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To convert joules to thermochemical calories, divide by 4.184. So 100 J = 23.9 cal, 1,000 J = 239 cal, and 4.184 J = 1 cal exactly. Note that the "calorie" in food science is actually a kilocalorie (kcal) — 1 food Calorie (capital C) = 4,184 J, not 4.184 J.
The calorie was historically defined as the heat needed to raise 1 gram of water by 1°C. Different calorie definitions exist (thermochemical, international table, 15°C calorie), differing by less than 0.1%. The thermochemical calorie (4.184 J) is the modern standard used in chemistry and physics.
Did you know? A single chocolate chip cookie contains about 50 food Calories — that's 50 kcal or 209,200 joules. Expressing everyday food energy in joules makes the numbers astronomically large, which is why nutritionists use kilocalories instead.
Chemistry and physics students convert energy between joules and calories for thermodynamics problems. Lab protocols for calorimetry experiments may report heat in calories, requiring conversion to SI joules for publication. Comparing energy content in scientific literature that mixes SI and non-SI units.