Unit price: how to actually find the better deal
Unit price is one division: price ÷ quantity. It strips away the package size and tells you what you're actually paying per ounce, per bag, per sheet — the only number that makes two different-sized packages honestly comparable.
The formula
Unit price = price ÷ quantity. Do it for every package under consideration, in the same unit, and the lowest number wins — regardless of which package looks bigger or costs more upfront.
A real example: peanut butter, two sizes
A 16 oz jar costs $4.99. A 40 oz jar costs $7.49.
- Small jar:
4.99 ÷ 16 ≈ $0.312/oz. - Big jar:
7.49 ÷ 40 ≈ $0.187/oz.
The big jar costs more upfront ($7.49 vs. $4.99) but is actually cheaper per ounce — about 40% less per ounce than the small jar. Here, "buy the bigger size" happens to be correct, but only the division proves it.
A real example: trash bags, and what the difference is worth
A 40-count box costs $9.99. A 90-count box costs $16.99.
- Small box:
9.99 ÷ 40 ≈ $0.250/bag. - Big box:
16.99 ÷ 90 ≈ $0.189/bag.
Say a household uses 360 bags a year. Buying only the small boxes: 360 ÷ 40 = 9 boxes × $9.99 = $89.91. Buying only the big boxes: 360 ÷ 90 = 4 boxes × $16.99 = $67.96. That's 89.91 − 67.96 = $21.95 saved over a year, from one recurring household item, just by buying the lower unit price consistently.
A real example: when the "value" size loses
A small spice jar, 1.5 oz, costs $3.49. A larger "value" jar, 5 oz, costs $12.99.
- Small jar:
3.49 ÷ 1.5 ≈ $2.327/oz. - Big jar:
12.99 ÷ 5 = $2.598/oz.
Here the bigger jar is the worse deal per ounce — about 12% more expensive per ounce despite the "value" label. Packaging and marketing size don't determine unit price; only the division does, and it goes against the bigger package often enough to be worth checking every time.
Why the shelf tag isn't always enough
Many stores print a per-unit price on the shelf tag, but the unit they use isn't always consistent between two sizes of the same product (per ounce vs. per 100 g, for instance), and sale prices don't always get the tag updated. Doing the division yourself on the two numbers actually in front of you — the price and the quantity on the package — is the only version that's guaranteed current and comparable.
What this doesn't capture
Unit price says nothing about whether you'll use the whole package before it spoils or goes to waste, or whether the larger size fits your storage space. A great per-unit price on a quantity you can't use before it goes bad isn't actually a deal — weigh the unit price against how much you'll realistically use.
Run your own numbers
The free Unit Price calculator runs this instantly for any two packages — entirely on your device.