Calories per serving from per 100 grams
For food listed per 100 g, multiply the number by your portion in grams and divide by 100. The same calculation applies to protein, fat and carbohydrate.
A weighed portion
Suppose a food has 240 kcal per 100 g and you eat 75 g. Your portion has 240 × 75 ÷ 100 = 180 kcal. If it has 12 g of protein per 100 g, the portion has 9 g. These are arithmetic examples, not measurements of a particular product.
Check what the label calls a serving
A US Nutrition Facts label commonly gives values per serving. A package may contain several servings, and sometimes a second column gives values for the whole package. Read that heading before entering a number. The listed serving size describes a reference amount; it is not a personal recommendation for how much to eat.
If the label gives 180 kcal for a 75 g serving, the value per 100 g is 180 × 100 ÷ 75 = 240 kcal. Do not enter 180 as the per-100-g value.
Grams and millilitres are different
Use the same unit as the source. A label per 100 ml can be scaled by millilitres, but converting it to grams requires the food’s density. Do not assume every drink weighs one gram per millilitre. A count such as “one slice” also needs a source weight or your own weighing.
Enter the source, then your weight
In Snapfork, open manual entry or select a food, check the values per 100 g, and enter the weight you actually ate. Changing weight recalculates the portion. A photographed portion is an estimate that you can correct.
Sources and limits
FDA: serving size on the Nutrition Facts label. The calculation above is ordinary proportional arithmetic. It inherits any uncertainty in the label and in your portion weight.
Prepared with AI for Snapfork. Responsible operator: Oleksandr Trifonov. Checked 2 October 2026; report corrections through app support.