About the Latent Heat Calculator
This latent heat calculator finds how much energy is absorbed or released when a substance changes phase at constant temperature — melting, freezing, boiling or condensing. Choose the substance and process, enter the mass, and it applies Q = m × L using the specific latent heat of fusion or vaporization. You can also run it backwards to find how much mass a given amount of heat can melt or boil.
It is aimed at physics and chemistry students working through heating-curve problems, and at anyone curious about the energy in everyday phase changes: why ice keeps a cooler cold, why steam burns are so severe, or how much energy it takes to boil a kettle dry. Results are given in kilojoules, joules, kilowatt-hours, kilocalories and BTU.
Latent heat covers only the phase change itself, at the melting or boiling point. Heating the substance up to that temperature (sensible heat) is a separate step — use the specific heat calculator for that part. The built-in values are typical textbook figures at normal pressure; latent heat of vaporization falls as the boiling temperature rises, so use a custom value for precise work.
How to use the latent heat calculator
- 1Choose whether to find the heat energy or the mass.
- 2Pick the substance, or choose custom and enter its latent heat.
- 3Select the phase change: melting, freezing, boiling or condensing.
- 4Enter the mass in kilograms (or the heat available in kilojoules).
- 5Read the energy in kJ, kWh, kcal and BTU, and whether it is absorbed or released.
Formula and method
During a phase change the temperature stays constant while energy is absorbed or released to rearrange the molecules. The heat involved is the mass m multiplied by the specific latent heat L of the transition: Lf (latent heat of fusion) for melting and freezing, Lv (latent heat of vaporization) for boiling and condensing. Melting and boiling absorb energy; freezing and condensation release exactly the same amount.
The built-in values are typical figures at atmospheric pressure (water: 334 kJ/kg to melt ice at 0 °C and about 2,257 kJ/kg to boil water at 100 °C). Unit conversions use 1 kWh = 3,600 kJ, 1 kcal = 4.184 kJ (thermochemical) and 1 BTU = 1.05506 kJ (International Table). To find the total energy to turn ice into steam, add the sensible heat (m × c × ΔT) for each warming stage.
- Q
- Heat absorbed or released (kJ)
- m
- Mass changing phase (kg)
- L
- Specific latent heat of fusion (Lf) or vaporization (Lv), kJ/kg
Worked examples
Melting 1 kg of ice
Q = 1 kg × 334 kJ/kg = 334 kJ, absorbed from the surroundings at 0 °C. That is about 0.093 kWh or 79.8 kcal — the same energy that would warm 1 kg of liquid water by roughly 80 °C.
Boiling away 0.5 kg of water
Q = 0.5 kg × 2,257 kJ/kg = 1,128.5 kJ (0.313 kWh). Boiling half a litre dry takes almost seven times the energy needed to melt the same mass of ice.
How much ice can 1,000 kJ melt?
m = Q ÷ L = 1,000 kJ ÷ 334 kJ/kg ≈ 2.99 kg of ice at 0 °C can be melted, leaving water still at 0 °C.
Condensing 2 kg of a refrigerant with L = 200 kJ/kg
Q = 2 kg × 200 kJ/kg = 400 kJ, released as the vapour condenses — this is the heat a refrigerator’s condenser coil rejects to the room.
Frequently asked questions
What is latent heat?+
Latent heat is the energy absorbed or released when a substance changes phase — solid to liquid or liquid to gas — at constant temperature. It is “hidden” because a thermometer shows no temperature change while it is being added.
What is the latent heat of fusion of water?+
About 334 kJ/kg (333.55 kJ/kg more precisely) to melt ice at 0 °C. The same amount is released when water freezes, which is why farmers spray crops with water to protect them from frost.
What is the latent heat of vaporization of water?+
About 2,257 kJ/kg at 100 °C and atmospheric pressure. It is larger at lower temperatures — roughly 2,450 kJ/kg near room temperature — which matters for evaporative cooling and sweating.
Why are steam burns worse than boiling water burns?+
When steam condenses on skin it releases about 2,257 kJ/kg of latent heat before it even starts to cool — more than eight times the energy given off by cooling the same mass of 100 °C water to body temperature (about 264 kJ/kg).
How do I calculate the total heat to turn ice into steam?+
Add the stages: warm the ice to 0 °C (m·c_ice·ΔT), melt it (m·Lf), warm the water to 100 °C (m·c_water·ΔT), then boil it (m·Lv). For 1 kg starting at 0 °C that is 334 + 418.6 + 2,257 ≈ 3,010 kJ.