Latent Heat Calculator
The energy needed to melt or boil something, and how long a given heater would take to do it.
Results
What this tool does
Heating something usually makes it hotter. Melting and boiling are the exceptions: while the change of state is happening, every joule goes into rearranging the material and the temperature sits perfectly still. The amount involved is far larger than most people expect — it takes more energy to boil away a kilogram of water already at a hundred degrees than to heat that water from freezing to boiling in the first place. Put in the mass and the latent heat for your material, and this page gives the energy and the time a heater of a given power would need.
Formula
Q = m × L (the temperature does not change during the change of state)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
m | Mass | kg |
l | Latent heat of the material | kJ/kg |
p | Power of the heater | W |
Q | Energy needed | kJ |
QJ | The same in joules | J |
QW | The same in kilowatt-hours | kWh |
T | Time at that power | s |
TM | The same in minutes | min |
EQ | That energy would heat the same water by | °C |
Worked example
- Mass2 kg
- Latent heat of the material334 kJ/kg
- Power of the heater2000 W
- Energy needed668.0000 kJ
- The same in joules668,000 J
- The same in kilowatt-hours0.185556 kWh
- Time at that power334.00 s
- The same in minutes5.567 min
- That energy would heat the same water by79.790 °C
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
Frequently asked questions
Why does the temperature not change?
Because the energy is going into breaking the bonds that hold the material in its current state, not into making its molecules move faster — and temperature only measures the second. Ice at zero degrees takes 334 kilojoules per kilogram to become water at zero degrees, and during the whole of that the thermometer does not move. This is why a drink stays cold as long as there is ice in it, and why the ice disappearing is the moment it starts warming up.
Which latent heat value do I use?
The one for the change you are making, and they differ enormously. Melting ice takes 334 kilojoules per kilogram; boiling the resulting water takes 2,260 — nearly seven times more, which is why a kettle that boils in two minutes would take a quarter of an hour to boil dry. Look up the figure for your substance and the transition you want; this page cannot supply it, because a value that depends on the material and the pressure is not a constant a static site should be inventing.