Price Per Unit: The One Number That Reveals the True Cost of Anything You Buy

Why the Sticker Price Lies to You

You're standing in the cereal aisle. There's a 500g box for £2.80 and a 750g box for £3.99. Both are on the same shelf, both look like reasonable value, and the larger box has a big "FAMILY SIZE — GREAT VALUE" flash on the front. Which one should you buy?

Without doing the maths, you're guessing. And supermarkets know it. The entire science of retail packaging is designed to make comparisons as difficult as possible — different weights, different volumes, different counts, labels in small type. Price per unit is the antidote: a single standardised number that lets you compare any two products on equal terms.

What Price Per Unit Actually Means

Price per unit is simply the cost of one standardised measure of a product — whether that's per gram, per millilitre, per sheet, per tablet, or per item. The formula is straightforward:

Price per unit = Total price ÷ Quantity

For the cereal example above: the 500g box works out to £0.56 per 100g, while the 750g box works out to £0.532 per 100g. The bigger box wins — but only just, and only if you'll actually eat 750g of cereal before it goes stale.

Real-World Examples Across Common Categories

Cereals and dry goods

Cereals are one of the best categories to apply price-per-unit thinking because pack sizes vary wildly — 375g, 500g, 720g, 1kg — and prices rarely scale proportionally. Always compare per 100g, which is the unit most supermarkets are legally required to display in the EU and UK (though compliance is inconsistent).

Toilet paper and paper products

Toilet paper is notoriously hard to compare because packs come in different sheet counts, different ply, and different sheet sizes. A "400-sheet double-ply" roll is not the same as a "200-sheet triple-ply" roll even though both could be marketed as equivalent. Here, price per 100 sheets (or per roll of equal ply) is your best comparison unit. Be suspicious of rolls described in vague terms like "mega" or "jumbo" without specific sheet counts.

Washing powder and detergents

Detergents are sold by weight, volume, and wash count — sometimes all three on the same shelf. Concentrated formulas use smaller amounts per wash, meaning a 1kg box of concentrate may genuinely outperform a 1.5kg box of standard formula. When comparing detergents, price per wash is more useful than price per kg, but you need the manufacturer's recommended dose to calculate it accurately.

Bottled drinks and personal care

Shampoos, conditioners, body wash, and bottled water are best compared per 100ml. A 400ml bottle at £3.20 costs 80p per 100ml. A 750ml bottle at £4.50 costs 60p per 100ml — a meaningful 25% saving per use.

Bigger Isn't Always Cheaper — Exceptions That Catch Shoppers Out

The assumption that larger packs always offer better value per unit is wrong often enough to be dangerous. Supermarkets regularly price "family size" or "bonus" packs at a higher price per unit than their standard counterparts, relying on the psychological assumption that shoppers won't check. Research from consumer watchdog Which? found that in a significant proportion of cases, the multipack or larger size was actually more expensive per unit than buying singles.

Premium "limited edition" or seasonal packaging is another trap — the same product in a novelty tin or gift box may cost 40–60% more per unit while appearing to be a generous gift-size portion.

Using Price Per Unit in Online Shopping

Online grocery shopping removes the physical effort of doing the maths but doesn't remove the need for it. Most major UK supermarkets (Tesco, Sainsbury's, Ocado) display unit prices online, usually in small grey text beneath the main price. The unit used is not always consistent across products in the same category — one pasta brand might show per 100g while another shows per kg — so visual scanning can still mislead you. Use the browser's search function (Ctrl+F) to find "per 100g" on a product page, or simply divide manually before adding to your basket.

The Unit Mismatch Problem

Comparing across different types of unit requires extra care. Weight and volume are not interchangeable unless you know the density of the product — a litre of olive oil weighs about 0.92kg, so a price-per-litre comparison and a price-per-kg comparison will give slightly different rankings. For most practical purposes the difference is small, but for expensive products like cooking oils or cleaning concentrates it adds up.

Similarly, comparing price per item only works when items are genuinely equivalent. Twelve standard bin bags versus fifteen "heavy duty" bags is not an apples-to-apples comparison even if you calculate price per bag.

Making It a Habit

The best shoppers treat price per unit as instinctive rather than effortful. Once you know your anchor prices for the staples you buy regularly — the benchmark cost per 100g of pasta you're happy with, the per-wash detergent cost you consider acceptable — you can spot a genuine deal at a glance and ignore the marketing noise around it.

A price per unit calculator removes all the mental arithmetic from this process. Enter the price and quantity for two competing products and get an instant comparison. Over a year of grocery shopping, consistently choosing the better value option on 20–30 product categories can save a household hundreds of pounds without any sacrifice in quality.