Grams to Calories

This calculator turns the weight of a portion into calories (kcal), as long as you know how many calories the food has per 100 grams.

How the calculation works

The math is a straight proportion: kcal = grams × (kcal per 100 g) ÷ 100. Labels in the EU, UK and many other countries list energy per 100 g by law, so you only need to weigh your portion and copy the figure from the pack. US Nutrition Facts panels are per serving instead: divide the calories by the serving weight in grams and multiply by 100 to get the per-100 g value.

"Grams" is the weight of what you will actually eat, in the same state the calorie figure refers to — dry pasta and cooked pasta differ hugely. "kcal/100g" is the energy value from the label or a food database such as USDA FoodData Central. If only kilojoules are given, divide by 4.184.

Label calories are normally derived with Atwater factors: 4 kcal per gram of protein and carbohydrate, 9 per gram of fat, 7 per gram of alcohol. The calculator does not apply those factors itself; it scales a ready-made energy value to your portion size.

Worked example

The defaults are 100 g of a food with 250 kcal per 100 g: 100 × 250 ÷ 100 = 250 kcal. A 180 g bowl of cooked white rice at about 130 kcal/100 g works out to 180 × 130 ÷ 100 = 234 kcal.

Things to keep in mind

• Weigh food in the same form as the calorie figure. Pasta and rice absorb water when cooked and end up roughly two to three times heavier, while calories per 100 g drop accordingly.
• Count oil, butter, sauces and sugar added during cooking as separate entries; they often add more than the main ingredient.
• Database values are averages. Meat varies with fat content and fruit with variety and ripeness, so a specific product's label is more accurate.
• If you follow a medical diet (diabetes, kidney disease and so on), use calorie counting alongside your doctor's or dietitian's advice, not instead of it.