Unit Price Calculator

What a unit price is, in one division

A unit price is the price divided by the amount in the package. That is the whole idea, and it is the only way to compare two packages that hold different amounts. A $3.49 jar holding 12 ounces works out to $3.49 divided by 12, which is $0.2908 per ounce. A $6.99 jar holding 24 ounces works out to $0.2913 per ounce.

Those two numbers are almost identical, and that is the point of doing the division. The bigger jar costs twice as much money and holds twice as much product, so the shopper who grabs it because it looks like the value buy has gained nothing. Unit price strips the package size out of the decision and leaves one number you can rank.

Supermarkets already print this number on the shelf tag in most states, usually in small type in the bottom corner, in dollars per ounce or cents per ounce. Reading it takes a second and it beats doing mental division in the aisle. The catch is that the tag does the math for one unit and rounds it, so two products that differ by a fraction of a cent can look tied.

Worked example: the 12 ounce and 24 ounce jar

Put the two jars side by side and do the long division.

• 12 ounce jar at $3.49: 3.49 / 12 = $0.29083 per ounce
• 24 ounce jar at $6.99: 6.99 / 24 = $0.29125 per ounce
• Difference: 0.29125 minus 0.29083 = $0.00042 per ounce

The small jar is cheaper per ounce by four hundredths of a cent, which is 0.14%. Buy 48 ounces of product either way and the totals are $13.96 from four small jars against $13.98 from two large ones. Two cents separates them, and the small jars give you four lids to open instead of two.

Now round the unit prices the way a shelf tag would. At 3 decimals both jars read $0.291, so the tag shows a tie. At 2 decimals both read $0.29, and the comparison disappears entirely. The ranking only appears at the fourth decimal, which is why a unit price column that rounds early is close to useless on cheap products.

The big pack is not always cheaper

Volume discounts are a pricing habit, not a rule. Retailers know that shoppers assume the larger package is the better deal, so the middle size is often the one with the worst unit price. The four sizes below are the same product on the same shelf, priced the way a store might price them.

The 24 ounce jar is the interesting row. It holds twice what the 12 ounce jar holds and costs exactly twice as much, so the store gave away nothing for the larger purchase. The 40 ounce jar at $9.99 breaks the pattern and lands at $0.2498 per ounce, 14% below the 12 ounce jar. A shopper who only compares the 12 and the 24 walks out with the worse of the two, and a shopper who compares all four buys the 40 ounce.

This pattern shows up most in the sizes stores push hardest: the one at eye level, the one with the coupon tag, the one in the special display at the end of the aisle. The unit price is the only number on the shelf that does not care where the product is sitting.

How many decimals you need before the ranking flips

Rounding is not a detail here, because the differences between packages are often smaller than a cent. The same two jars produce a different answer at every rounding level.

A workable rule for the aisle: use 4 decimals below $1 per unit, 3 decimals from $1 to $10, and 2 decimals above $10. That keeps the gap visible whether the product costs 29 cents an ounce or $12 a pound. The calculator on this page follows that rule automatically, which is why the two jars above read $0.2908 and $0.2913 rather than a pair of identical $0.29 tags.

Two cautions. A gap under 1% is not worth a second trip or a bigger pantry, so treat near ties as ties. And a rounded number should never be multiplied back out to a total, because 40 ounces at a displayed $0.25 is $10.00 while the real total is $9.99.

Weight ounces and fluid ounces are not the same

The ounce on a bag of flour and the ounce on a bottle of oil are different units that share a name. A weight ounce is 28.35 grams. A fluid ounce is a volume, 29.57 milliliters. Comparing the two directly is the most common unit price mistake in the grocery store, and it is why a 16 fluid ounce bottle can look like a worse deal than a 16 ounce jar when the two hold different amounts of material.

Water is the one case where the numbers almost match: 16 fluid ounces of water weighs about 16.7 ounces, because a milliliter of water weighs a gram. Oil is lighter, at roughly 0.92 grams per milliliter, so 16 fluid ounces of oil weighs about 15.3 ounces. A syrup at 1.33 grams per milliliter goes the other way, and 16 fluid ounces of it weighs about 22.2 ounces.

If you need to cross the line, convert one side first: fluid ounces times 29.57 gives milliliters, and grams divided by 28.35 gives ounces. The NIST unit conversion reference lists the exact factors. If you do not know the density of the product, do not compare across the two families at all, because the answer depends on what is inside the package.

What the shelf tag is required to show

Unit pricing rules are state law, not federal law, so what you see on the tag depends on where you shop. The model most states start from is the NIST Office of Weights and Measures handbook, which defines how a unit price is computed, which units are allowed, and how the number is displayed. States that adopted a unit pricing regulation usually require the price per unit in a type size that can be read from arm's length.

Three things vary by state and by chain. The unit chosen can be an ounce, a pound, 100 sheets or a single item. The rounding can be to the cent or to a tenth of a cent. And the rule may apply only to stores above a certain size, which is why a small market can leave the unit price off entirely.

Where the tag is missing, the division is easy enough on a phone, and the number is worth having. Grocery prices move for reasons that have nothing to do with package size, and the USDA food price outlook tracks those moves at the category level. A product that was the value buy last month can lose that spot after a single price change, because the ranking depends on both the price and the size.

Counts, sheets, loads and other units that hide the math

Not every product is sold by weight or volume, and the packages that are sold by count are where unit price does the most work. The trick is to pick one unit and convert both packages to it before comparing.

Paper towels are the classic trap. A package that says 6 rolls tells you nothing, because roll sizes range from 40 sheets to 160. Six rolls of 120 sheets is 720 sheets, so $9.49 divided by 720 is $0.0132 per sheet, or $1.318 per 100 sheets. Toilet paper at 12 rolls of 264 sheets is 3,168 sheets, which works out to $0.378 per 100 sheets and about $1.00 per roll. The two products are not comparable to each other, but two paper towel packages are, once both are converted to sheets.

Laundry detergent is the easiest case in the store, since the load count is printed on the front. It is also the easiest to fake: a concentrated formula that claims 64 loads at a smaller scoop is not the same as 64 full washes. Read the load count and the dose size together, and treat a shrinking dose as a price increase hiding in plain sight.

When unit price is the wrong tool

Unit price answers one question: which package costs less per unit. It does not answer whether you should buy the package at all. Four cases where the cheapest unit price loses.

• Spoilage. A 40 ounce jar at the best unit price is a bad buy if half of it goes in the trash. The real unit price of a jar you throw away is double what the tag says.
• Storage. Bulk only pays if you have the shelf, the freezer space or the pantry to hold it. A second refrigerator costs more than a year of small-package premiums.
• Coupons and buy-two deals. A buy-two-get-one-free offer changes the effective unit price, and the discount usually applies to the smaller size. Work out the unit price after the discount before you decide.
• Price per use. Two brands can match on price per ounce and differ on how much you use per task. That is a cost-per-use question, not a unit price question.

Set the two packages you are holding into the calculator, pick the unit from the list, and read the gap. If the gap is under 1%, buy whichever one you will finish. If the gap is 10% or more and the product keeps, the math is worth following.

Prices and package sizes change constantly, so treat the output as a comparison of the numbers you entered, not as a claim about any store's current price. All calculations run in your browser, and nothing you type is uploaded or stored.