Compare Unit Prices: Which Size or Brand Is Really Cheaper?
Two cereal boxes sit side by side, one at $4.20 and one at $5.85. Which is the better buy? Without the weights the question cannot be answered, and even with them it takes a short calculation. The tool that settles almost every such comparison is the unit price: the cost of one standard unit of the product.
This guide shows how to compute it, how to handle packs that use different units, why bigger is not automatically cheaper, and how to notice when a familiar product quietly gets smaller.
What a unit price is
unit price = total price ÷ quantityQuantity is weight, volume, count or number of uses.
Pick a unit that makes sense for the product, such as per 100 grams, per liter, per sheet or per wash, and use the same one for every option. Many shelf labels already show a unit price, but different products may use different units, so check that two labels match before comparing them.
Worked example: a 500 g cereal box at $4.20 costs 4.20 ÷ 5 = $0.84 per 100 g. A 750 g box at $5.85 costs 5.85 ÷ 7.5 = $0.78 per 100 g. The larger box is cheaper by (0.84 − 0.78) ÷ 0.84 = 7.1%. The percentage calculator is useful for that last step.
When the packs use different units
Convert to a single unit first. Say a drink comes in a 750 ml bottle at $3.60, a 1 liter bottle at $4.50 and a 1.5 liter bottle at $6.30. A 750 ml bottle is 0.75 liters, so the prices per liter are:
| Pack | Size in liters | Price | Price per liter |
|---|---|---|---|
| Small bottle | 0.75 | $3.60 | $4.80 |
| Medium bottle | 1 | $4.50 | $4.50 |
| Large bottle | 1.5 | $6.30 | $4.20 |
The volume converter converts between volume units, so a bottle in milliliters can be compared with one in fluid ounces. Be careful not to compare weight with volume: a price per ounce of weight and a price per fluid ounce are different measures.
Bigger is not always cheaper
Large packs usually cost less per unit, but not always, and shoppers who assume they do can end up paying more.
Worked example: a 12 oz jar costs $3.00, or $0.25 an ounce. A 16 oz jar costs $4.40, or $0.275 an ounce. The bigger jar costs 10% more per ounce. Only the unit price reveals it.
Counts work the same way. A pack of 12 rolls with 200 sheets each at $9.60 holds 2,400 sheets, which is $0.004 a sheet. A pack of 6 rolls with 450 sheets each at $9.45 holds 2,700 sheets, or $0.0035 a sheet. Compared per roll, the first looks cheaper ($0.80 against $1.575), yet the second costs less per sheet, which is what you actually use.
Spotting a shrinking pack
A price can stay the same while the contents shrink, so the unit price rises without any visible price change. Suppose a 500 g pack at $3.60 becomes a 450 g pack at $3.60. The unit price moves from 3.60 ÷ 5 = $0.72 per 100 g to 3.60 ÷ 4.5 = $0.80 per 100 g, an increase of 0.08 ÷ 0.72 = 11.1%.
Remembering a unit price, or photographing the label, makes it easier to notice. A drop in quantity of 10% leads to a price-per-unit rise of 11.1%, because the same money buys less. You can check changes like this with the percent decrease calculator.
When the lowest unit price is not the best buy
- Waste. A cheaper bulk pack of fresh food is no bargain if part of it spoils before you use it. Divide by what you will actually use, not by what is in the pack.
- Strength or concentration. Concentrated products need less per use, so compare cost per use rather than per volume.
- Storage and cash flow. A very large pack ties up money and space. For an item you use slowly, the savings may not justify it.
- Quality differences. Two brands with the same unit price may not perform equally. Unit price tells you the cost, not the value.
Combine unit price with a discount and the result changes again. If you want to price a coupon on top of a size, see dollars off or percent off for how to compare the offers.
These examples use invented prices and are for illustration only. Always compare the actual prices and quantities on the shelf or the website.
- Percentage Calculator: Pick the question you need answered, enter two numbers and get the result instantly. The guide below shows the formula behind each mode.
- Volume Converter: Pick a number and two units to convert between 25 metric, US and imperial volume units, from cubic kilometers down to teaspoons. The answer updates as you type.
- Percentage Decrease Calculator: Choose whether you want to take a percentage off a number or find the percentage drop between two values, fill in the fields and press Calculate. The guide below shows the formula and several worked cases.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
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