Use the per 100g column to compare similar foods on equal terms. Then use the portion information to work out what you would actually eat. Both figures help, but they answer different questions.
That distinction matters when two packets use different serving sizes. The smaller number on the front may simply describe a smaller helping. You do not need to become a nutrition expert to untangle it: check the heading above the numbers before comparing the numbers themselves.
What does “per 100g” mean?
It means the amount of a nutrient in 100 grams of the food. For drinks, the comparison is usually per 100 millilitres. UK nutrition labels provide these standard quantities and may also show values per portion. The NHS points out that the manufacturer's portion may differ from yours (NHS: how to read food labels).
The standard column removes one variable: serving size. Comparing two breakfast cereals per 100g is like checking the price per kilogram. It gives you the same starting point even when the packs differ.
It does not mean you are expected to eat 100g of everything. A spoonful of dressing and a bowl of soup are different quantities, used in different ways.
How a smaller serving can change the comparison
Imagine these two cereals. The figures below are invented to demonstrate the calculation; they are not results for real products.
| Label information | Cereal A | Cereal B |
|---|---|---|
| Sugars per 100g | 8g | 6g |
| Stated serving | 30g | 45g |
| Sugars per stated serving | 2.4g | 2.7g |
Looking only at the last row, A appears to contain less sugar. That is true for those two stated servings. But B contains less sugar for the same weight of cereal.
If you served 40g of each, A would provide 3.2g of sugars and B would provide 2.4g. The order changes because the quantities are now equal.
This comparison tells you about sugar. It does not settle which cereal suits you overall. You might also compare fibre, salt, ingredients, price and whether you enjoy eating it.
Calculate the amount in your portion
Use this calculation:
Amount in your portion = amount per 100g × portion weight ÷ 100.
For another invented example, a soup containing 0.6g of salt per 100g would provide 1.8g of salt in a 300g bowl: 0.6 × 300 ÷ 100.
For a drink, use the volume in millilitres with the per 100ml figure. A hypothetical drink containing 4g of sugars per 100ml would provide 10g in 250ml.
These are calculations from label values, so treat the results as estimates. They are useful for understanding a serving, without suggesting that every spoonful has been measured in a laboratory. If you use the stated portion column, check whether it describes one item, half a pack or the whole pack.
Check whether the food is prepared or unprepared
Equal weights only help when they describe the same state of the food. Read any wording such as “as sold”, “as prepared” or “with milk”. Government guidance allows nutrition information to describe food after preparation when suitable instructions are provided (DHSC technical guidance on nutrition labelling).
For porridge, comparing dry oats with a bowl prepared using milk mixes two different things. Added liquid changes the weight, and milk adds nutrients too. Match dry with dry or prepared with prepared. If a serving includes an added ingredient, check which ingredient and how much the label assumes.
Where traffic lights fit
UK front-of-pack traffic lights offer a quick view of fat, saturates, sugars and salt. Their colours are based on set criteria; the scheme also uses additional red thresholds for larger servings. Do not assume every colour comes solely from the grams printed beside it (DHSC front-of-pack nutrition labelling guidance).
Use the colours as an initial comparison, then read the quantity. They work alongside the nutrition table and ingredient list, each adding a different piece of information.
Put it into practice on your next shop
Choose two products you would use for the same meal. Compare the same nutrient per 100g or 100ml, check the preparation instructions, then consider your portion. That gives you a useful answer without trying to rank every food against every other food.
Cull can help you explore a product's nutrition and processing information after a barcode scan. Open the explanation and check it against the packet. See how Cull works, or read our guide to NOVA classification for the separate question of how a food is processed.