What this calculator does
Unit price is price divided by quantity, and it is the only fair way to compare packs of different sizes. The larger pack is usually but not always the better rate, and the exceptions are common enough that checking is worth the few seconds it takes. Shelf labels carry unit prices for this reason, though they are not always in matching units between products.
What the comparison cannot tell you is whether the bigger pack is the better buy. A lower unit price on a size you will not finish is a false saving, and that is a judgement about your own use rather than arithmetic. The numbers here settle the rate; whether the rate is the right thing to optimise is a separate question.
The formula
Each product price is divided by its quantity to give a price per unit, and the two rates are compared directly. The saving is the difference between the rates as a percentage of the larger one. The final line prices the second product at the first product rate, which shows the comparison as a cash amount rather than a percentage.
| Term | Meaning |
|---|---|
| Unit price | Price divided by quantity, the cost of a single gram, millilitre or item. |
| Price per 100 | The same rate scaled to 100 units, which is how most shelf labels present it and easier to read than a long decimal. |
| Saving | How much cheaper the better rate is, as a percentage of the worse one. |
The inputs explained
| Field | What to enter |
|---|---|
| Product A: price ($) | The price of the first product. |
| Product A: size (g/ml) | The size of the first product, in grams, millilitres or item count. It must be the same kind of unit as the second product. |
| Product B: price ($) | The price of the second product. |
| Product B: size (g/ml) | The size of the second product, in the same unit as the first. Comparing grams against millilitres will produce a number, but not a meaningful one. |
When to use it
Comparing two pack sizes of the same product
The standard case: a small pack against a large one where the larger is not proportionally cheaper. The rate settles it, and the cash figure at the end shows what the difference actually amounts to.
Checking a multi-buy or special offer
Offers are usually presented as a discount off the pack rather than as a rate. Converting to a unit price shows whether the offer genuinely beats the everyday price of a different size.
Comparing across brands at different sizes
Two brands rarely use the same pack size, which makes the sticker prices incomparable. Reducing both to a unit price is the only way to see which is actually dearer.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
At what price does the larger pack stop being better value?
A 500 g pack at $4.50 held fixed, compared against a 1 kg pack at four different prices.
| Price of the 1 kg pack | Better value | B: per unit | Saving |
|---|---|---|---|
| $6.50 | Product B | 0.0065 per unit ($0.65 per 100) | 27.8% cheaper |
| $7.80 | Product B | 0.0078 per unit ($0.78 per 100) | 13.3% cheaper |
| $9.00 | Product B | 0.0090 per unit ($0.90 per 100) | 0.000% cheaper |
| $10.50 | Product A | 0.0105 per unit ($1.05 per 100) | 14.3% cheaper |
Questions
Is the bigger pack always cheaper per unit?
No. It usually is, but promotions on the smaller size, different packaging costs and straightforward pricing decisions all produce exceptions. The table above shows the crossover directly, and anything above it makes the large pack the worse rate.
What if the two products are measured in different units?
Convert one of them first. Comparing a price per gram against a price per millilitre gives a number with no meaning. For products sold both by weight and by volume, pick whichever unit both labels can be converted into.
Does a lower unit price always mean I should buy it?
No. A cheaper rate on a quantity you will not use before it spoils is a loss, not a saving, and the same goes for anything you have nowhere to store. The rate is one input to the decision rather than the decision itself.
Why does the calculator show the price per 100 as well?
Because raw unit prices come out as long decimals that are hard to compare at a glance. $0.0090 per gram and $0.0078 per gram are easier to read as $0.90 and $0.78 per 100 g, which is also the convention most shelf labels use.
For a straight cost-per-unit figure on a single product, see the cost per unit calculator. For comparing round items where area rather than weight is what you are buying, see the pizza value calculator.