Latent heat calculator
Choose melting or boiling, enter the mass and get the heat needed for the phase change at constant temperature. Water's latent heats are built in; for another substance enter its specific latent heat. You can also find the mass or the latent heat itself.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose what to find: the heat, the mass or the latent heat.
- Choose melting or boiling, and water or another substance (then enter its L in kJ/kg).
- Enter the mass, or the heat, and pick the units.
- Heat: Q = m × L.
- Mass: m = Q ÷ L. Latent heat: L = Q ÷ m.
- Water: L_f ≈ 334 kJ/kg (melting at 0 °C), L_v ≈ 2257 kJ/kg (boiling at 100 °C).
Worked example
To melt 2 kg of ice at 0 °C: Q = 2 × 334 = 668 kJ, or 0.1856 kWh.
To boil away 500 g of water at 100 °C: Q = 0.5 × 2257 = 1,128.5 kJ. In kilocalories that is 1,128,500 ÷ 4184 = 269.7181 kcal.
Measured: 1130 kJ evaporated 0.5 kg of a liquid, so L = 1130 ÷ 0.5 = 2260 kJ/kg.
Tips
- The temperature must already be at the melting or boiling point. Heat for warming up is separate.
- Latent heat of vaporization changes with pressure; 2257 kJ/kg is for normal atmospheric pressure.
- Units: 1 kcal = 4.184 kJ, 1 kWh = 3600 kJ.
Frequently asked questions
What is latent heat?
It is the heat a substance takes in or gives off while it changes phase, melting or boiling, without changing temperature. Ice at 0 °C must absorb heat to become water at 0 °C.
What are the latent heats of water?
About 334 kJ/kg to melt ice at 0 °C and about 2257 kJ/kg to boil water at 100 °C and normal pressure. Boiling takes almost seven times as much heat as melting.
How do I find the total heat to turn ice into steam?
Add the stages. Warming uses Q = m × c × ΔT (the specific heat calculator), each phase change uses Q = m × L. For 1 kg of ice at 0 °C to steam at 100 °C, 334 + 1 × 4.18 × 100 + 2257 ≈ 3009 kJ, with c of water about 4.18 kJ/(kg·K).
Is the heat released when water freezes or steam condenses?
Yes, the same amount comes back out. That is why steam burns are worse than burns from boiling water and why freezing water releases heat that can protect crops from frost.