Unit price explained: how to tell which pack is really cheaper

A unit price is the price of one standard amount of a product: per 100 g, per kilogram, per litre or per item. You get it by dividing the price by the quantity, and it is the only fair way to compare packs of different sizes. A 250 g pack at 2.50 and a 750 g pack at 6.80 look hard to compare until you see that one costs 1.00 per 100 g and the other about 0.91. I built the unit price calculator to do that division for several products at once, and this guide explains the method so you can do it in your head in a shop.

What a unit price is

Retailers sell the same thing in many sizes, and pack sizes are often chosen to make comparison awkward: 400 g against 454 g, 6 cans against 8, 1.5 litres against 2. A unit price strips out the size and leaves only the cost of the stuff itself. Once every option is expressed per 100 g, per litre or per item, the cheapest is simply the smallest number. It works for anything sold by weight, volume or count: food, detergent, nappies, printer paper, screws.

The formula

Unit price = price ÷ quantity, scaled to a convenient base. For a per-100 g price, divide the price by the weight in grams and multiply by 100:

• Small pack: 2.50 ÷ 250 g × 100 = 1.00 per 100 g.
• Big pack: 6.80 ÷ 750 g × 100 = 0.907 per 100 g.

The big pack wins, and the calculator reports the small one as about 10% more expensive per gram (1.00 ÷ 0.907 is 1.10). The base does not change the answer, only how readable the numbers are: per kilogram the same two packs are 10.00 and 9.07. For items sold by count, divide by the number of items: 12 rolls for 6.00 is 0.50 a roll, and 9 rolls for 4.20 is about 0.47.

Comparing different units

Things get harder when one pack is labelled in ounces and another in grams, which is common with imported goods. Convert everything to one unit first. One ounce is 28.35 g and one pound is 453.6 g, so a 16 oz jar at 3.99 holds 453.6 g and costs about 0.88 per 100 g; a 1 kg jar at 7.50 costs 0.75 per 100 g, so the 16 oz jar is roughly 17% dearer. The calculator converts g, kg, oz, lb, ml, litres and fluid ounces automatically, and refuses to rank weight against volume, because 100 g and 100 ml of most products are not the same amount. One limit worth knowing: it uses the US fluid ounce of 29.57 ml, while the old imperial fluid ounce used in the UK is 28.41 ml, so for imperial labels convert with the unit converter first. Metric vs imperial: how we got two systems explains why the two differ.

Is the bigger pack always cheaper?

Usually, but not always. Bulk packs often carry a lower unit price because packaging and handling cost less per gram, but shops also run promotions on smaller sizes, and multi-buy deals can make two small packs cheaper than one large one. Branded large packs are sometimes beaten by a store brand in a small size. The only way to know is to compare the unit prices on the day. When a pack is on offer, work out the sale price first with the discount calculator, then compare per unit; How to calculate a discount, a sale price and the real percentage off shows that step.

Shelf labels and their gaps

Many shops print a unit price on the shelf label, and in the UK and EU unit pricing is generally required for goods sold by weight or volume; in the US it depends on the state. The labels are helpful but inconsistent. One shelf may show a price per 100 g and the next a price per kg, multipacks may be priced per item while loose versions are priced per kg, and promotional prices are not always reflected in the unit figure. Toilet rolls and kitchen towels are a classic trap: priced per roll, while rolls differ in sheet count, so the fair comparison is per sheet or per metre. Check that the units on two labels match before trusting them.

When the cheapest is not the best buy

A lower unit price only saves money if you use what you buy. A family-size bag of salad that goes off half-eaten costs more per portion eaten than the small bag. The same applies to anything with a short shelf life, to products you are trying for the first time, and to bulk buys that strain the monthly budget or tie up storage space. Quality matters too: a cheaper detergent that needs a double dose has a higher cost per wash. For things you keep for years, such as shoes, tools or appliances, the better measure is cost per use, which the cost per use calculator works out from price, lifespan and how often you use it. The calculator here answers one narrow question honestly: which option is cheaper per gram, millilitre or item. Whether you will finish it is your call.