Start with the right line

On a US Nutrition Facts label the line you want is Total Carbohydrate. Dietary fibre, total sugars and added sugars are listed beneath it as parts of that total, so you don’t add them on top. The FDA’s sample lasagna label shows 34 g of total carbohydrate, with 4 g of fibre and 6 g of sugars shown underneath.

On UK and EU labels the line is Carbohydrate, usually followed by “of which sugars”. Under Regulation (EU) No 1169/2011, carbohydrate means any carbohydrate the body metabolises, and that includes polyols. Fibre is defined separately and is an optional extra in the nutrition declaration, so when a label shows it, it appears on its own line.

That means labels from different regions are not strictly like for like: a US Total Carbohydrate figure includes dietary fibre, while the EU defines carbohydrate and fibre separately. Use the carbohydrate line printed on your label, as printed. The calculator and the app use only that line. Neither makes fibre or “net carb” adjustments, and neither does this guide.

Check what the number describes

Before you multiply anything, read the column heading. The same row can describe three different amounts.

Heading says The number is for You scale by
Per 100 g or per 100 ml 100 g or 100 ml of the food your grams or ml ÷ 100
Per serving or per portion the serving size printed on the label number of servings
Per package the whole pack the fraction you eat

The FDA notes that a serving size reflects what people typically eat, not what they should eat, and that some packs carry two columns, one per serving and one per package. If a label has two columns, make sure you’re reading the one you mean. Per 100 g versus per serving goes into the difference in detail.

Work out your portion

Three examples cover most labels. The first uses the FDA’s own sample label. The other two use invented, rounded numbers chosen to make the arithmetic clear. They are not nutrition data for any real product.

A label that gives carbs per serving

The FDA sample label lists one serving as 1 cup (227 g), with 34 g of total carbohydrate. Its two-cup column shows 68 g.

2 servings: 34 × 2 = 68 g

If you weigh 150 g instead of counting cups, scale by the gram figure:

34 × 150 ÷ 227 = 22.47 g

That’s about 22.5 g. The label’s own numbers are rounded, so treat the result as a good estimate, not a measurement.

A label that gives carbs per 100 g

Illustrative example. A packet says 62 g carbohydrate per 100 g. You weigh out 40 g.

62 × 40 ÷ 100 = 24.8 g

A drink labelled per 100 ml

Illustrative example. A bottle says 10.6 g carbohydrate per 100 ml. You pour 250 ml.

10.6 × 250 ÷ 100 = 26.5 g

Use millilitres for a per-100-ml label and grams for a per-100-g label. Don’t convert one into the other.

Slips worth checking

  • Reading the sugars line. Sugars are already part of the total. Use the total.
  • Mixing up the basis. A per-serving figure scaled as if it were per 100 g gives the wrong answer, often by a lot.
  • Weighing the food in a different state from the label. If a label describes dry pasta, use the dry weight. A cooked weight needs a cooked value.
  • Rounding too early. Keep the full result until the last step, then round once.

Let a tool do the arithmetic

The sum is simple, but it’s easy to slip when you’re hungry. The free carb calculator takes the per-100 figure and your portion and shows the working. If you do this often, the Android app scans the barcode or nutrition label for you, so you only type the portion.

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.