Latent Heat Calculator
When a substance changes phase, it absorbs or releases a large amount of energy without any change in temperature. This energy is called latent heat, and it is what makes boiling water such an effective way to transfer heat in cooking and industrial processes, and why melting ice keeps drinks cold. The formula that governs it is simple: Q equals mass multiplied by the specific latent heat of the substance, where Q is in joules, mass is in kilograms, and the specific latent heat L is in joules per kilogram. The specific latent heat of vaporisation (liquid to gas) is different from the specific latent heat of fusion (solid to liquid), and both vary by substance. Water, for example, has an unusually high latent heat of vaporisation at about 2260 kJ/kg, which is why it is so effective as a coolant and why steam burns are so dangerous. The latent heat of fusion for water is about 334 kJ/kg. This calculator supports vaporisation, condensation, melting, and freezing. You choose the phase change type, enter the mass and the specific latent heat for your substance (common presets are provided to help), and the tool returns the total heat energy in joules and kilojoules. The result tells you how much energy must be supplied to drive the phase change, or how much is released if the process goes in the reverse direction.
Latent heat values vary with pressure and temperature. Standard values are given at atmospheric pressure and the substance's normal phase-change temperature.
How it works
The formula is Q = m × L, where Q is heat energy in joules, m is mass in kilograms, and L is the specific latent heat in joules per kilogram (entered here in kJ/kg and converted internally). For vaporisation and fusion (melting), energy is absorbed from the surroundings. For condensation and freezing, energy is released. The direction field indicates which applies. Per-gram values are calculated by dividing kJ/kg by 1000.
Worked example
You want to boil away 1 kg of water at 100 °C. The specific latent heat of vaporisation of water is 2260 kJ/kg. Using Q = m × L: Q = 1 × 2260 = 2260 kJ (2,260,000 J). The heat is absorbed (energy must be supplied to turn the water into steam). For comparison, melting 1 kg of ice uses L = 334 kJ/kg, so Q = 334 kJ, which is far less. These match the default values in the calculator above.
Related calculators
- Heat Transfer Calculator: find the rate of heat flow through conduction or convection.
- Enthalpy Calculator: compute sensible heat and enthalpy changes for reactions.
- Thermal Expansion Calculator: calculate how solids and liquids expand with temperature.
- Energy Converter: convert joules, calories, BTU, and other energy units.