Two columns, two questions

A per-100 column answers “how much carbohydrate is in a fixed amount of this food?” A per-serving column answers “how much is in the amount the manufacturer decided to call a serving?”

Per 100 g or 100 ml Per serving
Who picks the amount A convention The manufacturer
Best for Scaling to any weight, comparing products Eating exactly that amount
To get your portion × your amount ÷ 100 × your amount ÷ serving size

Where each convention comes from

Labels follow local rules, so the layout you see depends on where the product was sold.

  • UK and EU. Regulation (EU) No 1169/2011 requires the nutrition declaration to be expressed per 100 g or per 100 ml (Article 32). A per-portion figure may be added when the portion is quantified on the label and the number of portions in the pack is stated (Article 33). So a per-100 column is normally there and a per-portion column is sometimes added, with a few narrow exceptions.
  • United States. The FDA bases the label on a serving, shown as a household measure such as a cup or slice followed by the metric amount in grams. The serving size reflects the amount people typically eat, not a recommendation. Some larger packs also have a second column for the whole package.
  • Elsewhere. Formats vary. Read the column headings and the stated serving size rather than assuming.

Converting between them

To turn a per-serving figure into a per-100 figure, divide by the serving weight and multiply by 100.

Illustrative example. A 125 g pot lists 20 g carbohydrate per serving, and the serving is the whole pot.

20 × 100 ÷ 125 = 16 g per 100 g

The FDA’s sample lasagna label lists 34 g of carbohydrate for a serving of 1 cup (227 g):

34 × 100 ÷ 227 = 14.98 g per 100 g

That is roughly 15.0 g. Because label values are rounded, a figure derived this way is an estimate. If the label also prints a per-100 value, use that one.

Going the other way, multiply the per-100 figure by the serving weight and divide by 100.

Illustrative example. A cereal lists 48 g carbohydrate per 100 g, and the label’s serving is 25 g.

48 × 25 ÷ 100 = 12 g per serving

Drinks work the same way, in millilitres.

Illustrative example. A can lists 8.4 g carbohydrate per 100 ml, and the can holds 330 ml.

8.4 × 330 ÷ 100 = 27.72 g per can

Using a per-serving number for a different amount

Say that cereal gives 12 g per 25 g serving and you pour 40 g. Scale by the serving, not by 100.

Method Working Carbs
Correct: scale by the serving 12 × 40 ÷ 25 19.2 g
Mistake: treat 12 as per 100 g 12 × 40 ÷ 100 4.8 g

The mistake gives a result about a quarter of the true one. The error is largest when the serving is small.

Illustrative example

Scale by what the number describes

This cereal says 12 g carbohydrate per 25 g serving. Change the portion to see why the denominator matters.

Correct: 12 × portion ÷ 2519.2 g
Mistake: 12 × portion ÷ 1004.8 g

The per-serving calculator mode uses the serving size as its denominator. Grams stay grams; a per-100 ml label needs millilitres.

Mistakes to watch for

  • Counting servings loosely. A pack might hold 2.5 servings. If a serving has 18 g, the whole pack is 18 × 2.5 = 45 g.
  • Scaling a cup figure by grams. US labels give both a household measure and grams. If you weigh your food, use the gram amount as the serving size.
  • Mixing grams and millilitres. A per-100 ml figure goes with millilitres. Don’t convert one unit to the other.
  • Reading the wrong column. Check whether a column is per serving or per package before you multiply.
  • Trusting derived numbers too much. Anything you convert from a rounded label value is approximate.

If you’d rather not do the conversion by hand, the carb calculator has a per 100 g or per 100 ml switch and shows the working. For the step before this one, finding the right line on the label, see how to calculate carbs from a food label.

Sources

This guide explains arithmetic. It isn't medical advice and doesn't cover insulin or treatment decisions. Packaging is the best source for a specific product, because recipes and labels change.