In more detail
Unit price = price / quantity, with the quantity converted to the standard unit first (1,000 mL = 1 L, 1,000 g = 1 kg, 16 oz = 1 lb). Many shops print it on the shelf label, but labels sometimes use different units for similar products, such as per ounce for one brand and per pound for another, so convert before comparing. A lower unit price only saves money if you use all of it: food that spoils or a pack too large to store can cost more in the end.
Why it matters
Pack sizes and multi-buy offers are designed to make comparison hard, and the bigger pack is not always the better value. Unit prices also reveal "shrinkflation", when the pack gets smaller but the price stays the same.
Example
| Pack | Price | Price per litre |
|---|---|---|
| 750 mL | $2.49 | $3.32 |
| 1.5 L | $4.29 | $2.86 |
The 1.5 L pack is about 14% cheaper per litre, but if half of it would be thrown away, the 750 mL bottle is the better buy.