Unit price calculator

The shelf price tells you what a package costs, not whether it's a good deal. A bigger box costs more in absolute dollars but might give you far more — or far less — for each dollar. The only honest comparison is the unit price: the cost of a single unit, like one ounce, one gram, or one item. Enter the price and quantity for two options and this calculator divides it out, tells you which one wins, and shows exactly how much you save by switching.

How this calculator works

For each option it divides the price by the quantity to get the unit price, then compares the two. Unit prices are often a small fraction of a dollar, so the result is shown to four decimal places — otherwise two close prices would round to the same number and look identical when they aren't. The option with the lower unit price wins, and the "Cheaper by" figure is the gap measured against the pricier option, so a 10% saving means the winner costs 10% less per unit than the loser.

A worked example: peanut butter

A 12 oz jar is $3.49 and a 28 oz jar is $6.99. The small jar works out to $0.2908 per ounce ($3.49 ÷ 12); the large jar is $0.2496 per ounce ($6.99 ÷ 28). The big jar is cheaper by 14.16%. On a single jar that's a few cents, but if peanut butter is a weekly staple, buying the size that's 14% cheaper per ounce is the difference between paying for ten jars or eleven over a year.

When the bigger size is the trap

"Buy the bulk pack" is good advice often enough that people stop checking — which is exactly when stores catch them. Take toilet paper: a 6-roll pack at $4.99 is $0.8317 per roll, while the 12-roll pack at $11.49 looks like the obvious bulk buy but lands at $0.9575 per roll. The smaller pack is actually cheaper by 13.14%. This happens because the small size is on promotion, because "value" packs carry a convenience markup, or simply because the bigger number on the front of the box does the persuading. The unit price is the only thing that settles it.

Watch the unit on the label

The single most common mistake is comparing two numbers that aren't in the same unit. A jar priced per ounce and a bag priced per gram will give you a nonsense answer if you type the raw numbers in. One ounce is about 28.35 grams, so a package priced at $0.50 per ounce is roughly $0.0176 per gram — the per-gram figure looks far cheaper even though it's the same product. Before you compare, get both options onto the same unit: ounces with ounces, grams with grams, sheets with sheets, items with items. Stores are not required to print unit prices in matching units, and they often don't.

Good to know

• The quantity can be anything countable: weight (oz, g, lb), volume (fl oz, mL, L), or count (sheets, capsules, individual items).
• For multipacks, the unit you usually care about is the smallest one — price per roll, per pod, per diaper — not price per pack.
• Coupons and loyalty discounts change the real price. Enter the price you'll actually pay at the register, not the shelf tag, when a discount applies to only one of the options.
• Unit price ignores quality, freshness, and whether you'll use it all. The cheapest per-ounce option is a bad deal if half of it spoils before you finish it.

Frequently asked questions

Last reviewed June 2026.