Latent Heat Calculator
Latent heat is the thermal energy transferred during a phase change without a change in temperature. When ice melts or water boils, energy is absorbed to break intermolecular bonds rather than increase kinetic energy. The formula is simply Q = m L, where Q is the heat in joules, m is the mass in grams, and L is the specific latent heat in joules per gram. This calculation is used in refrigeration engineering, climate science, food processing, and chemistry laboratory experiments. Common values: water fusion L = 334 J/g; water vaporization L = 2,257 J/g at 100 degrees C.
Latent heat formula
Q = m × L
Q is heat in joules (J), m is mass in grams (g), L is specific latent heat in J/g. The same formula applies to fusion (melting/freezing) and vaporization (boiling/condensation). Heat is absorbed (positive Q) during melting and boiling, and released (negative Q) during freezing and condensation.
Specific latent heat values (NIST)
- Water, fusion (0 degrees C): 334 J/g (333.55 kJ/kg)
- Water, vaporization (100 degrees C): 2,257 J/g
- Ethanol, vaporization (78.4 degrees C): 841 J/g
- Nitrogen, vaporization (-196 degrees C): 199 J/g
- Iron, fusion (1,538 degrees C): 247 J/g
Frequently asked questions
What is latent heat?
Latent heat is the heat absorbed or released during a phase change at constant temperature. Unlike sensible heat, latent heat does not change the temperature of the substance; it changes its phase (solid to liquid, or liquid to gas).
What is the latent heat of fusion?
The latent heat of fusion is the energy required to melt 1 gram of a solid at its melting point. For water (ice to liquid): 334 J/g. For iron: 247 J/g. These values are tabulated by NIST.
What is the latent heat of vaporization?
The latent heat of vaporization is the energy required to vaporize 1 gram of liquid at its boiling point. For water: 2,257 J/g at 100 degrees C and 1 atm. This is why boiling water requires much more energy than melting ice.
Does latent heat apply to condensation and freezing?
Yes. The same amount of heat is released during condensation (gas to liquid) and freezing (liquid to solid). The process is reversible; the same value of L applies, but heat is given off rather than absorbed.
What units does this calculator use?
Mass in grams (g), latent heat in joules per gram (J/g), and the resulting heat Q in joules (J). To convert to kilojoules, divide Q by 1,000. To convert L from kJ/kg, multiply by 1 (since 1 kJ/kg = 1 J/g).
Official sources
- NIST Chemistry WebBook: Thermophysical Properties of Fluid Systems.
- NIST: NIST Standard Reference Database 69.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.