Advanced options
Use the same unit for both (g vs g, ml vs ml, sheets vs sheets: mixing g and ml only works for water). Shelf-edge labels DO show unit prices by law, but famously in mismatched units (one brand per 100g, its neighbour per kg, the special offer sometimes with no unit price at all): this calculator is the defence. The two standing rules: bigger is USUALLY cheaper per unit but not reliably (the "bulk trap" is real, especially on offer weeks), and the supermarket own-brand's unit price is the benchmark everything else must beat.
What The Unit Price Calculator Works Out
- Product A and Product B: each price divided by its size, shown in pence per 100 units (100g, 100ml or 100 items). Keep both in the same unit: grams against grams, millilitres against millilitres, and per-item for teabags, nappies, dishwasher tablets and sheets of paper, where weight misleads.
- Verdict: which product is cheaper per unit and by what percentage. Routine 10% to 30% gaps between adjacent shelf options compound across a year of shopping into three figures, with no brand downgrade needed, only a size correction.
- Why it matters: shelf-edge labels must show unit prices by law but arrive in mismatched units (per 100g beside per kg, per wash beside per litre), and promotional tickets sometimes omit them. Bigger packs are usually cheaper per unit but not reliably: the bulk trap shows up most on offer weeks when the small pack is repriced.
- Size units (advanced): tell the calculator that one size is in kilograms or litres and the other in grams or millilitres and it converts both to the base unit before comparing, so a 1.5 kg bag compares cleanly with a 750 g one.
- Offer or loyalty discount (advanced): a percentage off either product (a loyalty-card price or a yellow-sticker reduction) applied before the unit price is worked out.
- Multibuy on A (advanced): the number of packs you take home for one pack's price, for "2 for 1" style offers. For a "3 for 2" deal, enter the total price paid and the total size in the basic boxes instead.
- Per kilogram or litre (advanced): the same comparison restated in pounds per kg or per litre, the other unit shelf labels use, so you can check the label directly.
- Break-even price (advanced): the shelf price the dearer product would need to drop to before it matches the cheaper one per unit: a quick test for whether a future offer is worth switching for.
- Saving per year (advanced): the per-unit gap multiplied by how many units you buy a year, which is the figure that decides whether the comparison is worth your attention for that product.