Per 100g vs per serving: compare food labels fairly
Put two packets on the same basis, calculate your own portion, and avoid mixing dry and prepared values or grams and millilitres.
On this page
Use per 100g to compare similar solid foods on the same basis. Use the amount you actually expect to eat to understand your portion. A manufacturer's suggested serving may be larger or smaller than yours.
The NHS explanation of food labels describes nutrition information per 100g or 100ml, sometimes with a portion column too. The columns describe the same food at different quantities.
Put two packets on the same basis
Here is an invented example:
| Fictional snack | Sugars per 100g | Suggested portion | Sugars per suggested portion |
|---|---|---|---|
| A | 12g | 20g | 2.4g |
| B | 10g | 30g | 3g |
A appears lower in sugar if you look only at the suggested portions. B contains less sugar per equal weight. Neither observation is a contradiction: the portions are different.
If you plan to eat 30g of either one, A supplies 3.6g sugars and B supplies 3g. You can now make a comparison that matches your intended amount. These numbers are teaching examples, not measured product data.
Calculate a portion
Value per 100g × portion weight in grams ÷ 100 = value in that portion.
For a food with 200 kcal per 100g, a 35g portion contains 200 × 35 ÷ 100 = 70 kcal. The same scaling works for grams of protein, fat, carbohydrate, sugars, fibre or salt.
Our portion calculator handles this arithmetic in your browser. If you only have a serving value, reverse it: value per serving × 100 ÷ serving grams. Round sensibly; nutrition labels are not laboratory measurements of your exact mouthful.
Check the basis before calculating
Dry or prepared? A panel may describe a food as sold or after preparation according to instructions. Added water changes the weight. Added milk, oil or other ingredients can also change nutrients. Read the heading and preparation note before comparing.
Grams or millilitres? Do not silently treat these as interchangeable. For a panel per 100ml, measure the portion in ml and apply the same formula. The calculator uses grams, so it is intended for labels expressed by weight.
One item or the whole pack? A multipack's front figure may describe one item. Check the item weight and number you expect to eat. “One serving” does not necessarily mean “one pack”.
Keep the claim separate from the portion
A high fibre claim is based on specified composition thresholds. A tiny portion can still supply little fibre. A high protein claim uses the share of energy from protein, which stays the same when you scale the portion.
This method helps you read a packet. It does not set a target intake, recommend a portion or judge whether a food fits a personal medical diet. For ingredients, allergens and storage instructions, read the other parts of the label too.
Sources checked 21 September 2026. Examples are fictional; the calculations explain labels and do not certify a product or prescribe a diet.