Latent Heat Calculator

This latent heat calculator gives the heat needed for a phase change: Q = m·Lf for melting or freezing and Q = m·Lv for boiling or condensing. Melting 1 kg of ice at 0 °C takes 334 kJ, boiling 1 kg of water at 100 °C takes 2256 kJ. A second mode adds up the whole way from ice at −20 °C to steam.

Example: Calculation: One phase change (Q = m·L) · Substance: Water · Phase change: Melting (solid → liquid) · Mass 1 · Unit: kilograms (kg) → Heat: Heat absorbed: 334 kJ. Source: OpenStax University Physics Volume 2, 1.5 Phase Changes – Q = mLf (Eq. 1.7), Q = mLv (Eq. 1.8), Table 1.4 Heats of Fusion and Vaporization (values at the normal melting and boiling points, 1 atm), heating curve of water from −20 °C (2090, 4186 and 2020 J/(kg·°C)), Example 1.9 (ice in soda → 13 °C) (read 10.10.2026). Updated: 10 October 2026.

Calculation

Substance

Phase change

Mass

Unit

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Result

Heat: Heat absorbed: 334 kJ
Working: Step Value
Substance Water
Melting point (1 atm) 0 °C
Heat of fusion Lf 334 kJ/kg
Mass m 1 kg
Q = m·L 1 kg × 334 kJ/kg = 334 kJ = 334,000 J; Calculated with Q = m·L and the values of OpenStax Table 1.4 (normal melting/boiling point, 1 atm). The temperature stays constant during the phase change.

Step	Value
Substance	Water
Melting point (1 atm)	0 °C
Heat of fusion Lf	334 kJ/kg
Mass m	1 kg
Q = m·L	1 kg × 334 kJ/kg = 334 kJ = 334,000 J

How it is calculated

The heat for a phase change is Q = m·Lf for melting or freezing and Q = m·Lv for vaporization or condensation, with m the mass and L the latent heat of the substance. The textbook OpenStax University Physics explains why it is called “latent” (hidden): during the phase change energy enters or leaves the substance without changing its temperature. Melting and vaporization absorb heat; freezing and condensation release the same amount.

Latent heat of common substances

Substance	Melting point (°C)	Lf (kJ/kg)	Boiling point (°C)	Lv (kJ/kg)
Water	0	334	100	2256
Ethanol	−114	104	78.3	854
Ammonia	−75	332	−33.4	1370
Mercury	−38.9	11.8	357	272
Nitrogen	−210	25.5	−195.8	201
Lead	327	24.5	1750	871
Aluminum	660	380	2450	11400
Silver	961	88.3	2193	2336
Gold	1063	64.5	2660	1578
Copper	1083	134	2595	5069

The values come from OpenStax Table 1.4 and apply at the normal melting and boiling point at 1 atm. In general Lv is larger than Lf. For water at body temperature (37 °C) the book gives a higher heat of vaporization, 2430 kJ/kg. The running text uses 333 kJ/kg for melting ice, the table 334 kJ/kg; this calculator uses the table value.

From ice to steam: the heating curve of water

OpenStax describes ice heated from −20 °C; per kilogram and with the table value for melting: the ice warms at 2090 J/(kg·°C) up to 0 °C, then melts with 334 kJ/kg at constant temperature, the water warms at 4186 J/(kg·°C) up to 100 °C, boils with 2256 kJ/kg, and the steam warms at 2020 J/(kg·°C). The second mode adds up these steps:

Mass	From → to	Heat Q
1 kg	−20 °C → 120 °C	3090.8 kJ
1 kg	0 °C → 100 °C	752.6 kJ
1 kg	100 °C → 110 °C	2276.2 kJ
0.5 kg	20 °C → 100 °C	167.44 kJ
2 kg	25 °C → 100 °C	627.9 kJ
0.2 kg	−10 °C → 0 °C	4.18 kJ
0.1 kg	−5 °C → 25 °C	44.91 kJ

The phase changes dominate: boiling 1 kg of water takes more than five times as much heat as warming it from 0 °C to 100 °C (2256 kJ against 418.6 kJ). OpenStax Example 1.9 shows the effect in a drink: three 6.0 g ice cubes at 0 °C cool 0.25 kg of soda from 20 °C to 13 °C. Steam gives off its 2256 kJ/kg when it condenses on skin – handle steam with the same care as boiling water.

The final temperature when two substances are mixed comes from the mixing temperature calculator; heating water without a phase change is covered by the energy to heat water calculator.

Frequently asked questions

What is the formula for latent heat?

Q = m·L. For melting or freezing L is the heat of fusion Lf, for boiling or condensing the heat of vaporization Lv. Water: Lf = 334 kJ/kg, Lv = 2256 kJ/kg.

How much energy does it take to melt 1 kg of ice?

334 kJ at 0 °C. The ice stays at 0 °C until it has completely melted.

How much energy does it take to turn 1 kg of water into steam?

2256 kJ at 100 °C and 1 atm – more than five times the 418.6 kJ needed to heat the water from 0 °C to 100 °C.

Does freezing release heat?

Yes. Freezing releases the same heat that melting absorbs, and condensation the same as vaporization. The calculator shows the sign as “absorbed” or “released”.

Why does the temperature not change while ice melts?

The heat is used to break the bonds between the molecules, not to raise their temperature. OpenStax calls this energy “latent”, meaning hidden.

Sources and legal basis

OpenStax University Physics Volume 2, 1.5 Phase Changes – Q = mLf (Eq. 1.7), Q = mLv (Eq. 1.8), Table 1.4 Heats of Fusion and Vaporization (values at the normal melting and boiling points, 1 atm), heating curve of water from −20 °C (2090, 4186 and 2020 J/(kg·°C)), Example 1.9 (ice in soda → 13 °C) (read 10.10.2026)

As of: 10 October 2026 · Editorial policy