Did you know that a snack can show fewer grams of sugar per serving than its neighbour and still contain more sugar for the same amount of food? The trick is not hidden in an ingredient. It may be in the serving sizes printed above the nutrition table.
A nutrition information panel reports nutrients for a stated quantity of food. Per serving tells you the amount in the manufacturer’s stated portion; per 100 g provides a common basis for comparing two solid foods, even if their servings differ. Read both: compare like quantities first, then calculate what you actually expect to eat. For drinks, use the corresponding per-100 mL basis.
Start with the question rather than the biggest number
Imagine a student choosing between two sealed snacks for a school outing. One question is, ‘Which has a lower sugar concentration?’ Another is, ‘How much sugar would be in the amount I plan to eat?’ A third is, ‘How much is in the entire packet?’ They are three separate calculations. A single figure from the front of a package cannot reliably answer all three.
The Singapore Health Promotion Board’s nutrition-labelling guidance distinguishes nutrient values per 100 g or 100 mL from values per serving. Its food-label reading guide also notes that manufacturers’ serving sizes can vary. These sources describe the label convention; the classroom numbers below are entirely invented and are not descriptions of actual products.
Worked example: two fictional snacks with different serving sizes
Snack A declares a 25 g serving, containing 5 g sugar, 110 kcal of energy and 100 mg sodium. Its imaginary package contains four 25 g servings, or 100 g in total. Snack B declares a 40 g serving, containing 6 g sugar, 160 kcal and 120 mg sodium. Its imaginary package contains two 40 g servings, or 80 g total.
A hurried reader might say A is lower in sugar because 5 is less than 6. That compares different weights. To make both labels speak about the same 100 g of food, multiply each nutrient amount by 100 ÷ serving size in grams. Units matter: grams of sugar divided by grams of food gives a ratio, then we scale to 100 grams of food.
- Snack A sugar: 5 g ÷ 25 g × 100 g = 20 g of sugar per 100 g of snack.
- Snack B sugar: 6 g ÷ 40 g × 100 g = 15 g of sugar per 100 g of snack.
- Snack A energy: 110 kcal ÷ 25 g × 100 g = 440 kcal per 100 g; B: 160 ÷ 40 × 100 = 400 kcal per 100 g.
- Snack A sodium: 100 mg ÷ 25 g × 100 g = 400 mg per 100 g; B: 120 ÷ 40 × 100 = 300 mg per 100 g.
On these fictional numbers, B has less sugar, energy and sodium per equal 100 g mass. That is a comparison of those stated attributes, not proof that B is the best choice in every nutritional respect. Other nutrients, ingredients, allergies, dietary needs and how much is eaten can matter. We have not established a health outcome by doing arithmetic.
The amount eaten changes the answer again
Suppose the student eats 50 g of either snack. A contains 20 × 50/100 = 10 g sugar; B contains 15 × 50/100 = 7.5 g sugar. The sugar comparison is now fair because the masses match. This calculation assumes the food is reasonably uniform and that the stated label values are suitable approximations.
But suppose the student eats an entire packet instead. A is 100 g, so its four declared servings provide 4 × 5 = 20 g sugar and 4 × 110 = 440 kcal. B is 80 g, so its two servings provide 2 × 6 = 12 g sugar and 2 × 160 = 320 kcal. Now the packages differ in total mass. Clearly label this as a whole-pack comparison, not an equal-weight comparison.
A one-minute learner diagnostic
- If a learner circles 5 g versus 6 g and immediately announces the ‘better’ snack, ask them to read the serving sizes aloud. The issue is an unequal denominator.
- If the learner writes 20 g sugar per 100 g for Snack B, ask them to calculate 6 ÷ 40 × 100 while retaining the units. That result is 15 g, not 20 g.
- If the learner multiplies by ‘servings per pack’ when asked about a 50 g portion, the error is answering a different question. Restore the requested amount before computing.
- If the learner treats 100 mg sodium as though it were 100 g, ask them to distinguish milligrams of nutrient from grams of food.
- If a learner calls an invented snack healthy solely because one comparison favours it, ask which nutrients and individual needs were not examined.
Why this misconception survives
Food labels often present tidy columns that make unlike quantities look visually equal. The phrase ‘per serving’ can feel as concrete as ‘per 100 g’, but a serving is a declared quantity, not a standard unit shared between brands. Larger packages also make ‘one packet’ sound like ‘one serving’, although a single container may hold several servings. The correction is to name three things every time: nutrient, quantity of food, and comparison purpose.
Progress from guided calculations to independent judgement
Stage 1 — match quantities
Place two mock labels next to one another and cover the big nutrient numbers. Ask the student to locate ‘serving size’, ‘servings per package’ and ‘per 100 g’. Let them explain why 25 g and 40 g are not directly comparable. Only uncover the nutrient figures when the denominator is clear.
Stage 2 — reconstruct the label
Give an entirely fictional Snack C: 30 g per serving with 3.6 g sugar. The conversion is 3.6 ÷ 30 × 100 = 12 g per 100 g. Give Snack D: 60 g per serving with 6 g sugar. Its result is 6 ÷ 60 × 100 = 10 g per 100 g. Students should identify D as lower in sugar per 100 g despite its 6 g per-serving number exceeding C’s 3.6 g.
Stage 3 — change the actual portion
A learner plans to eat 75 g of C. At 12 g sugar per 100 g, the estimated sugar is 12 × 75/100 = 9 g. If C’s fictional packet contains four 30 g servings, that packet weighs 120 g and contains approximately 4 × 3.6 = 14.4 g sugar. Ask students to explain why 9 g and 14.4 g can both be correct: they answer different portion questions.
Stage 4 — create a comparison rule
Without a template, have the student compare two new product labels. They must write the intended purpose, show a matching denominator, include the unit, and say one limitation. Parents can check reasoning without debating which snack a child ‘should’ eat. Teachers can use label photographs with brand information removed so the arithmetic, rather than marketing, drives discussion.
How to check work independently
The first check is proportionality: a 50 g portion is half of 100 g, so its nutrient amount should be half of the per-100 g figure, assuming uniform composition. The second is an approximate sense-check: a 25 g serving is one-quarter of 100 g, so its sugar grams should be about one-quarter of its per-100 g sugar figure. The third is dimensional: mg of sodium should remain mg of sodium even when the food mass is described in g.
Track progress with two fresh label puzzles each week. Count how often the learner correctly names the denominator before dividing, gets the unit right, and distinguishes per-serving, per-100 g and entire-package answers. Later present a label showing only per-serving figures and see whether the learner can recover an equivalent 100 g comparison unaided.
What nutrition panels cannot settle by themselves
Label figures may be rounded and estimated; products differ in nutritional composition, and consumers have different needs. A per-100 g comparison does not by itself account for customary portion sizes. A food can have lower sugar per 100 g but a greater amount of something else. Products may also express liquids per 100 mL rather than per 100 g, and those are not automatically interchangeable: converting between mass and volume requires density information. For medical dietary restrictions, ask a qualified professional rather than deriving a treatment plan from this school exercise.
Questions parents and learners often ask
Should we always compare the smaller number?
No. First match the food mass or volume and identify the nutrient. A lower number for one nutrient does not establish a universal ranking of foods.
Is one packet always one serving?
No. Read ‘servings per package’. A packet may contain multiple labelled servings, and people may eat less or more than one.
Why are per-100 g and per-serving sugar values so different?
Because the quantities of food are different. They describe the same concentration at different scales, subject to rounding.
Could the printed values differ slightly from my calculation?
Yes. Rounding, manufacturer declarations and label presentation can create small apparent differences. Keep sensible precision and do not invent hidden exact values.
Use the same mathematics in a new setting
The larger learning goal is to translate a number into a truthful comparison: What was measured, per how much, and for which decision? The Core Aim guide to unit prices applies the same denominator discipline to cost; the mixed-units spreadsheet guide shows why units matter in calculations. Parents wanting broader learning support can explore the Education Hub and the Clementi Mathematics learning reference. When a student can explain the denominator before choosing a number, they are reading evidence rather than merely copying labels.