💧 Latent Heat Calculator (Q = mL)
Enter a mass and the specific latent heat (or pick a preset) to get the energy for the phase change, in kJ, J and kcal.
Heat for the phase change (Q = mL)
668 kJ
= 668,000 J · 159.66 kcal
Latent heat is the energy a phase change needs at constant temperature: Q = m·L, where L is the specific latent heat of fusion (melting) or vaporisation (boiling). Water's huge latent heat of vaporisation (2,260 kJ/kg) is why steam burns and sweating cools. 🔒 In your browser.
How the latent heat calculator (q = ml) works
A phase change — melting or boiling — absorbs energy at constant temperature: Q = m × L, where m is the mass and L the specific latent heat (of fusion for melting, of vaporisation for boiling). The tool multiplies them and shows the heat in kilojoules, joules and kilocalories. Presets include water's latent heat of fusion (334 kJ/kg) and vaporisation (2,260 kJ/kg), plus a few other materials.
Latent heat is separate from the sensible heat that raises temperature (q = mcΔT). During a phase change the temperature holds steady while the energy goes into breaking or forming bonds. Water's exceptionally large latent heat of vaporisation is why steam scalds so badly and why sweating cools you so effectively.
Frequently asked questions
What is latent heat?
The energy absorbed or released during a phase change (melting, boiling, freezing, condensing) at constant temperature. It goes into rearranging molecular bonds rather than changing temperature.
How do you calculate latent heat energy?
Q = m × L, where m is the mass in kg and L the specific latent heat in J/kg. Melting 2 kg of ice (L = 334,000 J/kg) needs 668,000 J = 668 kJ.
What is the difference between latent and specific heat?
Specific heat (q = mcΔT) is the energy to change a substance's temperature; latent heat (Q = mL) is the energy to change its phase at constant temperature. A full heating curve uses both in turn.
What is the latent heat of vaporisation of water?
About 2,260 kJ/kg — much larger than its latent heat of fusion (334 kJ/kg). That's why boiling a pot dry takes far longer than melting the same mass of ice, and why steam carries so much energy.
Why does the temperature stay constant during a phase change?
Because the added energy breaks intermolecular bonds (melting, boiling) rather than speeding molecules up. Only once the phase change is complete does further heat raise the temperature again.