About

Energy is universal, but the labels we use to measure it are not. In the United States, food packaging predominantly uses Calories (which are technically kilocalories, kcal). In Europe and scientific contexts, the standard unit is the Kilojoule (kJ). This discrepancy often confuses consumers trying to track intake or students converting physics problems involving heat and work.

This tool performs the precise conversion between these units defined by the thermochemical calorie definition. It is suitable for both nutritional planning (reading foreign food labels) and thermodynamics homework. The conversion is based on the specific heat capacity of water.

Formulas

The definitions of the Calorie are based on the heating of water. The standard "thermochemical calorie" is defined exactly in Joules.

1 kcal ≡ 4.184 kJ

Energy_kJ = Energy_kcal × 4.184

Conversely, to get kcal from kJ:

Energy_kcal = Energy_kJ ÷ 4.184

Reference Data

Item / Activity	Energy (kcal)	Energy (kJ)
Apple (Medium)	95	397
Egg (Large)	78	326
Rice (1 cup, cooked)	205	858
Big Mac Burger	563	2,355
Running (30 mins)	~300	~1,255
Cycling (1 hour)	~500	~2,092
Daily Intake (Men)	2,500	10,460
Daily Intake (Women)	2,000	8,368

Frequently Asked Questions

Yes. In nutrition, the word "Calorie" (capital C) is shorthand for Kilocalorie (1,000 small calories). So, a 2,000 Calorie diet is actually 2,000 kcal or 2,000,000 small calories.

The Kilojoule is the SI (International System) unit of energy. Most of the world (including Australia, New Zealand, and the UK) requires food labels to state energy in kJ, although kcal is often listed alongside it.

It is a quick estimation for macronutrients: Protein and Carbs provide ~4 kcal/g (~17 kJ/g), while Fat provides ~9 kcal/g (~37 kJ/g).