One kilocalorie is exactly 4.184 kilojoules. This is the conversion on the back of every food package in Europe, where both figures are printed side by side because the law requires kilojoules and habit requires calories.
About the units
The kilocalorie is the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius. It is also, confusingly, what nutrition labelling means by "calorie": the food Calorie with a capital C is a kilocalorie, a thousand of the small calories used in chemistry. A chocolate bar listed at 250 calories contains 250 kilocalories, which is 250,000 physics calories. The ambiguity dates to the nineteenth century and has never been resolved, only papered over by context. The kilojoule is the SI-derived alternative, and EU food labelling regulation requires it to be shown, with kilocalories permitted alongside.
The exact factor
The exact value is 4.184 kJ per kcal, which defines the thermochemical calorie. Other calorie definitions exist — the 15 °C calorie at 4.1855 J and the International Steam Table calorie at 4.1868 J — and they differ by about a tenth of a per cent, which is far below the accuracy of any nutritional measurement. Reference points: a typical daily intake of 2,000 kcal is 8,368 kJ; a banana at 105 kcal is 439 kJ; and a gram of fat at 9 kcal is 37.7 kJ, against 4 kcal or 16.7 kJ for a gram of protein or carbohydrate.
Where you meet this conversion
Nutrition labelling throughout Europe, where both units appear on the same panel, and in Australia and New Zealand, where kilojoules dominate and calories are secondary. Fitness trackers, dietary apps and exercise machines report in one or the other depending on their market of origin, so comparing a Swedish food label to an American calorie counter requires the conversion. Clinical nutrition and metabolic research use kilojoules in publications and kilocalories in patient-facing material.