How to calculate unit price
Unit price is the cost per single unit, and it is the honest way to tell which product is actually cheaper. Here is the formula, a worked comparison, how to handle different pack sizes, and the traps that hide the real bargain.
Compare two products by their price per unit to see which is truly the better deal.
The price on the shelf tells you what a product costs, not whether it is good value. For that you need the unit price: the cost per single unit, such as per litre, per 100 grams or per item. Comparing unit prices is the only reliable way to tell which package is genuinely cheaper, because a bigger box is not always the better deal. The maths is one division, and once you do it a few times you will spot the real bargains without thinking. A free unit price calculator does it instantly, but the method below is quick to learn.
The unit price formula
Unit price is simply:
Unit price = total price ÷ quantity
The quantity is however the product is measured: grams, kilograms, litres, millilitres, sheets, or individual items. Whatever unit you choose, use the same one for everything you are comparing, and the lower unit price is the better value. That last point is the whole game: once two products are expressed as a price per identical unit, the cheaper one is obvious.
A worked comparison
Say Product A is priced 5.00 for 500 g, and Product B is priced 3.00 for 250 g. Work out each price per gram. Product A is 5.00 ÷ 500 = 0.01 per gram, and Product B is 3.00 ÷ 250 = 0.012 per gram, which is the same as 1.00 and 1.20 per 100 g. Product A is cheaper per gram, even though it costs more on the shelf, so the bigger pack really is the better deal here: A is about 17 percent cheaper than B (and B costs 20 percent more than A). Swap the numbers around and you can just as easily find the bigger pack is worse value, which is the point of checking.
Comparing different pack sizes and units
The trap is comparing prices that use different units. A tin priced "per 100 g" and another priced "per kg" look unrelated until you convert them to the same unit (there are 1,000 grams in a kilogram, so a price per kg divided by 10 is the price per 100 g). Supermarket shelf labels often show a unit price in small print precisely to make this comparison possible, but they do not always use the same unit for similar products, so it is worth doing the division yourself when it matters.
Pick whichever unit is natural for the product and stick to it: per 100 g for food, per litre or 100 ml for drinks and cleaning products, per item for things like batteries or nappies. The unit you choose does not change which product wins, only the size of the numbers.
Multibuys, loyalty prices and usable quantity
Unit price also cuts through the offers designed to look like savings. For a "3 for 2" deal on a 1.50 item, you pay 3.00 for 3 items, which is 1.00 each, a real unit price a third below the shelf price. A "bulk" bag is only cheaper if its price per unit beats the smaller one, which it sometimes does not. And a low unit price is no bargain if you cannot use the quantity before it spoils: the effective unit price of food you throw away is infinite. Factor in what you will actually use, not just what you buy.
Common unit price mistakes
- Comparing different units. Per 100 g against per kg, or per item against per pack, is not a comparison until you convert to the same unit.
- Assuming bigger is cheaper. Retailers sometimes price a mid-size or "value" pack at a higher unit cost than the small one. Always check rather than assume.
- Ignoring the offer maths. Work out the real price per unit after a multibuy or member discount, not the headline shelf price.
- Forgetting waste. The cheapest unit price only helps if you use the quantity. Count usable units, not just purchased ones.
Divide price by quantity, keep the unit the same for everything you compare, and the lower number wins. When you want it done at once for two products, the free unit price calculator works out the price per unit for each and tells you which is cheaper and by how much, and the percentage calculator helps with the discount maths behind an offer.
Frequently asked questions
How do you calculate unit price?
Divide the total price by the quantity. For a 500 g pack priced 5.00, the unit price is 5.00 divided by 500, which is 0.01 per gram. Use the same unit for everything you compare, and the lower unit price is the better value regardless of pack size.
Is the bigger pack always cheaper per unit?
No. Retailers sometimes price larger or mid-size packs at a higher cost per unit than the small one. The only reliable way to tell is to work out the price per unit for each and compare, rather than assuming the big pack wins.
How do I compare products in different units?
Convert them to the same unit first. For example, a price per kilogram divided by 10 gives the price per 100 g, so you can compare it with a product labelled per 100 g. Shelf labels try to help, but they do not always use matching units.
How do I work out the unit price of a multibuy?
Calculate what you actually pay for the number of items you get. A 3 for 2 deal on a 1.50 item means paying 3.00 for 3 items, which is 1.00 each, so the real unit price is 1.00, not 1.50.