Kilocalories to kilojoules (kcal to kJ)

One kilocalorie is exactly 4.184 kilojoules. The thermochemical calorie was defined as exactly 4.184 joules by convention in 1935, so this is a definition rather than a measurement of how much heat warms water.

Worked through: 500 kcal x 4.184 = 2,092 kJ.

Going the other way, one kilojoule is 0.2390057361 kilocalories.

For mental arithmetic: Multiply by 4.2.

Food labeling, and the capital letter that causes all the trouble. A food 'Calorie' with a capital C is a kilocalorie (a thousand of the physicist's calories), so a 500 Calorie meal is 500 kcal, 2,092 kJ, or two million of the small ones. European labels print kJ first and kcal second; American ones print only Calories. A 2,000 Calorie daily intake is 8,368 kJ. The factor 4.184 is the only number needed and it does not change.

Precision is worth matching to the input. A measurement given as 100 kcal carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.