How it works
Bigger packs are not always cheaper. Shops label packs in different sizes — 450 g, 500 g, 1 kg — so the only fair comparison is the unit price: the price of the same amount of product. Enter both sizes in the same unit (grams, millilitres, pieces, tablets, sheets…).
unit price = price ÷ quantity · per 100 = unit price × 100 saving % = (higher unit price − lower) ÷ higher × 100Worked example. Coffee A: 500 g for 18.50 → 18.50 ÷ 500 × 100 = 3.70 per 100 g. Coffee B: 1,000 g for 32.00 → 3.20 per 100 g. B is cheaper by (3.70 − 3.20) ÷ 3.70 = 13.5%. Buying two packs of A for 1 kg would cost 37.00 instead of 32.00.
Tips for smart comparisons
- Same unit on both sides: convert kilograms to grams (× 1,000) and litres to millilitres (× 1,000) first.
- Offers: for “buy 2 get 1 free”, enter the total price paid and the total quantity you take home.
- Concentrates: compare washing liquid by washes and tissues by sheets, not by bottle or box.
- Waste: a cheaper unit price only saves money if you use it all before it spoils.
Many countries require shelf labels to show unit prices; this calculator helps when they do not, or when you compare across shops and online. For any “if this costs that” question, the rule of three calculator also works, and for discounts the percentage calculator.