How this calculator works
Package sizes change and so do prices, so the shelf price says little about value. A unit price converts every package to the same basis: dollars per kilogram for weight, dollars per litre for volume, or dollars per item for a count. Comparing two unit prices is then a simple comparison, and the percentage difference tells you how much better one really is. The basis must match: comparing a price per kilogram with a price per litre, or a weight with a count, gives a meaningless answer. This is why the calculator asks which kind of product you are comparing.
Formula
unit price = price / size, where size is in kilograms, litres or items. difference = |unit price A - unit price B| / the larger unit price.
Worked example
A 500 g pack at $4.50 costs $9.00 per kilogram. A 750 g pack at $6.00 costs $8.00 per kilogram. The second costs $8.00 per kilogram against $9.00, so it is 11.1 percent cheaper per kilogram, a saving of $1.00 for every kilogram bought.
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Frequently asked questions
Can I compare a price per kilogram with a price per litre?
Only if you know the density of the product, which converts between them. For water one litre is one kilogram, but for oil, honey or flour the two are different. Compare like with like.
Is the biggest pack always the best value?
No. The unit price can be higher on a large pack, especially with promotions on smaller sizes. Check the unit price on the shelf label, which is required in many Canadian stores, rather than assuming.
Does this account for waste?
No. A large pack that goes stale or gets thrown away costs more per usable portion. If you will not use it all, compare the cost per serving rather than per kilogram.
What about multi-buy offers?
Enter the total price and the total size of the offer. Two for $5 becomes a price of $5 with double the size, and the unit price calculation handles it.