In short: melting 1 kg of ice already at 0°C takes about 334 kJ (0.093 kWh, ~317 BTU) — that's before any energy to raise its temperature further. Ice colder than 0°C, like ice straight from the freezer, needs extra energy first just to warm it up to the melting point.
These figures are planning aids based on standard thermodynamic data. For refrigeration, HVAC or process design, confirm with a licensed engineer.
How to calculate the energy to melt ice
Melting ice takes a specific, fixed amount of energy per kilogram, called the latent heat of fusion. Unlike heating water, this energy doesn't raise the temperature at all — it goes entirely into breaking the bonds that hold ice in its solid crystal structure. Once you know the mass of ice, the core calculation is a single multiplication:
This calculator converts your amount of ice to kilograms, applies the latent heat of fusion (334 kJ/kg), and — if you set a starting temperature below 0°C or a target temperature for the melted water — adds the extra sensible-heat terms on top. The result is shown in kJ, kWh, BTU, kcal, MJ and Wh.
Worked example. Take 3 kg of ice already at its melting point of 0°C. Multiply: 3 kg × 334 kJ/kg = 1,002 kJ (1.0 MJ), which is about 0.278 kWh or roughly 950 BTU. That single multiplication is the whole calculation — the complexity only shows up once the ice starts out colder than 0°C.
Latent heat vs. sensible heat, in one line. Latent heat changes a substance's state without changing its temperature (ice at 0°C becoming water at 0°C); sensible heat changes temperature without changing state. Melting ice always involves the first; ice from a freezer involves both.
Why starting temperature changes the answer
Ice fresh from a freezer is almost always colder than 0°C — a typical home freezer runs at about -18°C. That ice can't start melting the instant it's exposed to heat: it has to warm up to 0°C first, and every degree of that warming needs its own energy, using the specific heat of ice (about 2.09 kJ/kg·K, roughly half that of liquid water). Only once the ice reaches 0°C does the 334 kJ/kg latent heat of fusion take over and start converting it to liquid, still at 0°C.
The two effects are independent, which is why this calculator treats them as separate terms: one for warming the solid ice, and one for melting it. If you also want to know the energy to warm the resulting water further — say, up to room temperature — the optional field adds a third term using the specific heat of liquid water (4.186 kJ/kg·K). You can read the full method on our methodology page.
Energy per kilogram, by starting temperature
How much energy a single kilogram of ice needs to fully melt, depending on how cold it starts. The "extra" column is sensible heat; the total column adds the fixed 334 kJ/kg of latent heat.
| Starting temperature | Extra sensible heat | Total energy to melt |
|---|---|---|
| 0°C / 32°F (melting point) | 0 kJ/kg | 334.0 kJ/kg |
| -5°C / 23°F | 10.5 kJ/kg | 344.5 kJ/kg |
| -10°C / 14°F | 20.9 kJ/kg | 354.9 kJ/kg |
| -15°C / 5°F | 31.4 kJ/kg | 365.4 kJ/kg |
| -18°C / 0°F (home freezer) | 37.6 kJ/kg | 371.6 kJ/kg |
| -20°C / -4°F | 41.8 kJ/kg | 375.8 kJ/kg |
| -25°C / -13°F | 52.3 kJ/kg | 386.3 kJ/kg |
| -30°C / -22°F (deep freeze) | 62.7 kJ/kg | 396.7 kJ/kg |
Quick reference: energy to melt common amounts of ice
Ice already at 0°C, no further warming of the melted water. Handy when you just need a number without opening the calculator.
| Amount of ice | Energy (kJ) | Energy (kWh) |
|---|---|---|
| 1 ice cube (30 g) | 10 | 0.0028 |
| 1 ice tray (500 g) | 167 | 0.0464 |
| 1 lb | 151.5 | 0.0421 |
| 1 kg | 334 | 0.0928 |
| 5 lb bag | 757.5 | 0.2104 |
| 10 kg block | 3,340 | 0.9278 |
| 1 US ton | 303,000 | 84.2 |
Worked examples
One ice cube, straight from the tray
10.0 kJ
A typical 30 g ice cube at 0°C takes about 10 kJ (0.0028 kWh) to melt completely.
A 5 lb bag from the freezer
842.8 kJ
Starting at -18°C, a 5 lb bag needs about 85.3 kJ just to warm to 0°C, then another 757.5 kJ to melt — 842.8 kJ (0.234 kWh) in total.
A 40 lb cooler bag of ice
6,060 kJ
Already at 0°C, a 40 lb bag needs about 6,060 kJ (1.683 kWh) to fully melt — roughly what a space heater uses in an hour.
1 kg, melted and warmed to room temperature
417.7 kJ
Melting 1 kg of ice at 0°C takes 334 kJ; warming the resulting water to 20°C adds another 83.7 kJ — 417.7 kJ (0.116 kWh) total.
What else affects the result
Impurities and dissolved salt. Salt lowers the freezing point of water and can slightly change the effective latent heat, but for drinking-water-quality ice the effect is negligible. This calculator models pure water ice.
Air pockets. Packed snow, crushed or "cloudy" ice can trap air, making a given volume weigh less than solid ice — but the energy still scales with the actual mass of water ice present, which is what this calculator asks for directly.
Heat losses. In the real world, some heat is lost to or gained from the surroundings before it reaches the ice. This calculator gives the theoretical energy required at the ice itself — actual appliance or process energy use will usually be higher due to inefficiencies. See our specific heat calculator for the water-only heating case, or the energy to heat water calculator for warming liquid water without any melting involved.
Frequently asked questions
How much energy does it take to melt 1 kg of ice?
About 334 kJ (0.093 kWh, or roughly 317 BTU) for ice that is already at 0°C. Colder ice needs extra energy first to warm it to the melting point before it can start melting.
Does colder ice take more energy to melt?
Yes. Ice must first warm to 0°C before it can melt, and that step is separate from the melting itself. Ice at -18°C, a typical home freezer, needs roughly 38 kJ per kilogram of extra sensible heat before the 334 kJ/kg of latent heat can even begin.
How much energy does it take to melt an ice cube?
A typical ice cube, about 30 grams, takes around 10 kJ (0.0028 kWh) to melt if it starts at 0°C. An ice cube straight from the freezer needs a little more, because it has to warm up first.
Is melting ice the same as heating water?
No. Melting is a phase change and uses latent heat, while heating water afterward uses sensible heat. Ice at 0°C absorbs 334 kJ/kg to become water at 0°C without changing temperature at all; only once it's liquid does adding more heat raise its temperature.
How is this calculator's accuracy verified?
The calculator uses the standard latent heat of fusion for water, 334 kJ/kg, and specific heats for ice and liquid water from NIST and CRC reference data. Unit conversions use exact internationally defined factors. Full detail is on our methodology page.
Sources & references
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems — enthalpy of fusion of water and ice.
- CRC Handbook of Chemistry and Physics — specific heat of ice and liquid water.
- International Bureau of Weights and Measures (BIPM). The International System of Units (SI).
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)