What this calculator works out
This calculator compares two products by unit price — price per gram, per millilitre or per item — and tells you which is genuinely cheaper and by how much.
It reports the price per 100 units as well as per single unit, because per-100 is the form UK shelf labels usually use and the form that is easy to compare at a glance.
Enter your details
Before you compare
- Use the same unit for both products. Grams against grams, millilitres against millilitres. Comparing 750g against 1 litre measures nothing.
- Use the drained weight for products in liquid where that is what you are buying.
- Account for concentration. A concentrated version at twice the strength is effectively half the unit price.
- Include any discount you will actually receive at the till, including loyalty pricing.
UK retailers must display a unit price, but the unit chosen can differ between neighbouring products — one per 100g, the next per kilogram — which defeats quick comparison. Doing the division yourself removes the ambiguity, and it is the only way to compare across shops.
How this calculator works
Two divisions, then a comparison:
Unit price = price ÷ quantity
Price per 100 units = unit price × 100
Saving = (dearer − cheaper) ÷ dearer × 100The per-100 figure exists because raw unit prices on grocery items are fractions of a penny and round to nothing at two decimal places. The single-unit price is shown to four decimals alongside it for cases where you need the precision.
Worked example: 750g against 1kg
Using the default figures — Product A at £3.50 for 750g, and Product B at £4.20 for 1,000g:
- Product A: £3.50 ÷ 750 = £0.0047 per gram, or £0.47 per 100g
- Product B: £4.20 ÷ 1,000 = £0.0042 per gram, or £0.42 per 100g
- Product B is 10% cheaper per unit
The larger pack costs 20% more at the till and is 10% cheaper per gram — which is exactly why the headline price tells you nothing useful. Note that 10% on a weekly grocery item is a few pounds a year, not a transformation. The value of unit pricing is cumulative across a whole shop rather than dramatic on any single product.
Common mistakes
- Mixing units. Comparing grams with millilitres or with item counts produces a meaningless result.
- Ignoring concentration. Double-strength products need their quantity doubling for a fair comparison.
- Comparing across quality tiers. Unit price says nothing about whether the products are equivalent.
- Buying more than you need to win on unit price. The cheapest per gram is not the cheapest overall if some is wasted.
- Forgetting the drained weight. Products packed in liquid can differ substantially from the pack weight.
Frequently asked questions
Are supermarkets required to show unit prices?
Yes, for most products sold by weight, volume or count, under UK price marking rules. The requirement has been tightened in recent years following criticism that promotional prices and inconsistent units made comparison difficult. Even so, doing the division yourself remains the only way to compare between retailers.
Does the cheapest unit price always win?
No. It is a comparison of price, not of value. Quality, taste, waste, storage and whether you will actually use the larger quantity all matter. Unit price answers one question well and says nothing about the others.
How much can this actually save?
Modestly but consistently. Applied across a weekly shop, choosing on unit price rather than headline price typically saves a small percentage of the total — which compounds into a meaningful annual figure without changing what you eat. It is one of the lowest-effort savings available.
What about own-brand versus branded?
Own-brand is usually cheaper per unit, sometimes substantially, and blind comparisons frequently find little quality difference in staples such as flour, tinned tomatoes and cleaning products. The gap tends to be larger on products where branding does most of the work.
Is what I enter stored?
No. Prices and quantities are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — price marking and unit pricing requirements for retailers
- MoneyHelper — practical guidance on reducing grocery spending
Sources are checked at publication and can change — how I choose and check references.