The grocery shelf maths that can make the bigger pack more expensive per 100 grams

Supermarket pricing can make one pack look cheaper before you compare the amount inside. A quick unit-price check can show which option costs less for the same quantity.

A larger packet can look cheaper because the total quantity is higher. The shelf price does not prove the cost per 100 grams, and the cheapest pack can change once both sizes use the same unit.

Annual food and non-alcoholic beverage inflation was 0.9% in July 2026, with price changes differing between food categories. Your supermarket comparison still comes down to the products in front of you and the amount each one gives you for the price.

The bigger pack is not automatically cheaper

For example, a 500g pack at R42 costs R8.40 per 100g, while a 1kg pack at R90 costs R9 per 100g. The 1kg pack gives you twice as much product, but each 100g costs 60 cents more. Pack size also has to fit how much food you can finish or freeze. A lower unit price loses its financial benefit if part of a fresh food purchase ends up unused.

Section 23 of the Consumer Protection Act deals with the disclosure of prices and includes unit price within its definition of price. A shelf ticket might already show a price per kilogram or per 100g, which can spare you the calculation.

Use the same unit before you compare

If the shelf ticket gives the pack price, divide the price by the number of grams and multiply by 100.
R42 ÷ 500 × 100 = R8.40 per 100g
R90 ÷ 1,000 × 100 = R9 per 100g

A price per kilogram can also be converted to a price per 100g by dividing by 10. A same-unit comparison removes the distraction of different pack sizes, brands and packaging formats. A larger pack wins on unit price only when the arithmetic supports it. Pack size, promo stickers and bold shelf labels do not change the cost per 100g. Your calculator gets the final vote. Discounts and loyalty rewards can change the amount charged, which means the calculation should use the price you would pay after the discount.

A discount can change the answer

You should do the same calculation with multi-buy deals. If two 500g packs cost R70 together, the combined 1kg costs R7 per 100g. Compare R7 with the unit price of the single pack and the larger pack before deciding. The same logic applies online. A larger basket to qualify for free delivery can increase the checkout total even when the delivery charge disappears.

Shelf and till prices can differ as well. The Consumer Goods and Services Ombud provides guidance on displayed price errors and the consumer protections linked to an incorrect displayed price.

The pack with more grams is not automatically the pack that gives you more for each rand. Compare the same unit, then decide whether the quantity fits what you will use.