If you want the fairest comparison between two packaged foods, start with the figures per 100g or per 100ml. That puts both products on the same basis, regardless of whether one brand calls a portion 30g and another calls it 45g. Portion information can still help, but only after you have checked that you are comparing like with like.

That is the core of how to compare nutrition labels in a UK shop. Read the front and back together, standardise the numbers first, then bring serving size and real-life eating habits back into the decision. Used in that order, labels are much less likely to mislead you.

When to use per 100g and when portion sizes help

On UK food labels, the nutrition panel usually gives values per 100g or per 100ml and may also give values per portion. The per 100g or per 100ml figures are the most reliable starting point because they are standardised. They let you compare two yoghurts, two cereals, or two ready meals even if the packs are different sizes and the suggested portions are not the same.

This matters because portion sizes are set by the manufacturer. They can be useful, but they are not a neutral unit of comparison. One packet of crisps may present a portion as half the bag, while another treats the whole bag as one portion. One breakfast cereal may use a 30g serving, another 40g. If you compare only the per portion line, you can easily conclude that one product is lower in sugar, salt, fat or energy when the serving sizes are simply smaller.

Per 100g or per 100ml is therefore the fairest basis when:

  • you are choosing between two brands in the same category
  • pack sizes differ
  • serving sizes differ
  • one label shows traffic lights per portion and another per 100g
  • you want to check whether a claim such as “lighter” or “less sugar” actually shows up in the numbers

Portion sizes become useful once you have done that standard comparison. They help answer a different question, not “which food is intrinsically lower per equal amount?” but “what am I likely to consume in one sitting?”

That is often the more practical question for foods eaten as a single unit, such as:

  • a ready meal
  • a cereal bar
  • a single yoghurt pot
  • a can of soup that you normally eat all at once
  • a snack pack that is clearly intended for one person

For drinks, the same principle applies, but use per 100ml as the standard unit.

In the UK, this standardised presentation is part of food information rules retained from EU law. The detailed legal framework still requires mandatory nutrition declaration per 100g or per 100ml, while additional per portion information is voluntary provided it meets the rules. The official UK legislation is the Food Information Regulations and retained Regulation (EU) No 1169/2011 on the provision of food information to consumers, as published on legislation.gov.uk: UK food information rules for nutrition labelling

For consumer-facing guidance, the NHS also advises using the nutrition label to compare products and explains the traffic light system: NHS guide to food labels

A simple label-reading order that avoids false comparisons

A lot of bad comparisons happen because people jump straight to one number, usually sugar or calories, without checking what is actually being measured. We use a simple order that reduces those mistakes.

1. Check the category first

Compare like with like. A plain yoghurt and a sweetened dessert pot are not the same type of food. A peanut butter spread and a low-fat soft cheese spread serve different purposes. Even within one aisle, products may not be direct substitutes.

Ask first, “Are these genuinely the same sort of thing?”

2. Check the unit

Look for whether the table is giving values per 100g, per 100ml, per portion, or all three. If you are comparing solids, use per 100g. If you are comparing drinks, use per 100ml.

If one product only highlights per portion on the front, turn to the back and find the per 100g or per 100ml line.

3. Check the serving size

Only once you have found the standard unit should you look at the manufacturer’s portion size. Note the gram or millilitre amount, not just “1 serving”. The weight matters.

For example, “1 serving” could mean 25g in one cereal and 50g in another. Without the gram amount, the comparison is not meaningful.

4. Check the traffic lights

Front-of-pack traffic lights can be a useful shortcut in the UK, especially for fat, saturated fat, sugars and salt. But they are a shortcut, not the whole analysis.

They may be shown per 100g or per portion depending on the food and packaging. If you rely on the colour alone without checking the basis, you can still end up comparing unlike quantities.

The NHS explains the traffic light approach and what red, amber and green mean in practice: how NHS traffic light labelling works

5. Check the ingredients list

The nutrition panel tells you how much of a nutrient is present. The ingredients list helps explain why.

Ingredients are listed in descending order by weight at the time of manufacture. If sugar, glucose syrup, honey, fruit juice concentrate or similar ingredients appear high up the list, that gives useful context, especially where two products have similar nutrition figures.

This is also where you can spot whether sweetness comes mainly from fruit, milk, added sugars, or a mix.

6. Check the pack weight

Before judging the product, check how much is in the pack and whether the stated portion matches how people actually eat it.

A “2 servings” soup that many people would finish alone needs to be interpreted differently from a family pasta sauce that really is used across several meals.

If you want a fuller walkthrough of the process, our guide to [comparing food labels step by step]( /compare-food-labels ) covers the order in more detail.

Worked example with two clearly fictional packets

Here is a fictional UK shopping example using two cereal packets. The numbers are invented purely to show the method.

Packet A, toasted oat clusters with dried fruit

  • Pack weight: 450g
  • Suggested serving: 45g
  • Nutrition per 100g:
  • Energy 420 kcal
  • Fat 10g
  • Saturates 1.5g
  • Sugars 18g
  • Salt 0.30g
  • Nutrition per 45g serving:
  • Energy 189 kcal
  • Fat 4.5g
  • Saturates 0.7g
  • Sugars 8.1g
  • Salt 0.14g

Packet B, crisp wheat flakes with fruit pieces

  • Pack weight: 500g
  • Suggested serving: 30g
  • Nutrition per 100g:
  • Energy 390 kcal
  • Fat 3g
  • Saturates 0.5g
  • Sugars 15g
  • Salt 0.60g
  • Nutrition per 30g serving:
  • Energy 117 kcal
  • Fat 0.9g
  • Saturates 0.2g
  • Sugars 4.5g
  • Salt 0.18g

If we compare per portion only, Packet B looks lower in everything important. But that is partly because the serving is much smaller, 30g rather than 45g.

So first we standardise.

Step 1. Compare per 100g

Per 100g, Packet B is:

  • lower in energy, 390 kcal versus 420 kcal
  • lower in fat, 3g versus 10g
  • lower in saturates, 0.5g versus 1.5g
  • lower in sugars, 15g versus 18g
  • higher in salt, 0.60g versus 0.30g

That already gives a fairer picture. Packet B is not simply “better”. It is lower in sugar and fat, but higher in salt.

Step 2. Convert to the same portion size

Now suppose you usually eat a 45g bowl, regardless of brand. To compare real intake, convert Packet B to 45g.

Packet B per 100g values multiplied by 0.45:

  • Energy: 390 × 0.45 = 175.5 kcal
  • Fat: 3 × 0.45 = 1.35g
  • Saturates: 0.5 × 0.45 = 0.225g
  • Sugars: 15 × 0.45 = 6.75g
  • Salt: 0.60 × 0.45 = 0.27g

Now compare both at 45g:

Packet A at 45g

  • Energy 189 kcal
  • Fat 4.5g
  • Saturates 0.7g
  • Sugars 8.1g
  • Salt 0.14g

Packet B at 45g equivalent

  • Energy 175.5 kcal
  • Fat 1.35g
  • Saturates 0.225g
  • Sugars 6.75g
  • Salt 0.27g

On a like-for-like 45g basis, Packet B still comes out lower in energy, fat, saturates and sugars, but higher in salt.

Step 3. Bring actual eating habits back in

Now ask what you will really eat.

If you know you pour generous bowls and usually eat around 60g, convert both to 60g instead.

Packet A at 60g:

  • Energy: 420 × 0.60 = 252 kcal
  • Sugars: 18 × 0.60 = 10.8g
  • Salt: 0.30 × 0.60 = 0.18g

Packet B at 60g:

  • Energy: 390 × 0.60 = 234 kcal
  • Sugars: 15 × 0.60 = 9g
  • Salt: 0.60 × 0.60 = 0.36g

Again, same pattern. Packet B is lower in energy and sugars, but its salt is double Packet A’s at the same eaten amount.

That is why the right order matters. Standardise first, then compare the amount you are likely to consume. If you skip the first step, smaller stated portions can create a false impression.

Why total sugars and free sugars are not the same thing

One of the most common label misunderstandings is assuming the “sugars” line tells you how much free sugar a food contains. It does not.

On a UK nutrition label, the “of which sugars” figure within carbohydrate includes all sugars present in the food, whether they occur naturally or are added. That means the total can include:

  • sugars naturally present in milk, such as lactose
  • sugars naturally present in fruit and vegetables
  • sugars from ingredients added during manufacture, such as table sugar, syrups or honey
  • sugars from fruit juice concentrate

So if a plain yoghurt contains sugars from milk, those sugars appear on the label. If a fruit yoghurt contains milk sugars plus fruit sugars plus added sugar, all of those are included too. The single sugars figure does not separate them.

That is why you cannot identify free sugars from the nutrition panel alone.

For interpretation in the UK, NHS guidance matters here. The NHS explains that free sugars are sugars added to food or drinks, and sugars naturally present in honey, syrups, and unsweetened fruit juices and smoothies. Sugars naturally found in milk, fruit and vegetables are not classed as free sugars.

The NHS guidance on sugar is here: NHS explanation of free sugars and daily guidance

In practice, when you are looking at a label:

  • use the sugars line to compare similar products on a like-for-like basis
  • do not assume a higher sugars figure automatically means more free sugars
  • read the ingredients list to see whether sugars, syrups, honey, juice concentrate or similar sweetening ingredients have been added
  • remember that foods such as milk and plain yoghurt can show sugars without containing added sugar

A simple example makes this clearer. A plain yoghurt may show several grams of sugar per 100g because lactose is naturally present. A fruit-flavoured yoghurt with the same total sugars figure might include added sugar, fruit puree, or juice concentrate. The number alone does not tell you which sugars are free sugars. The ingredients list provides the missing context.

How to make a practical choice in the aisle

Once you have standardised the numbers and checked the ingredients, you still need to decide what matters most for that purchase.

Start with the eating context.

Is it a meal or a snack?

For a meal, energy, protein, fibre, salt and total portion size may all matter. A ready meal that is lower in calories but very small may leave you hungry and lead to extra snacking later. A soup that looks modest per 100g can still add up if the whole tub is 600g and you eat it all.

For a snack, the full pack amount often matters more than the manufacturer’s serving suggestion. If a snack pack is small and eaten in one go, the per pack figures may be more useful after you have checked the per 100g comparison.

How much will you actually eat?

Be honest about your usual amount. If you never split the sharing bag, assess the whole bag. If you always add a large pour of granola, compare the amount you really use, not the neat serving on the side panel.

Which nutrient is the priority for this choice?

There is rarely a single “best” option across every measure. One product may be lower in sugars but higher in salt. Another may be lower in fat but higher in energy. The right choice depends on the role of the food in your wider diet.

That is why we prefer a calm comparison rather than a good-food bad-food verdict.

Use labels alongside cost and usefulness

A practical shopping decision also includes value, waste, and whether the product suits the way you eat. A larger pack with slightly better figures is not automatically the better buy if half of it goes stale. Equally, a product with tidy per portion numbers may be poor value if the stated portions are unrealistically small.

If budget is part of the decision, it helps to weigh nutrition and cost together rather than separately. Our budget guide to everyday healthy eating is a useful companion when you are balancing label figures with what you can actually afford.

And if you want a quick refresher in the shop, keep our guide to [comparing food labels step by step]( /compare-food-labels ) in mind: category first, standard unit next, then serving size, traffic lights, ingredients and pack weight.

The short version is this. Use per 100g or per 100ml to compare foods fairly. Use per portion to understand what you are likely to consume. Read the sugars line with care, because it does not isolate free sugars. Then make the final choice in context, based on the kind of food, the amount you will really eat, and the nutrients that matter most for that purchase.