What this calculator does
Find out which package is the better deal by comparing what each one costs per ounce, pound, liter or item. A bigger package isn't always cheaper, and a sale on a small size can beat the family pack. The only fair way to compare is the unit price: what you pay for each ounce, liter, pound or item. This calculator works that out for up to four products at once. It tells you which one is cheapest per unit, and by how much.
You can enter sizes in different units. Say you're comparing a 1-pound bag with a 20-ounce bag, or a 2-liter bottle with a 64-ounce jug. The calculator converts them to the same unit before comparing. So you can type what's printed on each label without converting anything by hand.
Where it's used
Why this math matters outside the classroom:
- Grocery shopping. A 24-ounce box for $4.80 costs $0.20 an ounce, cheaper than a 16-ounce box for $3.50 at about $0.22 an ounce.
- Mixed units. Converting 1.5 pounds to 24 ounces lets a shopper compare a package priced by the pound with one priced by the ounce.
- Bulk buying. Shoppers check whether a larger size really costs less per unit before buying it.
- Math class. Students find unit prices from store flyers and decide which size is the better deal.
Grade level and standards: Grade 6; Common Core State Standards for Mathematics 6.RP.A.3.b, 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter the price and the size of product A and choose its unit. That unit becomes the one every price is shown in.
- Enter the price and size of product B, and of products C and D if you have more to compare. Leave both boxes blank for a product you don't need.
- Use the same kind of unit for every product: all weights, all volumes or all item counts.
- Read the best value, its price per unit and how much cheaper it is than the next best option. The table lists every product side by side.
What the inputs mean
- Product A price
- The shelf price you'd pay for the whole package, after any discount or coupon you want to include.
- Product A size
- The amount of product inside, as printed on the package (the net contents). Product A's unit sets the unit for the comparison. Grams and milliliters are compared per 100 g or 100 mL, which keeps the numbers easy to read.
- Product B size
- This can be in a different unit from product A, as long as it's the same kind of measure. Ounces here mean ounces of weight; use fluid ounces for liquids.
- Product C price
- Optional. Fill in both the price and the size to include a third product.
- Product D price
- Leave the price and size blank to compare fewer products.
Formula and method
size in common unit = size × (size of one entered unit ÷ size of one common unit) unit price = price ÷ size in common unit percent cheaper = (other unit price − best unit price) ÷ other unit price × 100
The common unit is product A's unit. Each size is first converted to it using exact official values. 1 pound is 16 ounces (453.59237 g), and 1 ounce is 28.349523125 g. 1 US fluid ounce is 29.5735295625 mL, and 1 US gallon is 3.785411784 liters. Then each price is divided by its size.
The percent cheaper is measured against the other product's unit price. So it answers the question “how much less do I pay per unit if I choose the best option instead?” If the best option is 10% cheaper, you get the same amount for 10% less money.
Worked example
Product A is a 16 oz jar for $4.29. Product B is a 28 oz jar of the same item for $6.99.
- Product A: $4.29 ÷ 16 oz = $0.2681 per oz.
- Product B: $6.99 ÷ 28 oz = $0.2496 per oz.
- Percent cheaper: (0.268125 − 0.249643) ÷ 0.268125 × 100 = 6.89%.
Result: Product B is the better value at about $0.25 per ounce, 6.89% cheaper per ounce than product A.
Understanding the result
The product with the lowest unit price gives you the most for your money, as long as you'll actually use it all. A bulk pack that goes stale or out of date before you finish it can cost more in real life than a smaller one.
Small differences, say under 3%, often matter less than other things. Those include storage space, the brand you like or how far you have to carry it. Big differences are worth acting on, especially for things you buy every week.
When two products tie, the calculator tells you so, rather than picking one at random.
Assumptions and limitations
- The products are equally good, so only price and amount matter.
- The sizes you enter are the usable contents, not the weight including the packaging.
- All prices are in the same currency. Either all of them include the same taxes and fees, or none do.
- Weights and volumes can't be compared with each other, because that would need the product's density (how heavy it is for its size). Compare liquids by volume and dry goods by weight, as printed on the label.
- Multi-buy offers such as “3 for $5” need to be entered as the total price for the total amount.
- The calculator doesn't account for waste, shelf life or the cost of storing large packs.
Frequently asked questions
Why does the calculator use product A's unit?
Every product has to be put in one common unit before prices can be compared. The first product's unit is the natural choice, because it's usually how you think about the item. Grams and milliliters are shown per 100 g or 100 mL, so the prices aren't tiny fractions of a cent.
Is the shelf tag's unit price always right?
Shelf tags are helpful, but products side by side are sometimes priced in different units. One tag might say per ounce, and the next per pound or per count. That makes them hard to compare at a glance. Entering the price and size from each package here puts them on the same footing.
How do I compare multipacks with single items?
Choose items as the unit if you're comparing things you count, such as rolls, pods or cans of the same size. If the multipack holds a different can or bottle size, compare by total volume or weight instead. To get that, multiply the count by the size of each one.
What is the difference between percent cheaper and percent more expensive?
They compare against different starting points. Say one item costs $0.80 per unit and another $1.00. The first is 20% cheaper than the second, but the second is 25% more expensive than the first. This calculator gives percent cheaper compared with the pricier option. The table shows how much more each product costs than the best one.
How this calculator is checked
Method: Definition of unit price (price divided by quantity) with exact unit conversions for the avoirdupois ounce and pound, the US fluid ounce, quart and gallon, and metric units. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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