Make the units agree
The checkout price tells you what you pay today. A unit price tells you what you pay for a measured quantity. Keep both figures in view, because a lower price per kilogram can still require more money than you have available for this shopping trip. Start with two similar products that already meet your needs.
Imagine oats priced at $2.40 for 750 grams or $3 for one kilogram. Dividing $2.40 by 0.75 gives $3.20 per kilogram for the smaller bag. The larger bag costs $3 per kilogram. These invented prices make the larger bag cheaper by weight, while the smaller bag needs $0.60 less at checkout.
Check what you are measuring
For food packed in liquid, decide whether the drained contents better represent the ingredient you need. For a concentrate, usable doses may be more relevant than bottle volume. Two packages can contain the same volume but produce different quantities when prepared according to their instructions.
Also consider waste. A large pack is not economical if much of it remains unused. Compare the amount you expect to consume, not just the amount printed on the label. Storage space and the household's preferences are practical constraints, not distractions from the arithmetic.
Keep a small price reference
Choose three frequently purchased products and note their usual size and unit price. You do not need a database of every item in the supermarket. A short list is enough to notice when a smaller pack has replaced the one you normally buy or a special offer is less generous than it first appears.
Check the receipt before recording the new price. Loyalty conditions, minimum quantities or an expired promotion may mean the shelf offer did not apply. Use the amount actually charged as your reference. Over time, this simple record helps distinguish a useful offer from a different package with a more attractive label.
For frequently purchased goods, keep the old receipt briefly so changes in package size and paid price can be checked together.
