Unit Price Calculator

How to use the unit price calculator

Enter a price, the size of a single item, the unit that size is measured in, and how many items are in the pack. Repeat for up to four options. Rows you leave blank are simply ignored, so comparing two packages takes no more setup than comparing four.

The tool converts everything to one base measure, ranks the options from cheapest to dearest per unit, and reports how much more the worst option costs both per unit and as a percentage. Units can differ between rows: grams against pounds, or millilitres against litres, all normalise onto the same scale.

The one input people get wrong is size. It is the size of a single item, not the total in the pack, because the pack count multiplies it out. A tray of 12 cans at 330 mL is entered as size 330 with a pack of 12, which the tool resolves to 3,960 mL. Typing 3,960 with a pack of 12 would multiply it twice.

The ranking is reported in one shared unit, chosen from the sizes you entered. Anything a kilogram or a litre and up is reported per kilogram or per litre. Smaller packages report per 100 grams or per 100 millilitres instead, so the difference between two spice jars does not disappear into a rounded cent.

The reason this is hard in the aisle rather than at home is the shelf tag. Unit-price labels exist, but the unit chosen varies between retailers and even between shelves in one store, so the same product can be tagged per 100 grams here and per kilogram there. Two tags in different units cannot be compared by reading them.

Is the bigger package actually cheaper?

Often, but not reliably, and the exceptions are not rare enough to ignore. The defaults on this page are one case: the 1.5 litre bottle at $2.09 works out to $1.39 a litre while the 2 litre at $2.89 works out to $1.45. Here is a second case, three box sizes of the same dry good.

Why shelf tags are hard to compare

Unit pricing on shelf labels is a real thing and a useful one, but the choice of unit is not uniform. One retailer may tag a sauce per 100 millilitres and another per litre; a store can tag pasta per pound on one shelf and per ounce on the next. The numbers are all correct, and none of them line up.

That is the whole difficulty of doing this in the aisle. A tag per 100 millilitres and a tag per gallon are not comparable by eye, the arithmetic to join them is not the kind anyone does standing up, and the two options are usually shelved a metre apart. Reducing everything to one unit is the only step that fixes it.

The formula, worked line by line

Unit price is one division, and the only difficulty is making sure the divisor is the whole package rather than one piece of it. Multiply the size of a single item by how many are in the pack, convert that to a base unit, and divide the price by it. Every option then carries one comparable number.

The reporting unit is picked from the sizes entered rather than fixed in advance. Once any package reaches a kilogram or a litre the ranking reports per kilogram or per litre; below that it reports per 100 grams or per 100 millilitres, so a small jar does not round to the same cent as its rival.

total = size of one item × items in the pack
base  = total × base units per entered unit    (g for weight, mL for volume)
unit price = price ÷ base
reported   = unit price × 1,000   (per kg or per L)
reported   = unit price × 100     (per 100 g or per 100 mL, small packages)
premium %  = (this unit price − best unit price) ÷ best unit price × 100

When unit price is the wrong question

Unit price answers one question only: which option costs least per unit of product. It says nothing about whether you will use it all, whether it fits your storage, whether the larger size will still be good by the time you reach the bottom of it, or whether two products at the same unit price are really the same thing.

Questions people ask