kJ per kilogram onto the skin before it even begins to cool.
Calculating a heating load
Any process involving melting or boiling needs both the sensible and the latent heat accounted for.
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 each state change take?
Common latent heat values applied to one kilogram.
| Specific latent heat | Heat energy (Q) | In kilowatt-hours |
|---|---|---|
| Melting ice (334) | 334.000 kJ | 0.092778 kWh |
| Boiling water (2,260) | 2,260.000 kJ | 0.627778 kWh |
Questions
Why does the temperature not change during melting?
Because the energy is going into breaking the bonds holding the solid structure together rather than into increasing molecular motion. Temperature measures that motion, so it stays put until the structure has fully given way.
Why is boiling so much more energy-intensive than melting?
Melting only loosens molecules enough to flow past each other. Boiling separates them completely against atmospheric pressure, which takes far more energy.
Why does sweating cool you down?
Because evaporating water absorbs its latent heat of vaporisation, and that energy comes from your skin. It is the same mechanism that makes a wet cloth feel cold.
Is the energy recovered when it freezes?
Yes, exactly the same amount is released. That is why water freezing in a container releases heat, and it is the principle behind phase-change thermal storage.
For heating without a state change, see the specific heat calculator. For heat flow through a material, see the thermal resistance calculator.