Latent Heat Calculator (Q = mL) — Melting & Boiling Energy
By Santaji GadepatilinUpdated October 30, 2026
Latent heat is the energy a substance absorbs or releases while it changes phase at constant temperature — melting, freezing, boiling or condensing. This latent heat calculator applies Q = mL to find the energy for a given mass, or the mass that a given amount of energy can melt or evaporate.
How to use the latent heat calculator
Enter the mass and choose the substance and phase change. The calculator gives the heat that must be added (melting, boiling) or removed (freezing, condensing) in kJ and kWh. To find how much mass a given amount of heat can melt or evaporate, clear the mass and enter the energy.
Latent heat formula
Q = m × L
Lf is the specific latent heat of fusion (solid ↔ liquid) and Lv the specific latent heat of vaporisation (liquid ↔ gas). This calculation covers only the phase change; add the sensible heat m c ΔT from the specific heat calculator to bring the substance to its melting or boiling point first.
Example
Boiling off 2 kg of water that is already at 100 °C:
- Q = 2 kg × 2,257 kJ/kg = 4,514 kJ
- = 4,514 ÷ 3,600 = 1.254 kWh
Energy per kilogram for each phase change
| Substance and change | Energy per kg | In kWh |
|---|---|---|
| Water — melting / freezing | 334 kJ | 0.09278 kWh |
| Water — boiling at 100 °C | 2,257 kJ | 0.6269 kWh |
| Ethanol — melting | 108 kJ | 0.03 kWh |
| Ammonia — boiling at −33 °C | 1,369 kJ | 0.3803 kWh |
| Aluminium — melting | 397 kJ | 0.1103 kWh |
| Copper — melting | 209 kJ | 0.05806 kWh |
| Iron — melting | 247 kJ | 0.06861 kWh |
| Lead — melting | 23 kJ | 0.006389 kWh |
Values are at the normal melting or boiling point at 1 atm; the latent heat of vaporisation of water falls as pressure (and boiling temperature) rises.
Notes
- Melting ice absorbs as much energy as heating the same water by about 80 °C — the basis of ice storage cooling.
- Steam tables give hfg for water at other pressures, for example about 2,015 kJ/kg at 10 bar(a).
- Use the heat duty calculator when a stream changes temperature at a steady flow rate.
Frequently asked questions
What is the formula for latent heat?
Q = m × L, where m is the mass and L the specific latent heat of fusion (melting) or vaporisation (boiling) in J/kg.
How much energy does it take to boil 1 kg of water?
About 2,257 kJ (0.63 kWh) once it is already at 100 °C — more than five times the 419 kJ needed to heat it from 0 to 100 °C.
Why does temperature stay constant during melting or boiling?
The added energy breaks the bonds between molecules instead of making them move faster, so the temperature holds until the phase change is complete.
Is energy released when steam condenses?
Yes — exactly the same amount, 2,257 kJ per kg at 100 °C. This is why steam is such an effective heating medium and why steam burns are severe.