Bigger isn't always a better deal. Enter price and quantity for each option to see the true cost per unit.
Use the same unit (oz, lb, ct, etc.) for every product you're comparing.
Add one row per package you are comparing, then type the price as it appears on the shelf and the quantity in the package. The calculator divides price by quantity for every row and ranks them, so the cheapest unit price is the one worth buying. The important step is unit consistency: if one row is in ounces, make every row ounces. A 2-pound bag entered as "2" against a 32-ounce bag entered as "32" will look like a sixteen-fold bargain that does not exist.
For multi-buys, enter the total you actually hand over. "3 for $10" is a price of 10 and a quantity of 3. For a product sold by count — trash bags, diapers, tea bags — the quantity is the count, and the unit price becomes cost per bag rather than cost per ounce. Either way the ranking works.
Three jars of the same peanut butter on one shelf:
| Package | Price | Unit price |
|---|---|---|
| 16 oz | $3.49 | $0.218 per oz |
| 24 oz | $4.99 | $0.208 per oz |
| 48 oz | $11.49 | $0.239 per oz |
The 24-ounce jar wins at 20.8 cents an ounce. The 16-ounce is 1 cent an ounce worse, which sounds trivial until you do the shopping-list version: 48 ounces costs $10.48 in 16-ounce jars and $9.98 in 24-ounce jars, a 50-cent difference on one item. The 48-ounce "family size" is the most expensive per ounce of the three, at 23.9 cents. That is the pattern worth watching for: big packaging is priced as convenience, not as a discount.
A unit price is a rate, and rates compare cleanly only when the thing being measured is the same. Two boxes of cereal at $0.31 and $0.28 per ounce are comparable; a box of cereal and a bag of flour are not, because you do not use an ounce of flour the way you use an ounce of cereal.
The second thing the number does not know is waste. A 48-ounce jar that goes stale half-finished has a real cost higher than 23.9 cents an ounce. If you use roughly 20 ounces a month, the 24-ounce jar is the right size and the 48-ounce is a slow-motion loss. The calculator tells you which package is cheapest per ounce; deciding whether you will finish it is the part that needs you.
Sales distort the ranking for exactly as long as the sale lasts. Re-enter the shelf price when the tag comes down, or the ranking you remember will be wrong on the next trip.
You can skip it. Tax is a percentage of the price, so it scales every row by the same factor and never changes which one wins. It only matters if the items are taxed differently, which happens with food versus non-food in some states.
Because shelf space and the assumption that shoppers will not divide are both worth money. The number on the tag is a price, not a promise of value.
Convert one to the other first. Sixteen ounces make a pound, so 2 pounds is 32 ounces and the two are directly comparable. Mixing pounds and ounces in the same comparison is the most common way to get a wrong answer.
Yes, and it is often better. For laundry detergent, enter the number of loads the bottle claims and the unit price becomes cost per load. Claims vary in honesty, so treat the bottle's number as the manufacturer's estimate.
Many stores charge the full unit price for a single item. If yours does, the honest comparison is one item at $1.29 against one item at $1.00 elsewhere; the 10-for-$10 price only exists if you take all ten.
Unit price is price divided by quantity. NIST’s handbook material is the weights-and-measures context for how stores are supposed to state quantity.
Last updated: November 2, 2026. Reviewed by David Liu.