Latent Heat Calculator
The energy to melt, freeze, boil or condense a given mass — Q = m·L — the heat that changes phase at constant temperature, plus how long it takes and what it costs.
The Heat That Hides
Heat a pot of ice water and the temperature sits stubbornly at 0 C until the last ice melts — only then does it start to climb. That pause is latent heat: energy going into breaking the bonds of a phase change rather than raising temperature. It is “hidden” because the thermometer does not move while it happens.
Q is the energy, m the mass, and L the latent heat of the substance — the energy per kilogram for that particular change. Melting and freezing use the latent heat of fusion; boiling and condensing use the latent heat of vaporization, which is usually much larger.
Why Boiling Costs So Much More
For water, melting takes 334 kJ/kg but boiling takes 2,257 kJ/kg — nearly seven times as much. Pulling molecules fully apart into vapor takes far more energy than just loosening them from a solid. It is why a little sweat evaporating cools you so effectively, and why steam burns are so severe: condensing steam dumps all 2,257 kJ/kg back into your skin.
Frequently Asked Questions
How is this different from sensible heat?
Sensible heat changes temperature and you can feel it (m x c x temperature change). Latent heat changes phase at constant temperature. A full problem – say ice at -10 C to steam – adds several sensible and latent steps together.
Why does ice keep a drink cold so well?
Because melting each gram of ice soaks up 334 joules without the ice getting warmer – far more than simply having cold water. The phase change does the heavy lifting.
Does latent heat depend on pressure?
Yes. Latent heat of vaporization in particular falls as pressure rises, reaching zero at the critical point. The presets here are typical values at one atmosphere.
Related calculators
- Sensible Heat Calculator — the temperature-change half of the problem.
- Heat Transfer Calculator — how fast that heat moves.
- Pipe Heat Loss Calculator — standing heat loss from pipe.
- Engineering Unit Converter — energy and power units.