Did you know that two snack packets can appear to have very different amounts of sugar simply because their manufacturers chose different serving sizes? One panel may show 4.8 g of sugar for a 30 g serving; another may show 7.5 g for a 50 g serving. Reading only those two figures suggests that the first snack contains less sugar. But the numbers describe unequal portions. Before comparing foods, pupils need to make the units and quantities equal.
“Per serving” means the stated nutrient amount in one declared serving size, whereas “per 100 g” means the nutrient amount in exactly 100 grams of the food. To compare concentration on a like-for-like mass basis, convert both products to per 100 g. To understand what someone actually eats, calculate the amount for the portion consumed. Those are different but complementary questions. The serving size is a measurement basis, not necessarily a recommendation for an individual.
Why nutrition panels need a common denominator
Imagine comparing two pupils’ maths results when one paper has 20 marks and the other has 50. The raw scores cannot be compared fairly without first stating the maximum. Nutrition labels pose a similar issue: a quantity of sugar belongs to a mass of food. If the masses differ, the sugar numbers cannot be placed side by side as though the portions were equal.
Singapore’s Health Promotion Board explanation of nutrition labelling, updated 23 September 2026, describes Nutrition Information Panel values as potentially shown per 100 g or per 100 mL and/or per serving, with serving size and servings per package required when reporting per serving. The Singapore Food Agency consumer guide also explains why comparing per 100 g or 100 mL figures can be clearer when serving sizes differ. These pages supply the label-reading context; our arithmetic examples below are invented and are not real products.
Read four things before making any calculation
- Food form: is the label for the dry food, the prepared food or a diluted drink? Comparing different preparation states can mislead.
- Serving size: how many grams or millilitres count as one serving on that particular label?
- Nutrient and units: sugar and fat may be in grams; sodium often appears in milligrams; energy may be in kilocalories or kilojoules.
- Basis: is the displayed figure per serving, per 100 g, per 100 mL or per whole package?
A gram is a unit of mass; a millilitre is a unit of volume. Do not automatically treat 100 mL as 100 g for every food. Their equivalence depends on density. For ordinary packaged solids, compare per 100 g; for drinks, per 100 mL can be useful. If the two labels use different bases and the necessary density or mass information is absent, record that limitation rather than inventing a conversion.
Worked example: two fictional packets of crackers
Fictional arithmetic teaching labels—not real nutrition analyses: suppose Packet A states serving size 30 g, sugar 4.8 g per serving, sodium 120 mg per serving and energy 135 kcal per serving. Packet B states serving size 50 g, sugar 7.5 g per serving, sodium 150 mg per serving and energy 210 kcal per serving. Assume the stated serving values describe equivalent ready-to-eat forms and that the calculations use these rounded label figures.
Step 1: compare the sugar on the original panels
Packet A says 4.8 g sugar for 30 g food. Packet B says 7.5 g sugar for 50 g food. A’s displayed serving has less sugar, but it also weighs 20 g less. Therefore the sentence “Packet A has less sugar per equal amount” is not yet established.
Step 2: convert each to exactly 100 g
Use nutrient per 100 g = nutrient per serving × 100 ÷ serving mass in grams. Packet A: 4.8 × 100 ÷ 30 = 16 g sugar per 100 g. Packet B: 7.5 × 100 ÷ 50 = 15 g sugar per 100 g. In this invented comparison, B is slightly lower in sugar per equal 100 g mass, even though its printed “per serving” sugar number is larger.
Step 3: translate that into a realistic shared portion
Suppose a class worksheet asks what 60 g of either cracker would contain, using only the fictional panel values. Packet A: 16 × 60/100 = 9.6 g sugar. Packet B: 15 × 60/100 = 9.0 g sugar. The conclusion remains B lower by 0.6 g sugar for a 60 g portion. The 60 g portion here is an invented calculation amount, not advice about how much a child should eat.
Step 4: do not ignore units for sodium
For sodium, Packet A: 120 mg × 100/30 = 400 mg per 100 g. Packet B: 150 mg × 100/50 = 300 mg per 100 g. For equal 60 g portions, A would contain 400 × 0.6 = 240 mg, while B would contain 300 × 0.6 = 180 mg. Notice that sodium stays in milligrams throughout the calculation; calling the result “400 g sodium” would be a thousandfold unit error.
Step 5: compare food energy independently
A’s energy is 135 kcal × 100/30 = 450 kcal per 100 g; B’s is 210 kcal × 100/50 = 420 kcal per 100 g. For our fictional 60 g sample, A is 270 kcal and B is 252 kcal. These figures are mathematical transformations of the hypothetical label values. They are not a nutrient analysis of real foods, and label rounding may mean a calculated figure is not the same as one separately printed on a real package.
Step 6: reverse-check the original servings
Convert the 100 g figure back. For A, 16 g sugar per 100 g × 30/100 = 4.8 g per 30 g serving. For B, 15 × 50/100 = 7.5 g per 50 g serving. The reversibility checks the arithmetic. It does not prove a manufacturer’s measurement is exact or that a rounded panel is perfectly consistent across columns.
Per 100 g answers one question; per serving answers another
A per-100-g comparison asks, “If I took the same mass of each food, which contains more of this nutrient?” A per-serving figure asks, “How much does the manufacturer’s declared portion contain?” A portion-consumed calculation asks, “Given an actual amount eaten, what is the estimated nutrient intake from these panel values?” Confusing these three questions leads to many apparent contradictions.
For instance, a 25 g serving of a concentrated snack can show fewer grams of sugar than a 60 g serving of a less concentrated one. Yet when normalised to 100 g, the concentrated snack may contain more. Both figures can be true at the same time because they describe different amounts of food.
A short learner diagnostic before formal teaching
- What does 6 g sugar per 30 g serving mean? It means the panel attributes 6 g of sugar to 30 g of that food, not to 100 g.
- How much sugar per 100 g does that represent? 6 × 100/30 = 20 g per 100 g.
- What about an actual 15 g portion? 20 × 15/100 = 3 g sugar.
- Does a 100 mL drink automatically have a mass of 100 g? No; that depends on density.
- Can you judge a whole diet from one nutrient on a packet? No. This exercise tests comparison and units, not overall dietary suitability.
If a pupil writes 6/100 instead of 6/30, the problem is identifying the base quantity. If the result is “20 mg” instead of “20 g,” it is a unit mistake. If the calculation is correct but the pupil declares one product “healthy” from sugar alone, the issue is overinterpreting limited information. Diagnosing these separately is far more helpful than simply saying the answer is wrong.
Mistakes that can reverse a comparison
- Ignoring serving size: comparing 4.8 g with 7.5 g as though both described the same food mass.
- Confusing a serving with a whole packet: a packet may contain multiple servings. If a fictional packet has 3 servings at 4.8 g sugar each, the estimate is 14.4 g sugar for the whole packet.
- Comparing dry and prepared values: adding water changes mass or volume and therefore changes per-unit concentration even when the total nutrient amount does not change.
- Comparing grams with milligrams: 1 g = 1,000 mg. Convert when necessary before dividing or ranking.
- Treating calories and kilojoules as additive: these are alternative energy units, not two separate energy contributions.
- False precision: printing 15.0000 g after conversion does not create accuracy beyond the original panel.
- Turning a classroom calculation into advice: food choices can depend on allergies, dietary needs, serving patterns and other nutrients beyond one comparison.
Progressive practice: easy fractions before real packaging
Stage 1: doubles and halves
Start with the fictional label “2 g fibre per 50 g.” Ask the learner what 100 g contains. Doubling both the portion and fibre gives 4 g fibre per 100 g. For 25 g, halving the original portion gives 1 g fibre. This builds the meaning of direct proportion before the general formula.
Stage 2: choose the comparison basis
Two imaginary cereals list sugar per serving: X has 5 g in 25 g; Y has 7 g in 50 g. The student first predicts which number looks smaller, then calculates X = 20 g per 100 g and Y = 14 g per 100 g. Encourage the pupil to explain in words why Y is lower on an equal-mass basis even though its serving shows more sugar.
Stage 3: a food package with several servings
A fictitious box states 40 g per serving and 5 servings per box. Its panel shows 3 g fat per serving. The pupil calculates a total box mass of 200 g, fat per 100 g of 7.5 g, and a whole-box amount of 15 g. The teacher then changes the question: if someone ate 80 g, the label-based estimate would be 6 g fat. The purpose is to track exactly which mass each quantity belongs to.
Stage 4: liquids and the right units
A fictional drink has 9 g sugar per 250 mL. For 100 mL, 9 × 100/250 = 3.6 g sugar per 100 mL. For 500 mL, it would be 18 g under the same concentration assumption. Ask the student to keep millilitres in the denominator and not silently convert volume to mass.
Stage 5: independent shopper-style analysis
Give pupils two anonymised label cards with different serving masses and require a six-line response: identify the nutrient; state each serving mass; convert each to 100 g; compare the common basis; calculate a stipulated equal portion; and write one limitation. Use labels without brand names initially, so marketing does not replace reasoning. Only then invite students to practise with real packages, checking whether the labels refer to similar food forms.
A practical checklist for parents and teachers
A parent can ask a child: “If we both ate 40 g, would this packet’s printed serving number still be the relevant number?” Invite the child to calculate using the labelled basis rather than copying the biggest or smallest figure. Teachers can use pairs of deliberately different serving sizes to check whether pupils understand proportional reasoning instead of memorising a single conversion pattern.
Observable progress appears when a learner can select a common basis without being told, keep grams and milligrams separate, reverse-check the calculation and explain why a smaller per-serving number can coexist with a larger per-100-g concentration. For a further challenge, ask them to state when a comparison is impossible without more information, such as a drink labelled only by volume and a paste labelled only by mass.
What the calculations cannot establish
Nutrition panels provide labelled estimates under stated rules; they do not give a complete account of a person’s nutrition, health or allergies. A lower sugar concentration does not, by itself, establish that a product is suitable for someone with a specific health condition. We also should not assume that a label is a perfectly precise laboratory measurement. An appropriate conclusion is mathematical: one invented product has a lower stated amount of one nutrient per equal food mass under the comparison assumptions.
Singapore’s nutrition requirements are not identical for every food category, and some products may not have a full panel. This article does not make compliance decisions from one photograph. The HPB guidance and SFA labelling requirements page are the appropriate starting points for checking current regulatory rules.
Frequently asked questions
Is “per serving” always the amount I should eat?
No. A declared serving is the quantity used for that panel’s arithmetic and labelling; it should not be interpreted as an individual medical or nutritional recommendation.
Why can one product look lower in sugar per serving but higher per 100 g?
The serving is smaller. A smaller mass can contain fewer total grams of sugar while still having more sugar per unit mass.
Can two products with different serving sizes be compared fairly?
Yes, if they are sufficiently comparable foods and the nutrient, food form and units can be put on the same basis, usually per 100 g or per 100 mL. State the assumptions.
What if a panel shows only per serving values?
If serving mass or volume is given, convert using direct proportion. If the serving quantity is absent, do not invent one; find a reliable label or manufacturer source.
Does changing 50 g to 100 g double every nutrient number?
For a uniform food under the same composition assumption, doubling the amount doubles the estimated absolute nutrient quantity. Values printed on real labels may differ slightly after rounding.
Are 100 mL and 100 g the same?
Not in general. Millilitres measure volume, grams mass. Their relationship depends on density, which differs among foods and preparations.
The aim: compare like with like before making a claim
The best food-label lesson is not a race to find the smallest number. It is a lesson in fair comparison: identify the question, name the amount being measured, make denominators equal, check the arithmetic and stop where the evidence stops. That reasoning transfers to shopping prices, science concentrations, recipe scales and every chart that compares unequal quantities.
For a related unit lesson, read the verified Core Aim guide to kilojoules and kilocalories on food labels and the Core Aim guide to food waste per meal. Follow the eduKate Education Hub for wider learning, and the immutable Clementi Mathematics teaching reference for diagnosis-first, explanation-first and independently checked practice.