Latent Heat Calculator
Enter a mass and a specific latent heat to get the energy in joules — plus kilojoules — that a substance absorbs or releases when it changes phase.
Joules and kilojoules at once
Enter the mass and the specific latent heat and the calculator returns the heat (Q = m·L) in joules and the same value in kilojoules together.
Use SI units
Mass in kilograms and latent heat in joules per kilogram give energy in joules — fusion of water is ≈ 334,000 J/kg, vaporisation ≈ 2,260,000 J/kg.
What is latent heat?
The energy of a phase change
This latent heat calculator finds the energy a substance absorbs or releases when it changes phase — melting, freezing, boiling, or condensing — without any change in temperature. During a phase change the energy goes into rearranging the bonds between particles rather than making them move faster, which is why a melting ice cube stays at 0 °C until it has fully melted. The calculator turns two measurements, the mass in kilograms and the specific latent heat in joules per kilogram, into the total heat in joules, alongside the same value in kilojoules. It is the number behind why sweating cools you, why steam burns are so severe, and why ice keeps a drink cold for so long.
Enter a mass in kilograms and a specific latent heat in joules per kilogram to get the heat in joules and kilojoules instantly.
Latent heat is simply the mass multiplied by the specific latent heat of the substance, and dividing by 1000 converts the joules into kilojoules.
Q = m × LThere is no temperature term in the formula because the temperature stays constant during a phase change. To melt 1 kg of ice you take its specific latent heat of fusion (334,000 J/kg) and multiply by the mass: 1 × 334,000 = 334,000 J, which is 334 kJ. Boiling the same kilogram of water away would take far more, because the latent heat of vaporisation is about 2,260,000 J/kg. Keep mass in kilograms and the latent heat in joules per kilogram and the answer comes back in joules.
The formula is exact, but a couple of practical points are worth keeping in mind.
Phase change only, with consistent units
Q = m × L covers only the energy of the phase change itself, at the substance's melting or boiling point. Warming a solid up to its melting point or heating a liquid before it boils is sensible heat (Q = m × c × ΔT) and is a separate calculation. Use the correct specific latent heat for your substance and phase change — fusion or vaporisation — and keep units consistent (kilograms and joules per kilogram), or the joules will be wrong.