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Latent Heat Calculator

Find the energy for a phase change.

Result

668.00 kJ

How it is calculated

Energy needed for, or released by, a change of state at constant temperature is Q = m · L. The tool gives the result in kilojoules, so enter the mass m in kilograms and the specific latent heat L in kJ/kg. Use the latent heat of fusion for melting or freezing and the latent heat of vaporization for boiling or condensing. The mass must be above zero. Q is the heat that changes the phase only: warming the substance up to its melting or boiling point needs a separate calculation with specific heat capacity.

Worked example

Melting 2 kg of ice that is already at 0 °C, with a latent heat of fusion of 334 kJ/kg. Q = m · L = 2 × 334 = 668.00 kJ. So 668.00 kJ of heat must be supplied to turn the ice into water at the same temperature. Freezing the same water would release the same amount.

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Common mistakes

  • Entering L in J/kg or the mass in grams. The result is labelled kJ, so use kg and kJ/kg, otherwise the answer is off by a factor of 1,000.
  • Mixing up latent heat of fusion with that of vaporization. Water's vaporization value is several times larger than its fusion value.
  • Forgetting that the temperature does not change during the phase change. The warming steps before and after it need Q = m·c·ΔT separately.
  • Using the value for the wrong substance. Each material has its own latent heat, so take it from a reliable table.

About this calculator

This calculator finds the heat energy Q absorbed or released during a phase change — melting, freezing, boiling, or condensing — using the equation Q = m · L, where m is mass and L is the substance's specific latent heat (of fusion or vaporization) in kJ/kg, so the result comes out in kJ. Unlike sensible heat, this energy changes a substance's state without changing its temperature, which is why an ice cube stays at 0°C while it melts.

Latent heat values are tabulated for common substances — water's latent heat of fusion is about 334 kJ/kg and its latent heat of vaporization about 2,260 kJ/kg — and the calculation underlies refrigeration and HVAC system design, steam boiler sizing, calorimetry experiments, and industrial processes like metal casting and food freezing.

Mechanical and chemical engineers, HVAC technicians, physics students working through thermodynamics problems, and food scientists calculating freezing loads all rely on this formula. The tool has no built-in substance list, so the latent heat value has to be looked up in a reference table first.

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Comments (1)

  • Editorial team

    A quick sanity check: for water, vaporizing takes roughly 6.8 times as much energy as melting the same mass, since 2,260 divided by 334 is about 6.8. If your boiling result is not far above your melting result for the same mass, a value was probably mixed up. The output is in kilojoules, so divide by 3,600 to get kilowatt-hours when you compare it with a heater's or chiller's energy use. Which substance are you working with, and where did you find its latent heat?

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