Heat of Fusion Calculator
Calculate heat of fusion, transferred heat or changed mass, with kg, grams and pounds and a stated latent-heat value.
Work out the energy exchanged when a substance melts or freezes, the mass changed by a known amount of energy, or its specific latent heat of fusion. Unlike a temperature-rise calculation, this model covers the phase change itself. Mass can be entered in kilograms, grams or pounds, and energy in joules, kilojoules or kilowatt-hours.
How to use this tool
- Choose heat energy, mass or specific latent heat as the unknown.
- Select absorbed heat for melting or released heat for freezing.
- Enter the other two quantities with their units.
- Use the latent heat appropriate to the material and conditions, then read the signed heat transfer.
Formula
|Q| = m × Lf; m = |Q| ÷ Lf; Lf = |Q| ÷ m
Lf is energy per unit mass, entered in kJ/kg. The default 334 kJ/kg is an approximate value for water melting at 0°C at ordinary atmospheric pressure. It is not a value for every solid. The sign describes energy entering or leaving the material.
Example
Melting 2 kg of ice already at its melting point with Lf = 334 kJ/kg requires 2 × 334 = 668 kJ. Freezing the same amount releases 668 kJ, displayed as −668 kJ transferred to the material. Heating ice from below its melting point would require an additional calculation.
Frequently asked questions
Why does temperature not appear as an input?
The chosen latent heat must already correspond to the phase-change conditions. Temperature changes before and after melting are separate sensible-heat terms.
Does heat of fusion mean nuclear fusion?
Here fusion means melting a solid into a liquid. Nuclear energy and changes in chemical composition are outside this calculation.
Can I use a different material?
Yes. Enter its measured or specified latent heat at the relevant pressure and temperature; the calculator does not infer that property from a material name.
Does this give heater electricity consumption?
It gives the ideal phase-change energy. Heat losses, equipment efficiency and warming containers can increase the required electricity.
Sources
Last updated: 2026-10-30