Water Heating Calculator — Energy & Time to Heat Water
Find how much energy is needed to heat water from one temperature to another, and how long your kettle, boiler or immersion heater will take, using the specific-heat equation q = m × c × ΔT.
kg
°C
°C
W
q = m × c × ΔT (c = 4 186 J/kg·°C for water)
- 1
Temperature change (ΔT)
100 − 20 = 80ΔT = Tf − Ti; negative means heat is removed. - 2
Heat energy (q = m × c × ΔT)
1 × 4 186 × 80 = 334,880 - 3
Heating time (t = q ÷ P)
334,880 ÷ 2,000 = 167.4
How does this calculator work?
Energy to heat water: q = m × 4 186 × ΔT joules (m in kg, ΔT in °C). Divide by 3 600 000 for kWh. Divide energy by heater power (W) for heating time in seconds. Boiling 1 L from 20 °C needs about 335 kJ; a 2 kW kettle takes roughly 2.8 minutes. Phase changes are not included.
Formula
How this is calculated
The energy required to heat a mass of liquid water is given by q = m × c × ΔT, where m is the mass in kilograms, c is the specific heat capacity of liquid water (4 186 J per kilogram per degree Celsius, effectively constant between 0 °C and 100 °C), and ΔT is the temperature rise in degrees Celsius (identical in magnitude to a rise in kelvin). Because the density of water is close to 1 kg/L at normal temperatures, you can enter the volume in litres directly as the mass in kilograms without significant error.
A negative ΔT (final below initial) gives a negative q, meaning heat is removed from the water rather than added. If you supply the heater's rated power in watts (joules per second), the calculator also outputs the theoretical heating time as time = q ÷ P. Real heating always takes longer due to heat losses to the container and the surrounding air; an open pot on a gas hob may reach only 50–70% thermal efficiency, while a sealed electric kettle typically exceeds 90–95%.
Results are given in joules (J), kilojoules (kJ), kilowatt-hours (kWh) and kilocalories (kcal). The specific heat 4 186 J/(kg·°C) applies to liquid water only; ice uses approximately 2 090 J/(kg·°C) and steam approximately 2 010 J/(kg·°C). Phase-change energy (melting or vaporisation) is not included — use a separate latent-heat calculation when heating through a phase boundary.
Frequently asked questions
From 20 °C to 100 °C is a ΔT of 80 °C. Using q = 1 kg × 4 186 × 80 = 334 880 J ≈ 335 kJ ≈ 0.093 kWh. A 2 000 W electric kettle would take about 167 seconds (roughly 2.8 minutes) under ideal conditions.
No. q = mcΔT covers only sensible heat — heating liquid water within its liquid phase. Converting liquid water to steam at 100 °C requires an additional 2 260 kJ per kilogram (latent heat of vaporisation). Use a latent-heat calculator if you need to account for boiling-off.
Heat losses to the environment — through the vessel walls, the air above the water, and steam evaporation — reduce effective efficiency. Multiply the ideal time by the inverse of your heater's efficiency (e.g. divide by 0.7 for 70% efficiency) to get a better real-world estimate.
Also known as
TG we-Calculate Editorial Team. (2026). Water Heating Calculator — Energy & Time to Heat Water [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/water-heating-calculator
TG we-Calculate Editorial Team. "Water Heating Calculator — Energy & Time to Heat Water." TG we-Calculate. 2026. https://we-calculate.com/calculator/water-heating-calculator.
TG we-Calculate Editorial Team, "Water Heating Calculator — Energy & Time to Heat Water," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/water-heating-calculator
@misc{wecalculate_water_heating_calculator, title = {Water Heating Calculator — Energy & Time to Heat Water}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/water-heating-calculator}}, year = {2026}, note = {TG we-Calculate} }
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