Unit Price Calculator: Compare Grocery Package Sizes
Learn how to compare grocery package sizes using price per 100 g, liter, or item. Work through a 12 oz versus 20 oz example and avoid common unit-price mistakes.
A unit price calculator tells you what two packages cost for the same amount. Divide each shelf price by its net quantity, after converting both quantities to the same unit. The lower price per unit is the cheaper purchase by quantity. Our grocery unit price calculator does the conversion and comparison in your browser.
We checked the calculator's default example on October 29, 2026: a 12 oz package for $3.49 and a 20 oz package for $4.99. The larger package has a higher shelf price, but a lower price per 100 g.
How to compare package sizes with a unit price calculator
Enter each package's price, quantity, and printed unit. The calculator accepts ounces, pounds, grams, and kilograms for weight; US fluid ounces, milliliters, and liters for volume; and item counts. It compares weight with weight, volume with volume, or count with count.
For the default example, both packages use weight ounces:
| Package | Shelf price | Net quantity | Price per ounce | Price per 100 g |
|---|---|---|---|---|
| A | $3.49 | 12 oz | $0.2908 | $1.0259 |
| B | $4.99 | 20 oz | $0.2495 | $0.8801 |
Package B costs about 14.2% less per 100 g than Package A. The displayed percentage compares the difference with the higher unit price: ($1.0259 − $0.8801) ÷ $1.0259 × 100. Rounded unit prices in the table are for reading; the tool compares the unrounded values.
The unit price formula
For packages labeled in the same unit, use:
unit price = package price ÷ net quantity
The 12 oz package costs $3.49 ÷ 12 = $0.290833… per ounce. The 20 oz package costs $4.99 ÷ 20 = $0.2495 per ounce. To compare ounces with grams or pounds, first convert both net quantities to a shared weight unit. The calculator uses 28.349523125 g per ounce and shows the result per 100 g.
For liquids, it converts US fluid ounces and liters to milliliters, then shows price per liter. For a pack of identical items, it shows price per item. NIST's Unit Pricing Guide recommends using a consistent unit within a product category so shoppers can compare prices meaningfully.
When the lower unit price is not the better purchase
Unit price compares quantity for money. It does not account for whether you can use the whole package before it spoils, storage space, differences in quality, or how much cash you need to spend today. A 20 oz pack can have the lower unit price while the 12 oz pack still better matches what you need.
Check that you entered the net quantity, not the shipping weight or serving size. Also check that the products are comparable. A weight label and a volume label cannot be converted without knowing the product's density; the calculator will decline that comparison. The NIST guide likewise emphasizes consistent units of measure for similar products.
If a coupon or markdown changes the price, enter the actual price you expect to pay. You can first use the discount calculator to find the sale price. If tax changes the final cost, the sales tax calculator can estimate it before you compare the final prices. The unit price calculator itself does not add tax, coupons, or shipping.
Unit price calculator FAQ
Is a larger package always cheaper per unit?
No. Package size alone cannot establish value. Compare the two prices after converting quantities to a common unit. A small package on sale may have the lower unit price.
Can I compare ounces with pounds?
Yes. Choose oz for one package and lb for the other. Both are weight units, so the calculator converts them and displays dollars per 100 g.
Does a lower unit price mean I will spend less at checkout?
Not necessarily. The package with the lower unit price can have a higher total shelf price, as in the default example. Use the unit price to compare value per quantity and the shelf price to check your immediate spending.
Can I compare fluid ounces with regular ounces?
No. US fluid ounces measure volume; regular ounces measure weight. Converting between them requires the product's density, which this calculator does not assume.