Unit Price Calculator

Compare the real cost per unit — per 100g, per litre, per item — of two or more product sizes so you can see which one is actually the better deal, regardless of package size.

How it works

  1. Enter the price and quantity for the first item (e.g. £2.50 for 400 g).
  2. Enter the price and quantity for the second item.
  3. The tool calculates the cost per unit for both and highlights the better value.
  4. Switch the quantity unit — weight, volume, or count — to match the product labels.

How the comparison works

Shop labels often use different bases, such as per kilogram on one product and per 100 grams on another, and pack sizes rarely match. This tool puts every item on the same footing. It converts each quantity to a base unit (grams for weight, millilitres for volume, single items for counts), then works out the price per 100 g, per 100 ml or per item. Items are ranked from cheapest to dearest per standard unit, and the saving is the gap between the best and worst, shown as an amount and as a percentage of the dearest option.

Conversions are exact: 1 kg is 1,000 g, 1 oz is 28.3495 g, 1 lb is 453.592 g and 1 litre is 1,000 ml. Prices are held internally as whole pence or cents, which avoids the small rounding errors that creep in when floating-point numbers are added up.

A worked example

A 500 g pack of rice costs 2.40 and a 1.5 kg pack costs 6.30. Per 100 g the small pack is 48 pence and the large pack is 42 pence. The large pack is 6 pence cheaper per 100 g, which is 12.5% less than the small pack, so it is the better value if you will use it all.

When the cheapest unit price is not the best deal

Unit price tells you what you pay for the amount, not whether you will use it. A larger pack of fresh food that goes off before you finish it costs more per portion eaten than a smaller one. Quality, brand and ingredients also differ, and the numbers say nothing about them.

  • Compare like with like. Weight, volume and count are not interchangeable, because a gram cannot be converted to a millilitre without knowing the product's density, so only compare items measured the same way.
  • For multi-buy offers such as three for two, enter the effective price and total quantity as one item.
  • Remember that the shelf label may use a different base unit from the one you are comparing, which is the situation this tool was built for.

Frequently asked questions

Why isn't the bigger pack always cheaper per unit?

Packaging, bulk discounts, and pricing strategy don't always scale together — a mid-size pack is sometimes priced to look like the best deal even when a different size is genuinely cheaper per unit.

What is the formula for unit price?

Unit price equals total price divided by total quantity, in whatever unit you want to compare — per gram, per litre, per item.