Converting energy units
Energy conversions run through the joule. Each unit is a fixed number of joules (a calorie is 4.184 J, a watt-hour 3,600 J, a BTU about 1,055 J), so the value is expressed in joules and divided by the target unit. The same physical quantity, just labelled differently.
Food, power and heating
The kilocalorie — the "Calorie" on nutrition labels — links diet and physics: a 2,000 kcal day is about 8.4 megajoules. Kilowatt-hours meter electricity, and BTUs rate heating and air-conditioning. Converting between them puts everyday numbers on one scale.
Calorie vs. Calorie
A food "Calorie" (capital C) is actually a kilocalorie — 1,000 small calories. The tool keeps them separate so you can convert either without the factor-of-1,000 mistake that trips people up.
Energy units and their joule value
Every energy unit is really a fixed number of joules, so a conversion just turns the value into joules and back out into the target unit — the physical amount never changes, only the label.
| Unit | In joules | Typical use |
|---|---|---|
| Joule (J) | 1 | SI base unit |
| Foot-pound (ft·lb) | 1.356 | Torque, mechanics |
| Calorie (cal) | 4.184 | Chemistry, physics |
| BTU | 1,055 | Heating & air conditioning |
| Kilojoule (kJ) | 1,000 | Food energy (metric) |
| Watt-hour (Wh) | 3,600 | Battery capacity |
| Kilocalorie (kcal) | 4,184 | Food "Calories" |
| Kilowatt-hour (kWh) | 3,600,000 | Electricity bills |
Common energy conversions
| From | Equals |
|---|---|
| 1 kilocalorie (kcal) | 4.184 kJ |
| 1 kilowatt-hour (kWh) | 3.6 MJ (3,600 kJ) |
| 1 BTU | 1.055 kJ |
| 1 watt-hour (Wh) | 3,600 J |
| 1 kilocalorie (kcal) | 3.966 BTU |
Everyday energy on one scale
Numbers help when you can picture them. An AA battery holds about 9 kJ, a phone charge roughly 40 kJ, a full day of food around 2,000 kcal (8.4 MJ), and a kilowatt-hour — the unit on your power bill — is 3.6 MJ. The "In the real world" panel above turns whatever you type into phone charges, minutes of jogging and more, instantly.
Frequently asked questions
How many joules in a kWh?
3,600,000 joules — one kilowatt sustained for one hour.
Is a food Calorie a kcal?
Yes. The "Calorie" on nutrition labels is a kilocalorie, equal to 1,000 small calories or about 4,184 joules.
How many joules are in a calorie?
One calorie (cal) is 4.184 J. The food 'Calorie' is actually a kilocalorie: 1 kcal = 4,184 J.
How much is 1 kWh in joules?
1 kWh is 3,600,000 J (3.6 MJ) — the energy of 1 kW running for one hour.
Is a calorie the same as a kilocalorie?
No — the calorie on food labels is really a kilocalorie (1,000 cal). 1 kcal = 4.184 J.
What is a joule?
The SI unit of energy. One joule is the work done by a force of one newton over one metre — about the energy to lift a small apple one metre.
What's the difference between kW and kWh?
A kilowatt (kW) is a rate of power; a kilowatt-hour (kWh) is an amount of energy — 1 kW used for one hour, or 3.6 MJ. Your bill charges for kWh, not kW.
How many joules are in a BTU?
About 1,055 joules. A BTU is the energy to raise one pound of water by one degree Fahrenheit, used mostly for heating and air conditioning.
How much energy is 1 kcal in everyday terms?
4,184 joules — roughly enough to heat a mug of water by about 4 °C. A day's food, near 2,000 kcal, is about 8.4 MJ.