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Unit Price Calculator
Compare up to four pack sizes and see the true cost per kg, per 100 g, per litre or per item — including multipacks, UK and US units, and the exact saving.
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Enter the shelf price and pack size for 2–4 products. Everything runs in your browser.
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What is unit price?
Unit price is what one standard amount of a product costs — one kilo, one litre, one item. It is the only fair way to compare packs of different sizes, because shelf prices alone tell you nothing: a bigger pack usually costs more at the till yet less for each kilo it contains.
The formula is one division:
unit price = total price ÷ total quantity
The arithmetic is trivial; the work is making both packages use the same unit first. A 750 g pack at £3.20 and a 1 kg pack at £4.00 can only be compared once both are expressed per kilo — £4.27/kg versus £4.00/kg — and only then does it become obvious that the bigger pack wins despite its higher sticker price.
How this calculator works
Each product’s total quantity is converted to a common base unit — grams for weight, millilitres for volume, single items for counts — using exact conversion factors (1 kg = 1,000 g, 1 oz = 28.3495 g, 1 lb = 453.592 g, 1 L = 1,000 ml, 1 US fl oz = 29.5735 ml, 1 UK fl oz = 28.4131 ml). Multipacks are handled by multiplying pack count by the size of one pack. The winner is chosen from full-precision numbers, never from rounded display values, and exact ties are shown as ties rather than an arbitrary winner.
When the bigger pack is not the bargain
- Promotions and coupons. £2 off the small pack can beat the everyday price of the large one. Enter the discounted shelf price and compare again.
- Shrinkflation. Pack sizes quietly shrink while prices hold. Unit price — not packet familiarity — catches it.
- Waste and storage. A cheaper per-kilo price means nothing if a third of the pack spoils or doesn’t fit your freezer. The cheapest unit you actually use is the only one that counts.
- Budget. A lower unit price can still mean more spending today. This calculator shows the shelf price next to every result so the trade-off stays visible.
Why you can’t compare grams with millilitres
Weight and volume measure different things. Converting between them needs the product’s density, and labels don’t print it. Treating a gram as a millilitre is roughly 9% wrong for cooking oil and about 30% wrong for honey — far bigger than the per-unit differences you are usually chasing. That’s why this tool only ranks products measured the same way, and tells you plainly when a comparison isn’t possible.
Fluid units have their own trap: a US fluid ounce (29.57 ml) and a UK fluid ounce (28.41 ml) share a name but differ by about 4%, and US and imperial pints and gallons differ far more. The unit list here keeps the two systems labelled and separate.
Use it while you shop
The calculator is built for recurring shopping trips: it works on a phone in the aisle, needs no login, shows no ads, and keeps everything on your device. Save a comparison to reuse next week, or copy its share link to settle a “which one is actually cheaper?” debate in the family chat.
Have a question the sections above didn’t answer — how to read a shelf label, whether multipacks really pay off, or why US and UK fluid ounces disagree? The full unit price FAQ covers it.