Latent Heat Calculator (Q = mL)

Latent heat is the energy a substance absorbs or releases while it changes phase at a constant temperature: ice melting at 0 °C, water boiling at 100 °C, steam condensing on a cold window. Because the temperature does not move, Q = mcΔT cannot capture it; you need Q = mL instead. Use this calculator for heat-of-fusion and heat-of-vaporization problems, cooling loads from ice, or evaporative cooling estimates.

How to use this tool

1. Choose whether to solve for the heat energy, the mass or the latent heat.
2. Pick a phase change for water, or choose Other and enter the latent heat in kJ/kg.
3. Enter the mass and its unit.
4. Enter the heat energy in kJ if you are solving for mass or latent heat.
5. For water, check the table to see how the energy compares with heating the liquid.

Formula

Q = m × L

Q is the heat in kJ, m the mass in kg and L the specific latent heat in kJ/kg (numerically the same as J/g). For water, L is about 334 kJ/kg for fusion and 2,256 kJ/kg for vaporization at 100 °C. Evaporation at lower temperatures needs more energy: about 2,442 kJ/kg at 25 °C.

Example

Boiling away 500 g of water at 100 °C takes Q = 0.5 × 2,256 = 1,128 kJ, or about 0.313 kWh. Melting the same 500 g of ice takes only 167 kJ, and warming the meltwater from 0 to 100 °C takes 209.2 kJ — boiling is about three quarters of the total energy to go from ice to steam.