How this calculator works
A price on its own says little without the quantity. Divide the total price by the number of units to get the unit price, then compare the unit prices of two packages. The lower unit price wins for any quantity, provided you can use the extra units before they spoil or before you run out of storage. The comparison also shows what package A's quantity would cost at the cheaper unit price and the difference from what you would actually pay for package A, which is the cash saving from choosing the better deal. If package A is already the cheaper option the saving is zero, because there is nothing to switch to.
Formula
Unit price = total price / quantity. For two packages, unit price A = price A / quantity A and unit price B = price B / quantity B. The lower unit price is the smaller of the two. Cost of A's quantity at that price = quantity A x lower unit price. Saving = price A - that cost.
Worked example
Package A contains 12 units for $36, so each unit costs $3.00. Package B contains 18 units for $45, so each costs $2.50. Package B is cheaper. Buying 12 units at $2.50 would cost $30, so choosing package B for that quantity saves $36 - $30 = $6. A single purchase of 10 units for $25 works out to $2.50 per unit.
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Frequently asked questions
What if the packages have different sizes?
That is exactly what the unit price handles. The packages can be any size, as long as both quantities are measured in the same unit, such as grams, millilitres or items.
Should I always buy the lower unit price?
Not always. The bigger package can cost less per unit but tie up cash or lead to waste. Compare the unit price with how much you will actually use before the product expires.
Why is the saving zero for the cheaper package?
The saving measures what you gain by switching away from package A. If package A already has the lowest unit price there is no cheaper rate to switch to, so the saving on package A is zero.