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Latent Heat Calculator

The latent heat calculator computes the energy released or absorbed during a phase transition like melting or vaporizing.

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Latent heat for the phase change2.256 MJ1 kg × 2,256,000 J/kg — at constant temperature, so it does not appear on a thermometer at all
Sensible heat required0 J1 kg × 4,182 J/(kg·K) × 0 K
Temperature change0 Kthe same number of degrees Celsius — a temperature DIFFERENCE is identical in K and °C, unlike an absolute temperature
Specific heat used4,182 J/(kg·K)Water
In kilowatt-hours0.626667 kWh
In food calories539.1969 kcal
In BTU2,138.2754
Total heat2.256 MJ
Latent share of the total100%
Equivalent temperature rise if that energy were sensible539.45 Kwhich is why boiling a pan dry takes so much longer than bringing it to the boil
Time at that heater power18.8 minutes1,128 seconds at 2 kW and 100% efficiency
Energy actually drawn2.256 MJ0 J of it wasted

The formula

Q = mcΔT for sensible heat, Q = mL for a phase change

Latent heat hides from the thermometer

Sensible heat changes a temperature and is what mcΔT computes. Latent heat changes a phase at constant temperature, so it does not register on a thermometer at all — which is why a pan of boiling water stays at 100 °C however hard you heat it, and why the water level falls instead.

The magnitudes are strikingly unequal. Heating water from 20 °C to 100 °C takes about 335 kJ per kilogram; boiling it takes 2,256 kJ per kilogram — nearly seven times as much. Melting ice takes 334 kJ/kg, almost exactly the same as heating the resulting water through the full 80 degrees, which is why ice is such an effective coolant.

Water's specific heat of 4,182 J/(kg·K) is anomalously high — several times most metals and about four times air. That single property moderates the planet's climate, makes water the default coolant in almost every engineering context, and is why coastal temperatures swing far less than inland ones.

Note that ΔT is a difference, so it is the same number in kelvin and Celsius. That is the one place in thermodynamics where Celsius is safe to use, and it is worth being deliberate about which kind of temperature a formula wants.