How to compare unit prices

Unit price is the price divided by the amount in the pack. It helps when two products use different pack sizes, such as 500g and 1kg, or 750ml and 1 litre.

Estimate only: check shelf labels, drained weights, delivery substitutions and checkout discounts before relying on the result.

Use matching units

Compare grams with kilograms, millilitres with litres, or count-based packs with the same count type.

Watch pack details

Some items show drained weight, usable sheets, washes or servings. Use the amount that matches how you will actually use the product.

Not always the best buy

The cheapest unit price only helps if you can use the larger pack before it expires or goes to waste.

Example unit price check

If one pack costs £2.50 for 500g and another costs £4.20 for 1kg, the smaller pack is £5.00 per kg and the larger pack is £4.20 per kg. The larger pack is cheaper per kilogram, but the calculator still leaves room for you to decide whether the bigger pack is practical.

Use the optional third item for a supermarket own-brand, a refill pack or a bulk size. For cleaning products, nappies, coffee pods, dishwasher tablets and similar items, compare by wash, sheet, pod or item rather than by pack weight if that better reflects how you use it.

Discounts and loyalty prices

Enter the price you will actually pay at checkout. If a loyalty price or voucher applies to only one item, compare the discounted price against the normal price for the other items.

Drained or usable weight

For tins, jars and frozen goods, the headline weight may include water, brine or packaging. Use drained weight when that is the fairest comparison.

Waste changes the answer

A lower unit price can still be poor value if the food expires, storage is limited, or the product is a size you will not finish.

Common unit price mistakes

Do not compare a ready-to-use product with a concentrate unless you adjust for dilution. Do not compare nappies, wipes, dishwasher tablets or washing capsules by pack weight when the useful count is the better measure. Do not ignore delivery charges if one item is online-only and the other can be collected in store.

For food, compare the amount you expect to eat rather than the largest number on the label. A multipack can include packaging, sauce, liquid or variety sizes that make the simple pack weight less useful.

When unit price is not enough

Unit price is a strong first check, but it should not be the only decision. A larger pack can be cheaper per kilogram while still being the wrong buy if it will expire, needs more storage, has lower quality, or locks up money you need elsewhere.

For subscriptions, delivery thresholds and multibuy offers, combine tools: use unit price to compare pack value, the multibuy calculator to check offer totals, and the free delivery calculator to see whether adding more to the basket really reduces the final cost.