What is a calorie? Why 1g of fat "weighs" twice as much as 1g of protein or carbs

A calorie is heat, quite literally

A calorie is a unit of energy, rooted in measuring heat.

An easy definition to remember: 1 kcal is the energy needed to warm 1 liter of water by 1°C.

On food labels, "calorie" almost always means kcal (kilocalorie). One kcal equals 1,000 small calories. People shorten it for convenience, so "a 200-calorie bowl of rice" is really 200 kcal.

Many labels also show kJ (kilojoules), the energy unit of the international system. Conversion: 1 kcal is about 4.2 kJ. The two numbers on the label say the same thing, just in different units.

Measuring calories by burning food

The instrument is called a bomb calorimeter. It sounds scary, but the principle is very simple.

1. Put a dried food sample into a sealed steel chamber filled with oxygen.
2. Burn the sample completely.
3. The steel chamber sits in water. Measure how many degrees the water warms up.

The more the water warms, the more energy the food holds.

There is a catch, though. Fire can burn everything, but your body cannot. Fiber burns beautifully in the steel chamber, yet your small intestine does not digest it. Part of the protein also leaves your body in urine.

In the late 19th century, the American scientist Wilbur Atwater corrected for this. He measured the part the body does not use, then subtracted it. The result is the set of numbers food labels still use today.

The 4, 9, 4 set (plus a 7)

These are the Atwater factors, per gram:

Fat is more than twice protein and carbs. Picture two tablespoons in your kitchen:

• One tablespoon of cooking oil (about 13g): roughly 120 kcal.
• One tablespoon of sugar (about 12g): roughly 50 kcal.

Same spoon, but the oil "weighs" twice the calories. That is why deep-fried dishes usually carry more calories than boiled ones, even with the exact same main ingredient.

Alcohol deserves a mention too: 7 kcal per gram, close to fat. So a can of beer is not "just water".

Fiber gives only about 2 kcal, because most of its energy is used by your gut bacteria. Some sweeteners, such as erythritol, are counted as close to 0 under European labeling rules.

Add up a label's calories: try it once for fun

Take any label. Multiply grams of protein by 4, grams of carbs by 4, grams of fat by 9, then add them up.

Say a tub of yogurt has 4g protein, 12g carbs and 3g fat per 100g:

• Protein: 4 × 4 = 16 kcal
• Carbs: 12 × 4 = 48 kcal
• Fat: 3 × 9 = 27 kcal
• Total: about 91 kcal

If the label says 90 or 92 kcal, it matches. Sometimes it is a bit further off. The usual reasons:

• Rounding: each line was rounded before printing.
• Fiber and low-calorie sweeteners: they sit inside the carb line but give less than 4 kcal per gram.
• The manufacturer's method: some use specific factors for each type of food.

This sum helps you see where a food's calories "come from". A 200 kcal food with 150 kcal from fat is very different from a 200 kcal food with most of its energy from protein.

Calories measure energy, not quality

200 kcal could be a medium bowl of white rice. It could also be one and a half cans of sugary soda.

Same number, completely different experience. A bowl of rice with fish and greens keeps you full for a long time. Soda slips down fast, and before long you feel hungry again.

Protein and fiber usually make a meal more filling. Sugary drinks barely fill you up at all, even though their calories are not small.

So calories are a useful number, but only one number. They tell you "how much energy". They do not tell you whether a food has vitamins, fiber or protein.

To learn how much energy your body needs each day, read what are BMR and TDEE.

Frequently asked questions