Unit Price Calculator

What this unit price calculator does

You are standing in front of two versions of the same thing. One is a big bag, one is a multipack, and they are priced so that comparing them in your head is annoying. That is not an accident.

This turns any pack into a price per unit, which is the only number that lets you compare them honestly. Enter the price, how many items are in the pack, and the size of each item, and it gives you the cost per unit of size. Work in pounds throughout and that is a price per pound.

Then run the other option and compare the two figures. That is the whole method, and it takes about fifteen seconds per product.

It runs in your browser. Nothing typed here is stored or sent anywhere.

How to use it

1. Total Price. What the whole pack costs at the till.
2. Count in Pack. How many separate items are inside. A single bag is 1. A six pack is 6.
3. Size. The size of each item, in pounds. Not the total weight of the pack, the weight of one of them.

Press Calculate. Press Reset to clear it.

The count and size fields together are what let it handle multipacks properly. A single 2 lb bag is count 1, size 2. Three 1.5 lb packets is count 3, size 1.5. Both give you a price per pound and the two are directly comparable.

One thing about what it accepts. The size unit is yours to choose, but it has to be the same one on both sides of a comparison, so anything measured differently needs converting first and there is a table below for that. Sizes below 1 are fine: a 12 ounce box is 0.75 lb, so enter count 1 and size 0.75 and you get the price per pound directly.

The one line of arithmetic

Unit price = total price ÷ count ÷ size

Or, the same thing said differently, the total price divided by the total quantity in the pack. The count and size are separated out only because packaging usually gives you the two numbers separately rather than the product.

What makes this worth doing rather than eyeballing is that supermarkets vary all three numbers at once. A slightly different pack size, a slightly different count, a price that ends in 99, and suddenly two products that differ by 20 percent in value look roughly the same. Reduce both to one number and there is nowhere for that to hide.

Three packs, one shelf

Same product, three ways to buy it:

The three pack is the best value by a wide margin, at roughly 62 percent of the price per pound of the large bag and less than half the small box. But it has the largest number on the shelf label, which is exactly why people reach past it.

The small box is more than twice the unit price of the three pack. That is the convenience premium, and on some products it is worth paying. The point is knowing what you are paying rather than assuming.

Getting everything into the same unit first

The single biggest source of wrong answers here is comparing a price per pound against a price per kilogram and not noticing. So convert first, then compare.

The pound and kilogram conversion is worth committing to memory, because a kilogram is a bit over two pounds and it is very easy to treat them as roughly the same. A price that looks 10 percent better per kilogram than per pound is actually about 45 percent worse.

For liquids you will meet fluid ounces and litres, and for anything sold by count you can ignore weight entirely and just use price divided by count. The method does not care what the unit is as long as both things you are comparing use the same one.

The big pack is not always cheaper

Most people assume larger equals better value, and most of the time that holds. It is not a rule, though, and supermarkets know that you think it is.

Three situations where the bigger pack loses.

A promotion on the small one. A two for one or a temporary cut on the standard size regularly beats the family pack, sometimes by a lot. The shelf label showing unit price is the giveaway, when the shop provides one.

Premium packaging on the large size. Resealable tubs, dispensers and handled jugs cost money and it lands in the price. You are paying for a container.

Waste. This is the one no calculator can price for you. A five pound bag at half the unit price of a one pound bag is only cheaper if you use all five pounds. If two of them go in the bin, your effective unit price was worse than the small bag and you also had to store it.

So the honest version of the method is: work out the unit price, then ask yourself whether you will genuinely use the whole thing. The first part is arithmetic and this tool does it. The second part is knowing your own household, and it matters at least as much.

Questions people ask

How do I handle a multipack?

Put the number of items in the count field and the size of one item in the size field. Six 1 lb bags is count 6, size 1. The tool multiplies them to get the total quantity.

The size field will not accept anything under 1. What do I do?

Scale both numbers up by the same factor. Four 12 ounce boxes at $4.29 each is $17.16 for 3 lb, so enter 17.16, count 1, size 3. The unit price comes out the same.

Does it have to be pounds?

The label says pounds but the arithmetic does not care. Enter kilograms, litres or ounces and read the answer as price per that unit. Just keep both products you are comparing in the same unit.

Which currency?

Any. The dollar sign is cosmetic and the division works identically in any currency.

What about things sold by count, like eggs or batteries?

Put the count in the count field and 1 in the size field. You will get a price per item.

Why not just read the shelf label?

Use it when it is there and legible. It is often in small print, sometimes uses a different unit from the product next to it, and on promotional stock it is not always updated. This is for checking.

References

A note on the conversions used above. The relationship between the pound and the kilogram is fixed by international agreement at exactly 0.45359237 kilograms to the pound, and the avoirdupois ounce is one sixteenth of that. Both are documented by the National Institute of Standards and Technology.

1. National Institute of Standards and Technology, Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811