Latent heat calculator (fusion and vaporization)
Choose the phase change or enter your own latent heat, then enter the mass to get the energy needed with no change in temperature.
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How to use it
- Enter mass, phase change and latent heat (for other value).
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Q = m × L, where L is the latent heat in kJ/kg. For water: fusion about 334 and vaporization at 100 °C about 2257.
Example
Inputs: Mass = 1 kg, Phase change = Melting ice (334 kJ/kg). Result: Heat required: 334 kJ; In joules: 334,000 J; In kilowatt hours: 0.0928 kWh.
Limits and caveats
- Water values are rounded (333.5 for fusion, 2256 for vaporization at normal pressure); heat of vaporization changes with pressure.
- It excludes the heat needed to reach the transition temperature; add that with the Q = mcΔT calculator.
Questions about this tool
Why does temperature stay flat while boiling?
The energy goes into breaking the bonds between molecules rather than making them move faster.
Why is vaporization heat so much larger than fusion?
Vaporization separates molecules completely, while melting only loosens them.
How much energy turns 1 kg of ice at 0 °C into steam?
About 334 to melt, 418 to heat to 100 °C and 2257 to boil, roughly 3009 kJ in total.