Rule of Three Calculator guide
How to Work Out Which Size Is Actually Better Value
To compare two sizes, convert both to the same amount using the rule of three. If 750ml costs £2.40, then 1000ml would cost (2.40 × 1000) ÷ 750 = £3.20. Compare that against the actual price of the litre bottle — whichever is lower is the better value.
Why the bigger box is not always cheaper
Shops rely on the assumption that buying bigger means paying less per unit. Often it does. Fairly often it does not, particularly on promotions, where a multi-buy on small packs can beat the large one.
The only way to know is to put both options onto the same footing, and that is a rule of three problem.
The method
A 750ml bottle costs £2.40. A 1 litre bottle costs £3.05. Which is better value?
Work out what 1000ml would cost at the small bottle's rate:
- 750 ml → £2.40
- 1000 ml → x
Reading the result
x = (2.40 × 1000) ÷ 750 = 2400 ÷ 750 = £3.20.
So at the small bottle's rate, a litre would cost £3.20. The actual litre bottle is £3.05, which is cheaper. The large bottle wins, by 15p per litre.
The key move is that you are no longer comparing £2.40 against £3.05 — those are prices for different things. You are comparing £3.20 against £3.05, which are prices for the same thing.
A faster version for the supermarket aisle
Dividing to get a price per single unit is quicker than a full proportion when you are standing in a shop.
£2.40 ÷ 750ml = £0.0032 per ml. £3.05 ÷ 1000ml = £0.00305 per ml. Lower wins, so again the litre.
Those numbers are awkward to read, so scale them to something human: per 100ml, that is 32p versus 30.5p. Same comparison, easier to hold in your head.
Most shops print exactly this on the shelf label in small type. It is worth knowing how it is calculated, both because the labels are sometimes in inconsistent units and because promotional prices often are not reflected in them.
When cheaper per unit is still the wrong buy
Unit price answers one question well, but it is not the whole decision.
The larger size is not better value if you will not finish it before it goes off. Half a wasted large bottle is worse value than a small one you used entirely, whatever the shelf label says.
It is also worth watching for changes in the product itself — the large size is occasionally a slightly different formulation or grade, in which case you are not comparing like with like at all.
