The formula
unit price = price ÷ quantity
Where it comes from
Divide the price by the quantity to get the price per gram, millilitre or unit, and compare. It is the sum supermarket labels are required to show, though in small print and sometimes in different units between neighbouring products, which is exactly what makes them hard to compare.
How to work it out by hand
- Divide the price of the first product by its quantity
- Do the same with the second
- Compare the two unit prices: the lower one is cheaper
What is worth knowing
The large size works out dearer per unit more often than you would think, particularly on three-for-two style offers. And there is a wrinkle the sum does not capture: if the product goes off or spoils before you finish it, the theoretical saving disappears. The big pack only wins if you are going to get through all of it.