Heat Energy Calculator (Q = mcΔT)

Heat energy and specific heat

The energy needed to change a material's temperature depends on three things: how much of it there is, what it's made of, and how big a temperature change you want. That's captured in one equation.

Q = m · c · ΔT

Q is the heat energy in joules, m the mass in kilograms, ΔT the temperature change in kelvin (which equals a change in °C), and c thespecific heat capacity — the energy to raise 1 kg of the material by 1 K. Water's is famously high (4186 J/kg·K), which is why it takes so long to boil and why it's used to store and move heat. Rearrange the equation to solve for any of the four values; this calculator does that for you.

Worked example: heating a kettle

Heating 1 kg of water (1 litre) from 20 °C to 100 °C is a ΔT of 80 K, so Q = 1 × 4186 × 80 = 334 880 J, about 0.093 kWh. A 2000 W kettle delivering that energy takes at least 334 880 / 2000 ≈ 167 seconds, a little under three minutes — real kettles take longer because of heat lost to the surroundings.

From energy to time and power

Power is energy per unit time, so once you know Q, the time to supply it at a steady power P is t = Q / P, and the power needed to do it in a set time is P = Q / t. Enter a power above and the calculator adds the heating time. Remember this is the ideal figure — no real heater is 100% efficient, and some heat always escapes, so allow a margin.

A note on limits

Q = mcΔT covers warming or cooling within a single state. It doesnot include the extra energy absorbed while something melts or boils (the latent heat of a phase change), and specific heat itself varies a little with temperature — the table gives typical near-room-temperature values.