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Lesson 2 of 7 6–9 min read
Calories & Energy: Understanding Food Energy
Calories appear on almost every food label. This lesson explains what they measure, where they come from, and what they don't tell you.
kcal and kJ
Food energy is measured in kilocalories (kcal) or kilojoules (kJ). When people say “calories” about food, they almost always mean kilocalories. Kilojoules are the metric (SI) unit and are used on labels in many countries, including the UK, the EU and Australia, often alongside kcal.
Conversion
1 kcal ≈ 4.184 kJ. To convert kcal to kJ, multiply by 4.184. To convert kJ to kcal, divide by 4.184.
Where food energy comes from
Most food energy comes from carbohydrate, protein and fat (alcohol also provides energy). Standard conversion factors used for food labelling are:
| Component | Approximate energy |
|---|---|
| Carbohydrate | 4 kcal per gram |
| Protein | 4 kcal per gram |
| Fat | 9 kcal per gram |
| Fibre | 2 kcal per gram |
Fat provides more than twice as much energy per gram as carbohydrate or protein. Note that labelling rules differ between countries — for example, how fibre is counted varies.
Worked example
Simplified example
A food contains 10g protein, 20g carbohydrate and 5g fat.
- Protein: 10g × 4 = 40 kcal
- Carbohydrate: 20g × 4 = 80 kcal
- Fat: 5g × 9 = 45 kcal
- Total: 165 kcal
This is simplified for learning. Real foods may contain fibre, alcohol or other components, and published values may be measured or calculated in different ways.
Reading energy on a label
Labels often show energy per 100g and per portion. Here is a fictional example:
Nutrition information (fictional example)
Per-100g values make it easy to compare products. Per-portion values tell you about the amount the manufacturer suggests — which may differ from what you actually eat.
Scaling to your portion
To work out energy for any portion: (kcal per 100g ÷ 100) × grams eaten. For example, a food with 200 kcal per 100g, eaten in a 150g portion: 200 ÷ 100 × 150 = 300 kcal.
Try it yourself
A food contains 120 kcal per 100g. How many kcal are in a 150g portion?
Use (kcal per 100g ÷ 100) × portion in grams.
How the body uses energy
The body uses energy to keep basic functions running (such as breathing, circulation and maintaining body temperature), to digest food, and for physical activity. How much energy a person needs varies with factors such as age, sex, body size, activity level and health. This course does not provide personal calorie targets — speak to a qualified healthcare professional for individual advice.
Calories aren't the whole picture
A calorie value tells you how much energy a food provides, not its full nutritional composition. Two foods with the same calories can differ greatly in protein, fibre, vitamins, minerals, salt and the types of fat or sugar they contain.
Put it into practice
Look up the energy and macronutrients of everyday foods using USDA FoodData Central data.
Adults who want a general starting estimate, not a personal target, can see how energy needs are estimated in the Macro Calculator for Weight Loss, which shows its working and limits.
Check your understanding
4 questions. Choose an answer for each, then check.
Answer all 4 questions to check your work.
Lesson summary
- Food energy is measured in kcal or kJ; 1 kcal ≈ 4.184 kJ.
- Approximate factors: carbohydrate 4, protein 4, fat 9, fibre 2 kcal/g.
- Scale label values with (kcal per 100g ÷ 100) × grams.
- Energy needs vary between people.
- Calories don't describe a food's full nutritional value.
Further reading & sources
- NHS (UK): Understanding calories (opens in a new tab)
- NHS (UK): Food labels (opens in a new tab)
- European Union: Regulation (EU) No 1169/2011 – food information to consumers (Annex XIV conversion factors) (opens in a new tab)
- FDA (US): How to Understand and Use the Nutrition Facts Label (opens in a new tab)
- USDA: FoodData Central (opens in a new tab)