A lower shelf price can hide a smaller package. Before deciding that one bag of rice is a bargain, put both options on the same scale: price per kilogram, litre, or another useful quantity. The calculation is the package price divided by the amount inside. You need a consistent unit, not a detailed spreadsheet or a shopping app.
Start with two products you would genuinely buy. If one costs 8 for 500 grams and another costs 14 for one kilogram, their prices per kilogram are 16 and 14. These are illustrative numbers, not current offers. The larger bag is cheaper by weight, but it still requires more money at checkout. A tight weekly budget can make the smaller purchase the workable choice.
Read the quantity before the discount
Check whether the shelf label already displays a unit price. If the units differ, convert them before comparing: one kilogram contains 1,000 grams. On a phone calculator, divide the price of a 750-gram package by 0.75 to obtain its price per kilogram. There is little benefit in calculating several decimal places; a rounded figure is usually enough to distinguish ordinary options.
The comparison also needs equivalent contents. A jar of olives with liquid is not directly comparable with the same weight of drained olives. Look for drained weight where relevant. Concentrated and ready-to-use products require another adjustment, and an assorted multipack may include flavours nobody wants. Packaging size alone does not tell you how much useful food reaches your plate.
Choose a stopping rule for differences that are small. If two acceptable products are close in unit price, decide using taste, available storage, and the quantity you will finish. Spending several minutes to save a tiny amount on one item can distract from a much larger difference elsewhere in the basket. Practise on staples you buy often, where the calculation is easiest to reuse.
Make the result useful at home
Record only a few comparisons that matter to your household. A note reading “usual rice, price per kilogram, store and date” is sufficient. It is a record of a purchase, not a promise about the next visit. Recheck the package weight when a familiar product changes appearance or an offer seems unusually attractive.
Finally, compare the amount you expect to use. A lower price per kilogram loses its advantage when a large share goes unused. Buy the larger pack when consumption, storage, and cash flow all fit. Otherwise, choosing less food can be the more economical decision even when the shelf arithmetic favours the biggest bag.