If the claim on the front is a comparison rather than a measurement, you need somewhere reliable to settle the question. It is on the back, in a column most people walk straight past.
Why per serve cannot be trusted for comparing
A serving size is chosen by the manufacturer. There is no independent body deciding that a serve of cereal is 30g rather than 45g, or that a serve of a sauce is one tablespoon rather than two.
Which means the per serve column can be made to look better without changing the food at all. A smaller stated serve makes every number on the panel smaller. The sugar figure drops, the kilojoule figure drops, and nothing inside the box has changed.
It also makes two products almost impossible to compare. If one brand states a 30g serve and another states 45g, their per serve columns are describing different amounts of food. Reading across them tells you nothing useful.
Why per 100g works
Per 100g is fixed. It means the same thing on every pack in the aisle, regardless of what the manufacturer decided about serve sizes, and it is required to be there.
So it is the one column where you are comparing like with like. Same quantity of food, both products, every time.
For liquids the equivalent is per 100mL, and the same logic applies.
The five-second version
Pick up both products. Ignore the front of the pack. Ignore the per serve column. Read across the per 100g column for whatever you actually care about, which might be sugar, or fibre, or protein, or sodium.
That is it. It takes about five seconds once the habit is in, and it is the single most useful thing you can do in a supermarket.
Try it once this week
Choose one product you buy regularly and one alternative sitting near it. Same category, two brands.
Compare them on per 100g and see whether the answer matches what the fronts of the packs led you to expect. Sometimes it does. Often enough it does not, and that is the moment the habit sticks.
If you would like more on reading a panel properly, including what the percentage daily intake figures are actually based on, we went through it in our guide to reading nutrition labels.
When per serve is still useful
It has its place. Per serve is the right column when you want to know what you are actually about to eat, rather than which of two products is better.
Two different questions, two different columns. Comparing, use per 100g. Portioning, use per serve.
If you would like somewhere obvious to practise this, we have written about where the sugar in a weekly shop actually hides, and those are the packs most worth turning over first.
The takeaway: Serving sizes are set by manufacturers, so the per serve column can be made to look better without the food changing. Per 100g is fixed across every product, which makes it the only column where you are comparing like with like. Use per 100g to choose between products, and per serve to understand what you are eating.
The science behind this post:
- Food Standards Australia New Zealand. Nutrition information panels. Panels must express values both per serving and per 100g, or per 100mL for liquids. Serving size is determined by the manufacturer. foodstandards.gov.au
- Australia New Zealand Food Standards Code, Standard 1.2.8: Nutrition information requirements. foodstandards.gov.au
- Food Standards Australia New Zealand. Percentage daily intake labelling is based on an average adult reference diet of 8700kJ. foodstandards.gov.au