Flip over an Australian or European snack package and the energy number is in kilojoules — 1,750 kJ for a chocolate bar — while the American version of the same product says 418 Calories. Both describe the same energy; the kilojoule is simply the metric energy unit scaled up by a thousand from the joule. One kilojoule equals 1,000 joules, 0.239 kilocalories of food energy, and 0.2778 watt-hours of electricity. It is the unit of nutrition labels across most of the world and of physics problems everywhere.

Type your value in the Kilojoules (kJ) box and four translations appear: Joules (J) at 1,000 each, Megajoules (MJ) at one-thousandth each, Kilocalories (kcal) at 0.2390057 each, and Watt-hours (Wh) at 0.2777778 each. The 10 kJ example gives 10,000 J, 0.01 MJ, about 2.39 kcal, and 2.78 Wh. That kilocalorie figure is the one dieters need: divide kilojoules by 4.184 to get food Calories, so a 2,000 kJ meal is about 478 Calories. The watt-hour column has a neat physical meaning too — 3.6 kJ runs a 1-watt device for an hour, which is why the factor 0.2778 is exactly 1/3.6. Students converting lab data, travelers decoding foreign labels, and athletes tracking intake in metric all use the same small set of factors.

With the pre-filled defaults — Kilojoules (kJ) 10 — the Kilojoule Converter returns Joules (J) 10,000.0, Megajoules (MJ) 0.0100, Kilocalories (kcal) 2.3901, Watt-hours (Wh) 2.7778. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.