Latent Heat CalculatorQ = mL
Latent heat is the energy absorbed or released during a phase change at constant temperature: Q = mL. Melting 1 kg of ice takes 334 kJ, and boiling 1 kg of water takes 2,257 kJ, nearly seven times more. Pick a substance and transition to get the energy, or run the full heating curve from ice below zero to steam above 100 °C, with each stage broken out.
Formula verified against OpenStax — University Physics, Volume 2 · HyperPhysics, Georgia State University
By Manzoor Ahmad · Pharmacist & calculator developerReviewed & updated
Substance & mass
Latent heats at normal melting or boiling points and 1 atm.
Step-by-step working
What is latent heat?
Latent heat is the energy a substance absorbs or releases while it changes phase, between solid, liquid and gas, without any change in temperature. “Latent” means hidden: a thermometer doesn’t register it.
It is why ice keeps drinks at 0 °C until it has melted, why sweating cools you, why steam burns are severe and how refrigerators and heat pumps move heat.
Quick answers
How much energy does it take to melt ice?
334 kJ per kg, at 0 °C. Melting a 2 kg bag of ice takes 668 kJ, the same energy as heating 2 kg of water by about 80 °C.
How much energy does it take to boil water?
2,257 kJ per kg at 100 °C, on top of the energy to bring it to the boil. Boiling off 1 L takes about 0.63 kWh.
Why does steam burn worse than boiling water?
When steam condenses on skin it releases its 2,257 kJ/kg latent heat as well as cooling, so it delivers far more heat than water at the same temperature.
What is the latent heat of sublimation?
The energy to go straight from solid to gas: about 2,834 kJ/kg for ice. Freeze-drying (lyophilisation) of foods and medicines relies on it. Work out the warming stages with the specific heat calculator.
Latent heat formula
Water: Lf = 334 kJ/kg, Lv = 2,257 kJ/kg; cice ≈ 2.11, cwater = 4.19, csteam ≈ 2.01 kJ/(kg·K).
| Symbol | Meaning | SI unit |
|---|---|---|
| Q | Heat energy | J |
| m | Mass undergoing the change | kg |
| L | Specific latent heat (fusion Lf or vaporisation Lv) | J/kg |
| c | Specific heat capacity | J/(kg·K) |
How to calculate latent heat
- Identify the phase change: melting/freezing uses Lf; boiling/condensing uses Lv.
- Convert the mass to kg.
- Multiply m × L.
- For a heating curve, add the sensible-heat stages (mcΔT) before and after each phase change.
Worked example: 1 kg of ice at −10 °C to steam at 110 °C
How much heat turns 1 kg of ice at −10 °C into steam at 110 °C?
Melt: 1 × 334 = 334 kJ
Water 0 → 100 °C: 1 × 4.186 × 100 = 418.6 kJ
Boil: 1 × 2,257 = 2,257 kJ
Steam 100 → 110 °C: 1 × 2.01 × 10 = 20.1 kJ
Total ≈ 3,050.8 kJ (≈ 0.85 kWh)
Melting just 500 g of ice takes 0.5 × 334 = 167 kJ, enough to heat the same water from 0 to 80 °C.
Latent heats of common substances
At 1 atm.
| Substance | Melting point | Lf (kJ/kg) | Boiling point | Lv (kJ/kg) |
|---|---|---|---|---|
| Water | 0 °C | 334 | 100 °C | 2,257 |
| Ethanol | −114 °C | 108 | 78 °C | 855 |
| Ammonia | −78 °C | 332 | −33 °C | 1,369 |
| Lead | 327 °C | 23 | 1,750 °C | 871 |
| Aluminium | 660 °C | 397 | 2,519 °C | 10,900 |
| Iron | 1,538 °C | 247 | 2,862 °C | 6,090 |
Common mistakes to avoid
- Adding a temperature change during melting or boiling. Phase changes happen at constant temperature.
- Mixing up Lf and Lv. Boiling water needs about 6.8× more energy than melting ice.
- Forgetting kJ versus J. 334 kJ/kg is 334,000 J/kg, or 334 J/g.
Manzoor Ahmad
Pharmacist turned Web Developer & Calculator Engineer · 13+ years in web developmentI’m a pharmacist by training who became a full-time tech geek, and I’ve spent 13+ years designing and coding web tools. Latent heat isn’t abstract to me: freeze-drying (lyophilisation) of vaccines and injectable medicines depends on the latent heat of sublimation, and I studied it as part of pharmaceutics.
I’m not a physicist, so I don’t ask you to take my word for it: every formula on this page is traced to an open university textbook or NIST reference data, and the calculator is tested against published worked examples before it goes live. Spot an error? Tell me and it will be fixed and dated on the next update.
Latent Heat Calculator: frequently asked questions
The energy absorbed or released by a substance during a change of phase (melting, freezing, boiling, condensing) at constant temperature.
Q = mL, where m is the mass and L the specific latent heat of the transition.
334 kJ/kg (about 80 kcal/kg) at 0 °C.
2,257 kJ/kg at 100 °C and 1 atm. It is higher at lower temperatures, about 2,450 kJ/kg at 20 °C.
The added energy breaks the hydrogen bonds holding the crystal together instead of increasing molecular kinetic energy, so the thermometer stays at 0 °C until all the ice has melted.
References & sources
- OpenStax — University Physics, Volume 2 — Ch. 1.6, Phase Changes
- HyperPhysics, Georgia State University — latent heats and the heating curve
- NIST Chemistry WebBook — enthalpies of fusion and vaporisation
Disclaimer: This calculator is an educational tool built on idealised textbook models. It is not a substitute for professional engineering or safety calculations.