How to use the unit price calculator
- Enter each item’s price, the price you would pay at the register, including any sale or coupon you will actually use.
- Enter the packs and the size of each. A single jar is 1 pack. A 6-pack of 12 fl oz cans is 6 packs of 12 fl oz. A bundle of 12 rolls with 90 sheets each is 12 packs of 90, measured by count.
- Choose the unit printed on the label: oz, lb, g or kg for weight; fl oz, pint, quart, gallon, mL or L for volume; or each for a count.
- Pick what to compare by, such as per oz or per 100 g. All items are converted to that one unit, the way a shelf tag should show them.
The results show each item’s unit price, which one is lowest, and the percentage more each of the others costs per unit. Add a fourth item, or remove one, with the buttons under the list.
The unit price formula
% more than the lowest = (unit price ÷ lowest unit price − 1) × 100
The calculator opens on three jars of peanut butter. A 16 oz jar at $2.79 costs 2.79 ÷ 16 = $0.174375 per ounce, shown as 17.4¢. A 40 oz jar at $5.99 costs 5.99 ÷ 40 = $0.14975, shown as 15.0¢. A twin pack of 28 oz jars at $9.50 is 9.50 ÷ 56 = 17.0¢ per ounce. The 40 oz jar is lowest, and the small jar costs 0.174375 ÷ 0.14975 − 1 = 16.4% more per ounce.
Switching the comparison to per lb multiplies each figure by 16: the 40 oz jar is 0.14975 × 16 = $2.396, or $2.40 per lb. The ranking doesn’t change with the unit, only the size of the numbers.
How unit prices are rounded
The National Institute of Standards and Technology (NIST) publishes a unit pricing guide for retailers, SP 1181. Its 2025 edition recommends showing unit prices of a dollar or more to the nearest cent, such as $1.03, and unit prices under a dollar either to the nearest whole cent (13¢) or to the nearest tenth of a cent (13.1¢), choosing one method and using it consistently. It also asks for standard rounding: 13.5¢ rounds up to 14¢ and 13.4¢ rounds down to 13¢.
The calculator follows the same rules, with a switch for tenths of a cent. Tenths help when items are close: two products at 15.04¢ and 15.46¢ both show as 15¢ in whole cents, but as 15.0¢ and 15.5¢ in tenths. The comparison percentages use the unrounded figures.
Ounces, fluid ounces and metric labels
Ounces and fluid ounces are different things. An ounce (oz) is a weight; a fluid ounce (fl oz) is a volume. Cereal and peanut butter are sold by weight, juice and shampoo by volume. The calculator won’t compare an item in oz with one in fl oz and marks it as not comparable, because the conversion depends on the density of the product.
Conversions use exact definitions. One ounce is 28.349523125 grams and one pound is 16 ounces, or 453.59237 grams. One US fluid ounce is 29.5735295625 milliliters, and a US gallon is 128 fl oz, or 3.785411784 liters. NIST lists these factors in Appendix B of its guide to SI units.
Example with mixed labels. A 64 fl oz bottle of juice at $3.99 against a 1 L carton at $2.19: 64 fl oz is 64 × 29.5735 = 1,892.7 mL, so the bottle costs 3.99 ÷ 1.8927 = $2.11 per L, which is 8¢ per liter less than the carton.
The NIST guide asks stores to use one unit throughout a product category, so every bottled water is priced per fluid ounce, say, rather than some per quart and some per 100 mL. When shelf tags in the same aisle use different units, enter the sizes here and compare in one unit.
Multipacks, counts and sale prices
Multipacks. Enter the number of packs and the size of one. Twelve 12 fl oz cans are 12 × 12 = 144 fl oz; the calculator does that multiplication.
Counts. Some products only make sense per piece or per 100 pieces. Paper towels are a good case: 6 rolls of 120 sheets for $8.99 is 720 sheets, or $1.25 per 100 sheets, while 12 rolls of 90 sheets for $14.49 is 1,080 sheets, or $1.34 per 100 sheets. The bigger bundle has more rolls but fewer sheets per roll, and costs more per sheet. Comparing by roll would have given the opposite answer.
Multi-buy offers. For “2 for $5” on a 12 oz item, enter a price of $5.00 with 2 packs of 12 oz: 5 ÷ 24 = 20.8¢ per ounce. A 20 oz package at $3.79 is 3.79 ÷ 20 = 19.0¢, still lower than the offer, by about 9%. A sale sign is no proof of a lower unit price than a larger regular-price size.
When the lowest unit price isn’t the better buy
- Waste. A larger size saves money only if you use it before it spoils, goes stale or expires. Paying 16% more per ounce for a jar you’ll finish can cost less than throwing out a third of a bigger one.
- Storage and cash. Bulk sizes take space and money up front. If either is tight, the small-size premium may be worth paying.
- Different products. Two detergents with different concentrations aren’t comparable by volume. Compare by loads or uses instead, by entering the number of loads as a count.
- Drained weight. Canned fruit and vegetables may list a drained weight as well as the net weight. Comparing drained weights shows what you actually eat.
For sellers: unit prices on your own labels
If you sell by weight or volume, the same calculation gives the unit price for a shelf tag or product page. NIST SP 1181 recommends using the word “per” between price and unit, as in $1.99 per lb, rather than a slash, and placing the unit price as close as practical to the selling price. The same applies online: on an e-commerce page the unit price should sit in a prominent spot near the price, so shoppers can compare sizes without doing the division. Some states require unit pricing in certain stores, so check your state’s weights and measures office for local rules.
Sources & assumptions
How this calculator gets its numbers. Full method: our methodology.
- Unit price = price ÷ (number of packs × size of each) in the unit you compare by.
- Ounces (oz) are weight and fluid ounces (fl oz) are volume; items measured by weight and by volume aren’t compared.
- Conversions use exact definitions: 1 oz = 28.349523125 g, 1 lb = 16 oz, 1 US fl oz = 29.5735295625 mL, 1 US gallon = 128 fl oz.
- Prices of $1 or more are shown to the cent; under $1, to the cent or a tenth of a cent, with 5 or more rounding up.
Sources
- National Institute of Standards and Technology, NIST SP 1181, Unit Pricing Guide (2025 edition). Unit price as the price per unit of measure; one consistent unit across a product category; unit prices of $1 or more to the nearest cent and under $1 to the nearest cent or tenth of a cent; 5 or more rounds up (13.5¢ → 14¢).
- National Institute of Standards and Technology, NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically. Conversion factors: 1 oz (avoirdupois) = 28.34952 g, 1 lb = 0.4535924 kg, 1 U.S. fl oz = 29.57353 mL, 1 U.S. gallon = 3.785412 L.
Last reviewed . A lower unit price only helps if you’ll use the larger amount before it spoils or expires.
Frequently asked questions
How do you calculate unit price?
Divide the price by the total quantity in one unit. A 40 oz jar at $5.99 is 5.99 ÷ 40 = $0.14975 per ounce, or about 15¢. For multipacks, multiply the packs by the size of each first.
Is price per ounce the same as price per fluid ounce?
No. Ounces measure weight and fluid ounces measure volume, so they can’t be compared directly. Compare weight items with weight items and liquids with liquids.
How do I compare a metric size with a US size?
Pick one unit and convert both. A 64 fl oz bottle is 1.8927 L, so at $3.99 it costs $2.11 per liter, which you can compare with a 1 L carton’s price directly.
How should unit prices under $1 be rounded?
NIST’s unit pricing guide recommends either the nearest whole cent or the nearest tenth of a cent, used consistently, with 5 or more rounding up. Prices of $1 or more go to the nearest cent.
Is the bigger package cheaper per unit?
Often, but not in every case. Sale prices on small sizes, bundles with smaller pieces, and multi-buy offers can all reverse it, so compare the unit prices rather than the package sizes.
How do I compare paper towels or toilet paper?
Use a count: enter the number of rolls as packs and the sheets per roll as the size, measured as each, then compare per 100 items. Comparing by roll ignores that rolls differ in size.