Example 1
m = 0.5 kg, Lₚ = 2.26×10⁶ → Q = 1.13×10⁶ J.
Latent Heat of Vaporization Calculator
Enter mass and the latent heat of vaporization. You get the energy that goes into the liquid-to-gas change at a steady boiling point. Half a kilogram of water at 2.26e6 J/kg is 1.13 MJ. A kilogram of ice melting is only 334 kJ.
Fusion: Q = m·Lₜ. Heating without boiling: Q = c·m·ΔT.
Result
Mass m (kg) and Lₚ (J/kg). The result shows up here.
How it works
While a liquid boils, temperature holds still again. The energy is not lifting the thermometer. It is pulling molecules into vapor. The formula is the same kind as fusion: Q = m·Lₚ. For water Lₚ ≈ 2.26e6 J/kg, so one kilogram of boiling water needs 2.26 MJ before the whole mass is steam.
Half a kilogram at that constant is 1.13 MJ. That is the calculator example: 0.5 × 2260000 = 1130000. Condensing steam gives the same heat back the other way.
Type mass in kilograms and Lₚ in joules per kilogram. The result is in joules. School 2.26e6 and 2260000 are the same number.
Before water boils you still have to warm it from tap temperature to 100 °C with Q = c m ΔT. Only then does this vaporization apply. A liter from 20 °C to 100 °C is about 335 kJ, then another 2260 kJ to make steam. The sum is much larger than Lₚ alone.
Melting ice is cheaper: 334 kJ per kilogram. That is why an ice cube changes a drink without taking as much energy as evaporating the same mass of water would.
The form has two fields. Type mass and Lₚ, then Calculate. For another liquid, take Lₚ from a table, not the water constant.
How to use
- Type the mass that is going to evaporate, for example 0.5 kg of water.
- Type the latent heat Lₚ. For water, school work usually uses 2.26e6 J/kg, that is 2260000.
- Click Calculate. 0.5 kg and 2260000 give 1.13 MJ, the energy for the whole change to steam at boiling.
- If the water is still cold, warm it to 100 °C first on the Q = c m ΔT page and add that result.
- Fusion is on the neighbouring page, with Lₜ. The energy per kilogram there is several times smaller.
Formula
Q = m · Lp
Unit: J. Temperature stays constant while boiling.
Letters in Q = m·Lₚ
Boiling also holds temperature, but the constant is Lₚ, not Lₜ. Half a kilogram of water at 2.26×10⁶ J/kg is 1.13 MJ of steam.
- Q
- Energy for the liquid-to-gas change. 0.5 × 2260000 = 1130000 J, the 1.13 MJ from the calculator example.
- m
- Boiling-water mass in kilograms. One kilogram at water Lₚ takes 2.26 MJ, about seven times the 334 kJ ice-melt figure.
- Lₚ
- Latent heat of vaporization in J/kg. Water is near 2.26×10⁶; this field does not want 334000 from ice.
Real-life examples
Example 2
m = 1 kg, Lₚ = 2260000.
Example 3
m = 0.2 kg at boiling.
Example 4
m = 0.05 kg.
Example 5
m = 2 kg.
Example 6
Same |Q|: m = 0.5 kg.
Example 7
m = 0.1 kg.
Example 8
m = 1 kg, Lₚ = 1000000.
Example 9
m = 0.01 kg.
Example 10
m = 5 kg.
Ways to use this calculator
- You estimate how much energy evaporating half a liter takes: 1.13 MJ, not 21 kJ from mixing this up with a 10 K rise.
- You build a kettle balance: warm to 100 °C, then Lₚ only if you really want steam, not just boiling water.
Frequently asked questions
How much energy evaporates 0.5 kg of water at Lₚ = 2.26 MJ/kg?
Q = 1.13 MJ. A full kilogram at the same Lₚ is 2.26 MJ.
Does temperature rise while water boils?
In a clean phase change it holds still until the whole mass has evaporated.
How is Lₚ for water different from Lₜ for ice?
Melting a kilogram of ice is 334 kJ. Evaporating a kilogram of water is 2260 kJ, about seven times more.
Should I compute Q = c m ΔT before vaporization?
Yes if you start below boiling. From 20 °C to 100 °C a liter is about 335 kJ, then Lₚ.
What unit do I type for Lₚ?
J/kg. 2.26 MJ/kg is 2260000. The result comes back in joules.
Does 2.26e6 work?
Yes. 2.26e6, 2.26×10⁶ and 2260000 are the same water constant.
What about condensing steam?
The same |Q|. Steam gives heat away, water takes it. The sign depends on the balance.
Does every liquid use 2.26 MJ/kg?
No. That is water. Alcohol and other liquids have their own Lₚ from a table.
How is this different from Q = c m ΔT?
Here temperature holds and the phase changes. There temperature rises and water stays water.
Where is fusion?
On Q = m Lₜ. The constant is smaller and it is about ice, not steam.
Knowledge sources
The formula is the school one. Units follow SI; NIST SP 330 and BIPM define the measures, not your result.
Page updated in 2026.