🛒 Unit Price Comparator

Shops price things so you cannot compare them: 340 g here, 12 × 30 g there, "3 for £5" on the shelf edge. Put in what is actually on the shelf and this reduces everything to one number. Handles multi-buys, which is where the maths most often surprises people.

Bigger is very often not cheaper. Supermarkets know this and price accordingly.
Three things this catches

Shrinkflation. The pack got 10% smaller and the price did not move, so the unit price rose by more than 10%. Comparing per 100 g makes it obvious in a way the shelf label does not.

Multi-buys you do not need. "3 for £5" is only a saving if you would have bought three at the single price. If you would have bought one, you have spent £5 instead of £2.

Weight versus volume. Oil, milk and cream are sold by volume but weigh differently; this normalises to the unit you choose, so pick grams for solids and millilitres for liquids rather than mixing them.