Unit Pricing in the Supermarket
The assumption that buying bigger saves money is wrong often enough to be worth checking every time — and the shelf labels that are supposed to let you check are frequently designed to make it difficult.
In short
- Larger packs are routinely more expensive per unit than standard ones.
- Unit prices on shelf labels use inconsistent bases, which blocks direct comparison.
- A multi-buy offer often loses to a single pack on promotion.
- The cheapest unit price is only a saving if you actually use what you bought.
Bigger is not reliably cheaper
The intuition that bulk buying is cheaper comes from a real economy — packaging and handling costs do not scale linearly with volume — but retail pricing does not have to pass that saving on, and frequently does not.
Large packs are commonly positioned as convenience or "family" items, where shoppers are less price-sensitive and less likely to check. Standard sizes are the ones used in price comparisons and promotions, so they are the ones kept sharp. The result is that the family pack can quietly carry a higher unit price than the ordinary one sitting next to it.
| Option A | Option B | Per 100 g | Actually cheaper |
|---|---|---|---|
| 2.49 for 500 g | 4.29 for 1 kg | 0.50 vs 0.43 | B, by 14% |
| 3.00 for 1 kg | 1.40 for 400 g | 0.30 vs 0.35 | A, by 17% |
| 1.20 for 250 g | 2.10 for 500 g | 0.48 vs 0.42 | B, by 13% |
| 5.00 for 6 items | 9.00 for 12 items | 0.83 vs 0.75 each | B, by 10% |
The second row is the one worth noticing: the kilogram pack loses to the 400 g pack. There is no rule of thumb that gets this right — only the arithmetic does.
Why the shelf labels do not help as much as they should
Most countries require a unit price on the shelf label, which in principle solves this entirely. In practice three things get in the way.
The first is inconsistent bases. One product is priced per 100 g, the one beside it per kilogram, a third per item. Comparing 0.50 per 100 g against 4.30 per kg requires a mental shift of the decimal point that most people will not perform while shopping.
The second is that promotional prices often do not update the unit price on the label, so the small print describes the shelf price and the large print describes the offer.
The third is drained weight. Tinned and jarred goods may be labelled by total or by drained weight inconsistently, and those are very different quantities for something packed in brine or oil.
Multi-buys are the least reliable of all
"3 for 2" and "2 for £5" offers are the hardest to evaluate because they change the effective unit price only if you buy the whole bundle, and they are frequently beaten by a single unit of a different size that happens to be on promotion.
A "2 for 5.00" on a 400 g pack is 0.63 per 100 g. A single 1 kg pack at 5.50 is 0.55 per 100 g. The offer is displayed prominently; the better deal is not displayed at all. Working out the per-unit figure for the bundle and comparing it against every size on the shelf is the only reliable method.
Shrinkflation and the moving base
Where a manufacturer reduces pack size while holding the price, the unit price rises without the shelf price changing. Because shoppers remember prices far better than they remember weights, this is largely invisible.
A pack going from 200 g to 180 g at an unchanged 2.00 is a 10% reduction in quantity and an 11.1% increase in unit price. Note that those two percentages differ — reducing the quantity by 10% raises the price per unit by 1 ÷ 0.9, which is 11.1%, not 10%. It is the same reversal trap that catches people when they try to remove a percentage by subtracting it.
When the cheapest unit price is the wrong choice
Unit price is a ratio, and optimising a ratio in isolation can make you worse off. Three cases where the larger pack is genuinely the wrong buy even at a lower unit price:
- Perishables you will not finish. Food thrown away costs its full price, not its unit price. A pack twice the size at 15% less per unit is a loss if you waste a quarter of it.
- Storage you do not have. Space has value, and a bulk pack that blocks a cupboard for six months has a real cost even if it never spoils.
- Cash you need now. Buying six months of something at a discount is a small return on money that is then unavailable for anything else.
The useful question is not "which is cheapest per unit" but "which is cheapest per unit that I will actually use". The first is arithmetic; the second requires knowing your own household.
A method that takes ten seconds
Pick one base and stick to it for the whole comparison — per 100 g and per 100 ml work well because most products land in a readable range rather than in fractions of a cent.
Convert everything to that base before comparing anything, including any multi-buy bundle, treating the bundle as a single larger pack. Then compare the numbers, and only afterwards apply the judgement about whether you will use the quantity involved. Doing the arithmetic first stops the packaging influencing the comparison.
Frequently Asked Questions
How do I calculate price per unit?
Divide the price by the quantity, then scale to a common base. For 2.49 covering 500 g: 2.49 ÷ 500 = 0.00498 per gram, which is 0.50 per 100 g. The important part is converting every option to the same base before comparing.
Is the bigger pack always cheaper per unit?
No. Larger and "family" packs are frequently dearer per unit than the standard size, particularly when the smaller one is on promotion. There is no reliable rule of thumb — only the arithmetic tells you which is cheaper.
Why are shelf unit prices hard to compare?
Because the bases are inconsistent — one label per 100 g, the next per kilogram, a third per item — and promotional prices often do not update the unit price shown. Drained versus total weight on tinned goods adds a third inconsistency.
If a pack shrinks by 10% at the same price, how much has the unit price risen?
By 11.1%, not 10%. The new unit price is the old one divided by 0.9. This is the same reversal that makes subtracting a percentage the wrong way to undo one.
Should I always buy the lowest unit price?
Only for things you will use. Food that spoils costs its full price rather than its unit price, so a larger pack at a lower unit price is a false economy if you waste part of it. Storage space and tied-up cash are real costs too.