You rarely use a whole pack. To cost only what you use: pack price × (amount used ÷ pack size), with both amounts in the same unit.
Examples
- 1kg rice for £2.00, 250g used → £0.50
- 500ml milk for £0.90, 100ml used → £0.18
- 12 eggs for £3.00, 3 used → £0.75
- 1kg cheese for £8.00, 150g used → £1.20
Converting units
1kg = 1000g, 1 litre = 1000ml, 1lb = 16oz ≈ 454g, 1oz ≈ 28.35g. You can't convert weight to volume reliably without knowing the ingredient's density, so keep them consistent.