How to use this tool
Add one row per product or package size. Type the shelf price, the quantity printed on the label and pick its unit; if it is a multipack, put the number of packs in the last column. As soon as two rows are complete the table ranks them from cheapest to most expensive per unit and shows how much more each one costs compared with the best option. Use the "show price per" menu to switch between per 100 g, per kg, per ounce, per litre, per item and so on; the ranking does not change, only the numbers you read.
Mixed units are fine. A 1.5 lb tub and a 750 g tub are both converted to grams before the price is divided, so the comparison is exact rather than a rough guess. If you enter a weight for one product and a volume for another the tool will warn you rather than produce a meaningless number.
Why use this instead of doing it in your head
Supermarket unit pricing is helpful when it is there, but it is often missing, printed in a different unit on neighbouring shelves, or absent altogether from online listings and warehouse stores. Comparing 12 oz to 340 g to 1 lb while standing in an aisle is exactly the kind of arithmetic people get wrong, and the "big pack is always cheaper" assumption fails more often than you would expect. Promotions on the small size, shrinkflation on the large one and multipack pricing all muddy the picture.
This calculator is deliberately quick to use on a phone: two taps to add a row, no account, and it keeps working with no signal once the page has loaded. It also handles counts as well as weights and volumes, so it works just as well for nappies, batteries and coffee pods as it does for rice and washing-up liquid.