Best buys
Compare two prices/quantities by computing unit cost (price per unit weight or volume).
Foundation-tier favourite. Divide price by quantity to find cost per unit; smaller = better value. Watch for different units.
Worked examples
500 g for £2.40 vs 800 g for £3.60: 0.48p/g vs 0.45p/g; the larger pack is better value.
2 L for £1.80 vs 750 ml for £0.80: 0.09p/ml vs 0.107p/ml; the 2 L is better.
Always compare in the same units before deciding.
Frequently asked questions
Reverse: cost per £?
Quantity per £ instead. 500 g for £2.40 = 208 g per £1. Larger = better. Same answer.
Tax/VAT?
Apply consistently before comparing. UK supermarkets quote inclusive prices.