The unit-price trap: why the big pack isn't always cheaper
Shelf prices hide the number that matters. How to compare price per kilo and per litre — and the four traps that make a 'deal' cost more.
Two packs. Same shelf. One says P89.99, the other says P49.99. Which is cheaper?
You cannot answer from the shelf price. You need the number underneath it: the unit price — what the pack costs per kilogram, per litre, per 100 grams.
Unit price is the only fair comparison between pack sizes, and it is the number most of us were never taught to read. Here are the four traps it catches.
The only number that compares
Unit price is simple arithmetic:
Unit price = price ÷ size
A 2 kg bag at P89.99 is P45.00/kg. A 1 kg bag at P42.99 is P42.99/kg. The smaller pack is cheaper per kilo — by P2.01, about 4.5%.
Supermarket shelf labels often print the unit price in small text under the shelf price. When they do, read it. When they do not, do the division on your phone. It takes five seconds.
Trap 1 — the big-pack premium
The biggest pack is usually cheaper per unit. Usually.
Sometimes it is not: the "family pack" carries a convenience premium, or the smaller pack is on promotion. The gap can be small per pack and large over a year.
Illustrative example (numbers illustrative, not real shelf prices):
| Format | Price | Unit price |
|---|---|---|
| 2 kg bag | P89.99 | P45.00/kg |
| 1 kg bag | P42.99 | P42.99/kg |
| 2 × 1 kg | P85.98 | P42.99/kg |
The 2 kg bag costs P4.01 more than buying two 1 kg bags. Not a disaster — but you would never know from the shelf.
The same applies to liquids: 750 ml vs 1 L, 2 L vs 2 × 1.5 L. Convert everything to the same unit before you compare.
Trap 2 — shrinkflation
A product stays the same price and gets smaller. The shelf price looks stable; the unit price rises.
Say your usual 1 kg pack becomes 900 g at the same price. The unit price goes from P45.00/kg to P50.00/kg — an 11% rise, with no change on the price tag.
This is why unit price belongs in your price diary, not just in your head. A receipt from three months ago will show it.
Trap 3 — the promotion illusion
"Was P59.99, now P52.99" is a real discount on that shelf — but not necessarily the cheapest price for that item.
If the shop next door sells the same item at P49.99 every day, the promotion is still P3 more expensive. A discount off a high price is still a high price.
Check the unit price against your own history, not against the "was" price.
Trap 4 — multi-buy maths
"3 for 2" only saves money if you would have bought three anyway — and if the unit price beats the alternatives.
Divide the multi-buy total by the total quantity. Compare that unit price with the single-unit price and with other shops. If the multi-buy lands at P44/kg and the single pack is P42.99/kg, the "deal" is not a deal.
When bulk genuinely wins
Bulk is not the villain — it is just not automatic. A bigger pack genuinely wins when:
- The unit price is lower — check it, do not assume it.
- The product keeps. Rice, maize meal, sugar, oil and tinned goods are good bulk buys. Bread, fresh produce and dairy near their date are not.
- You will actually use it. A 5 kg bag that goes stale is more expensive than two 2.5 kg bags you finish.
- Storage is not costing you. Space, heat and pests — a "cheaper" sack that spoils is not cheaper.
The rule: bulk wins when the unit price is lower and the product survives to the last use.
How to do it in your head
- Same unit, always. Convert to per kg, per litre or per 100 g before comparing.
- Move the decimal, not your thumb. P89.99 for 2 kg is roughly P90 ÷ 2 = P45/kg.
- Compare like with like. Same brand, same variant, same pack type. A 2 L bottle and a 2 L refill pouch may not be the same product.
- Write it down. Item, pack size, unit price, shop, date. Three entries make a pattern.
When the unit price isn't on the label
Some shelves print it; some do not. When they do not:
- Use the pack size on the front. It is usually accurate to the gram or millilitre.
- Divide price by size. P89.99 ÷ 2 kg ≈ P45/kg; P52.99 ÷ 750 ml ≈ P70.65/L.
- Convert to the same unit. 1 kg = 1,000 g; 1 L = 1,000 ml. Compare like with like.
- Match the product exactly. Same brand, same variant, same pack type. "Original" and "lite" can be different products at different costs.
Your receipt is the backup: Breadwinner stores the unit cost of every line, so three months from now you can still see what "the big pack" actually cost you per kilo. For why shelf prices move at all, see Reading inflation in your own trolley.
Where Breadwinner fits
Every line on a Breadwinner receipt is read with its unit cost, so your history is already in unit prices — not shelf prices. That is what makes pack-size changes and "deals" visible over time. Your last 7 days are free; Premium keeps 90 days, enough to see a full quarter of drift.
Scan your first slip — free, 10 slips a month, no card: app.breadwinner.deals
Example figures in this article are illustrative, not real shelf prices. Method applies regardless of the numbers.
Check your own slips
Breadwinner reads any receipt and shows what you actually pay, item by item. Free to start: 10 slips a month, no card required.
Keep reading
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From slip to insight in seconds
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Press and editorial enquiries: hello@breadwinner.deals