How to compare food labels with different serving sizes
Work through a cereal example, convert both labels to 100 g, then calculate the amount in your own bowl.
A cereal with 6 g of sugar per serving can have a higher sugar concentration than one with 8 g. The missing detail is the serving weight. Compare equal weights first, then calculate the amount you actually plan to eat.
Copy the denominator before the nutrient
The US serving size describes an amount typically consumed; it is not a prescribed portion for you. Nutrient quantities generally refer to that serving. FDA serving-size guidance. EU nutrition declarations use a 100 g or 100 ml basis, with portion information sometimes added. European Commission nutrition-labelling guidance.
For each package, write down three things: the nutrient name, its amount, and the weight that amount belongs to. Keep the unit attached to every number. In this worked example, both products are dry cereal and the nutrient is total sugars. The numbers are invented for arithmetic practice; they are not a review of any brand.
A smaller number can hide a larger concentration
| Example cereal | Serving weight | Sugar per serving | Sugar per 100 g |
|---|---|---|---|
| A | 30 g | 6 g | 20 g |
| B | 50 g | 8 g | 16 g |
Use this calculation: nutrient per 100 g = nutrient per serving ÷ serving weight × 100. For A, 6 ÷ 30 × 100 = 20. For B, 8 ÷ 50 × 100 = 16. B contains 4 g less sugar per 100 g in this example. That comparison answers a concentration question, not a complete nutrition question.
Put the comparison back into your bowl
If you weigh out 45 g of either cereal, A supplies 20 × 45 ÷ 100 = 9 g of sugar and B supplies 16 × 45 ÷ 100 = 7.2 g. If you add milk, fruit, or toppings, calculate those separately. The dry-cereal figures do not describe the whole breakfast.
You might eat different portions of the two products. In that case, keep two views: an equal-weight comparison for the products and an actual-portion calculation for your meal. Do not replace one with the other.
Three checks before saving the result
- Compare total sugars with total sugars, not with added sugars.
- Use grams of food for both denominators. Do not substitute a millilitre measurement without a supported conversion.
- Check whether each panel refers to dry food, prepared food, or a serving with other ingredients added.
Label values have limited precision, so keep sensible decimal places. A calculated 7.2 g is useful for comparison; displaying many extra digits would imply more certainty than the inputs provide. Use the comparison tool for the calculation, then return to the package for the rest of the decision.