How to Read Nutrition Labels — What Actually Matters
A nutrition label is the only honest thing on a food packet — the front of the pack is marketing, the back is data. The problem is that the data is presented in a way that makes comparison hard: per-serving figures with serving sizes nobody eats, traffic-light colours that flatter the product, and ingredient lists ordered to hide the sugar. Learning to read the label takes about ten minutes and then saves you from being misled for the rest of your shopping life.
The headline rule is to compare foods per 100 g, not per serving, because serving sizes are chosen by the manufacturer and vary between brands. Per 100 g, a useful set of cut-offs is: sugars above 22.5 g are high and below 5 g are low; saturated fat above 5 g is high and below 1.5 g is low; salt above 1.5 g is high and below 0.3 g is low; fibre above 6 g is high (UK Food Standards Agency traffic-light criteria). Everything else on the label is detail — those four numbers decide most of the comparison.
Use the calculator
Start with your own numbers: try our meal calorie calculator to work out your figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculators use your real weight, height, age and goal.
Related reading: Carbs Explained, Sugar and Weight Gain, Fibre and Weight Loss. You can also browse the full guides section for the rest of the series.
The key points
Per 100 g is the only fair comparison. A cereal that claims 120 kcal 'per serving' may define a serving as 30 g while you pour 60 g; a 'low fat' yoghurt may define a serving as half a pot. Per 100 g removes the manufacturer's choice from the equation and lets you compare two brands directly. Use per-serving figures only after you have decided the product is worth eating, to work out what your actual portion contains.
The ingredient list is ordered by weight. Ingredients must be listed in descending order of weight, so the first three ingredients tell you what the food mostly is. If sugar, glucose syrup or palm oil appears in the first three, the product is mostly sugar or fat whatever the front of the pack says. Sugar also hides under more than fifty names — sucrose, dextrose, maltose, honey, agave, fruit juice concentrate — and manufacturers split it across several names so none appears first.
Traffic lights help, but they are not the whole story. The UK traffic-light system rates fat, saturated fat, sugars and salt as green (low), amber (medium) or red (high) per 100 g. It is a good quick screen, but it says nothing about protein, fibre or micronutrients: a food can be all green and still be low in protein and fibre, and a food with one red — nuts for fat, for example — can be a genuinely healthy choice. Use traffic lights to compare similar foods, not to judge foods in isolation.
Health claims on the front are not evidence. 'High protein', 'no added sugar', 'natural' and 'light' are regulated terms in some countries and marketing in others, and none of them guarantees a healthy product. A 'high protein' bar can carry 300 kcal and 20 g of sugar; a 'no added sugar' juice can carry the sugar of four oranges. The label on the back is the check on the claim on the front — always verify.
At a glance
| Nutrient | Green (low) | Amber (medium) | Red (high) |
|---|---|---|---|
| Fat | 3 g or less | 3.1–17.5 g | More than 17.5 g |
| Saturated fat | 1.5 g or less | 1.6–5 g | More than 5 g |
| Sugars | 5 g or less | 5.1–22.5 g | More than 22.5 g |
| Salt | 0.3 g or less | 0.31–1.5 g | More than 1.5 g |
| Fibre | — | — | 6 g or more is high |
For drinks the cut-offs are lower (sugars: green 2.5 g, red 11.25 g per 100 ml). A product with mostly green and amber is usually a reasonable everyday choice; a product with several reds is best kept occasional. These cut-offs come from the UK Food Standards Agency.
The formula behind it
There is no single formula for a label, but two calculations cover most of the maths. First, your portion's calories = (calories per 100 g ÷ 100) × portion weight in grams — so a cereal at 380 kcal per 100 g eaten as a 60 g portion is 228 kcal, not the 114 kcal the '30 g serving' suggests. Second, salt (g) = sodium (g) × 2.5, because some labels list sodium rather than salt; 0.6 g of sodium is 1.5 g of salt, which is the red threshold. The traffic-light cut-offs in the table above come from the UK Food Standards Agency and are the standard most shoppers use.
Worked example — comparing two breakfast cereals. Cereal A: 380 kcal, 8 g sugar, 1 g saturated fat, 8 g fibre per 100 g, serving size 30 g. Cereal B: 420 kcal, 24 g sugar, 0.5 g saturated fat, 2 g fibre per 100 g, serving size 30 g. Per serving both claim about 115–125 kcal, which looks similar. Per 100 g the difference is clear: cereal B is red for sugar (24 g) and low in fibre, while cereal A is amber for sugar and high in fibre. A realistic 60 g portion of B carries 14 g of sugar — nearly half the WHO's suggested daily limit for free sugars at 2,000 kcal (25 g) — while A carries under 5 g. Same shelf, same serving claim, very different food.
Your plan, step by step
- Turn the pack over and find the per 100 g column before reading anything on the front.
- Check the four traffic-light numbers — fat, saturated fat, sugars, salt — and compare them with the cut-offs in the table above.
- Read the first three ingredients: if sugar or a fat is among them, the product is mostly that ingredient.
- Check fibre and protein, which traffic lights ignore: aim for at least 3 g of fibre and meaningful protein per serving.
- Work out your real portion with the per-100-g maths, and log that number rather than the manufacturer's serving.
How to use this guide
Use this skill on the five foods you buy most often first — breakfast cereal, bread, yoghurt, snacks and sauces — because those repeat every week and small swaps there compound. Once you know the per-100-g numbers for your regulars, you only need to check new products. Keep the traffic-light cut-offs on your phone for the first month; after that you will recognise a red product at a glance.
Common mistakes
Trusting the serving size. Manufacturers choose serving sizes, and they choose small ones. A 40 g bag of crisps labelled as two servings, or a 30 g cereal serving nobody pours, makes the numbers look half as bad as they are. Per 100 g is the honest comparison.
Judging a food by one red light. Nuts are red for fat and cheese is red for saturated fat, yet both can fit a healthy diet in sensible portions. Traffic lights compare similar foods — breakfast cereals with breakfast cereals, yoghurts with yoghurts — they do not rank all foods on one scale.
Ignoring the ingredient list. A product can be low in sugar per 100 g and still be mostly refined starch, or high in protein and still be mostly sugar. The ingredient list shows what the food actually is; the nutrition table shows how much of each nutrient it contains. You need both.
What this guide does NOT tell you
Labelling rules differ by country: traffic lights are used in the UK, Nutri-Score in parts of Europe, and the US uses a different format with added sugars listed separately. The principles in this guide — per 100 g comparison, ingredient order, and checking the back before believing the front — apply everywhere, but the exact cut-offs and claim rules are local. Labels also carry rounding and permitted tolerances, so figures are close rather than exact, and they say nothing about how a food affects your individual blood sugar or digestion.
Frequently asked questions
What is the most important thing on a nutrition label?
The per 100 g column, and within it the four traffic-light numbers: fat, saturated fat, sugars and salt. Those decide most of the comparison between similar foods. After that, check fibre and protein, which the traffic lights ignore, and the ingredient list, which shows what the food actually is. The front of the pack is marketing; the back is the product.
How do I compare two products with different serving sizes?
Ignore the serving sizes and compare per 100 g. Serving sizes are chosen by the manufacturer and are often unrealistically small. Once you have chosen the better product per 100 g, use the per-100-g maths — (value per 100 g ÷ 100) × your portion in grams — to work out what you actually eat.
What does 'no added sugar' mean?
It means no sugar was added during manufacture, but the product can still be high in naturally occurring sugars — fruit juice, for example, is 'no added sugar' and still carries about 10 g of sugar per 100 ml. Check the total sugars line on the label, not the claim on the front.
Are traffic lights reliable?
They are a good quick screen for comparing similar foods, using standardised cut-offs from the UK Food Standards Agency. Their limits are that they ignore protein, fibre and micronutrients, and that a single red does not make a food unhealthy — nuts are red for fat and are a healthy food. Use them as a guide, not a verdict.
Why do some labels list sodium instead of salt?
Some countries list sodium, the element, rather than salt (sodium chloride). Convert with salt = sodium × 2.5: 0.6 g of sodium is 1.5 g of salt, which is the red threshold per 100 g. Most UK and EU labels now list salt directly.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.