How to Compare Unit Prices
A $4 package costs less at checkout than a $6 package. That does not tell you which one gives you more for your money. If the first contains 10 ounces and the second contains 20 ounces, the higher-priced package gives you twice as much product for only half as much additional spending.
Unit price separates the amount you pay from the amount you receive. It answers a specific question: what does one ounce, one liter, or one item cost? Once both options use the same measure, you can compare their prices directly. The examples below use illustrative prices, not current store offers.
Start with price divided by quantity
Unit price = price ÷ total quantity
For the 10-ounce package, $4 ÷ 10 = $0.40 per ounce. For the 20-ounce package, $6 ÷ 20 = $0.30 per ounce. The larger package costs 10 cents less for every ounce. If you need and use 20 ounces, buying two small packages would cost $8, compared with $6 for one large package.
This distinction matters: the large package is cheaper per ounce, but it still takes $2 more out of your budget today than one small package. Unit price measures value by quantity. It does not decide how much you need or how much you can spend.
Use the price that buys the amount you enter. A price for an entire case must be paired with the contents of the entire case. A price for one bottle must be paired with that bottle's contents. Mixing a case price with one bottle's volume makes the result far too high.
Choose a measure that describes the product
Weight works for products sold by net weight, such as rice or flour. Volume works for liquids labeled in liters, milliliters, or fluid ounces. Count works when individual items are meaningfully comparable, such as identical batteries or dishwasher tablets. For ribbon or similar products, price per length may be useful.
Read the quantity label carefully. An ounce of weight and a fluid ounce of volume are different measures. You cannot compare them by simply removing the word “fluid.” Likewise, a bottle is a container, not a fixed volume. A 500 mL bottle and a 750 mL bottle should normally be compared per mL or liter, rather than per bottle.
Count comparisons also need judgment. Two rolls can contain different numbers of sheets, and two sheets can have different dimensions. A lower price per roll is not automatically a lower price for the same amount of paper. Use the most useful shared measure the packaging provides, then consider product differences separately.
Convert different units before dividing
Ounces and pounds
There are 16 ounces in one pound. Suppose a 12-ounce bag costs $3.60 and a 1-pound bag costs $4. The first costs $3.60 ÷ 12 = $0.30 per ounce. Convert the second bag to 16 ounces, then divide $4 ÷ 16 = $0.25 per ounce. The pound bag has the lower unit price.
You could instead compare both per pound. The 12-ounce bag is three-quarters of a pound, so $3.60 ÷ 0.75 = $4.80 per pound. The other bag is $4 per pound. Either common unit gives the same winner.
Grams and kilograms
A kilogram contains 1,000 grams. A 750 g bag priced at $4.50 costs $0.006 per gram, or $0.60 per 100 g. A 1 kg bag priced at $5.80 costs $0.0058 per gram, or $0.58 per 100 g. The kilogram bag is slightly cheaper by quantity.
Small units can produce awkward decimal values. Multiplying both prices by 100 to express them per 100 g makes them easier to read without changing their relationship. Do not compare one shelf label stated per 100 g with another stated per kilogram as though the numbers use the same basis.
Liters and milliliters
A liter contains 1,000 milliliters. A 750 mL bottle at $3 costs $4 per liter because $3 ÷ 0.75 L = $4/L. A 1.5 L bottle at $5.40 costs $3.60 per liter. The larger bottle wins on volume, provided the products are suitable substitutes.
The calculator converts compatible units automatically. It supports ounces with pounds and grams with kilograms, but does not automatically bridge U.S. weight and metric weight. For a bag marked only in grams and another marked only in ounces, first express both quantities in one supported weight group. See the methodology for exact supported groups and volume conventions.
Compare counts and multipacks without counting twice
A pack of 18 identical items for $5.40 costs $0.30 per item. A pack of 24 for $6.48 costs $0.27 per item. You can enter each total count directly as Amount in Unit with the multipack option off.
If a bundle contains four boxes with 18 items in each, its total quantity is 4 × 18 = 72 items. With “This is a multipack” enabled, enter 4 under “Number of items in pack” and 18 under “Amount in Unit.” Enter the price for all four boxes. Do not enter 72 per box and also multiply by four.
Decide whether a small difference matters
Return to the rice example: $0.60 versus $0.58 per 100 g is a difference of two cents per 100 g. Over one kilogram, that becomes 20 cents. That is a more useful decision figure than treating any lower unit price as a major saving.
When looking at the calculator, remember that displayed unit prices round upward to the next cent. Two displayed prices can look identical even though the underlying values differ. The winner is chosen from the unrounded calculations. Our methodology explains this display behavior; it is especially noticeable with grams and milliliters.
Use the result alongside your shopping needs
The cheapest unit price may belong to a quantity that will spoil, a product you dislike, or a package that does not fit your storage space. A concentrated cleaner may also need a different dose, so price per bottle alone does not tell you cost per use. Unit price is strongest when the products deliver comparable usefulness.
Try the Which Deal's Better? calculator with the prices and quantities on your packages. For a size decision, continue with bigger versus smaller packages. If the price depends on a Buy X Get Y promotion, use the BOGO calculator or read the multipack and BOGO guide.