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MathA 500 g box for $4.80 or a 750 g box for $6.90? Different brands, different sizes, different prices — the shelf makes honest comparison nearly impossible by design. Unit price cuts through it: one number that makes every option comparable.
This guide shows the two-second formula, how to handle mismatched units, and the retail traps that unit pricing exposes.
Try it yourself: Compare any two prices and sizes side by side. Percentage Calculator → · Convert units with our Unit Converter →
The formula
Unit price = total price ÷ quantity
— in a consistent unit: per 100 g, per oz, per liter, per roll —
— in a consistent unit: per 100 g, per oz, per liter, per roll —
The 500 g box at $4.80: 4.80 ÷ 5 (hundred-gram units) = $0.96 per 100 g. The 750 g box at $6.90: 6.90 ÷ 7.5 = $0.92 per 100 g. The bigger box wins — but only just, and now you know exactly how much better it is: 4 cents per 100 g, or about 40 cents on the full purchase.
Reading the shelf tag
Most US and many EU retailers already print unit prices on the shelf edge — a small line like "$2.15 / lb" or "$0.80 / 100 g" under the headline price. It is legally required in much of Europe and many US states. The catch: online stores and warehouse clubs often omit it, and the units differ product to product (per oz on one brand, per lb on another), which defeats the comparison.
When the tags don't cooperate, normalize to the same unit before dividing. Per 100 g or per liter is usually cleanest for groceries; per roll or per sheet for paper products.
Three traps unit pricing exposes
- Shrinkflation: the package keeps its size and price but quietly drops from 500 g to 450 g. Headline price unchanged; unit price jumps 11%. Only the unit-price line catches this.
- The bulk illusion: the biggest pack isn't always cheapest per unit. Retailers sometimes price the mid-size as the loss leader while the jumbo costs more per ounce. Never assume — compute.
- The "bonus pack" trick: "20% more free!" on a product whose unit price was quietly raised the same week. Compare unit prices before and after, not slogans.
Beyond groceries: the same math everywhere
Unit pricing generalizes to any "price ÷ amount" comparison:
- Cost per use: a $24 razor lasting 60 shaves ($0.40/use) vs a $12 one lasting 20 ($0.60/use) — the expensive option is 33% cheaper per use.
- Price per GB: comparing cloud storage and phone plans by dollar-per-gigabyte.
- Cost per mile: running costs of two cars, or per-day cost of a subscription you'll use twice a month.
Pair it with sales tax math when comparing taxed totals, and watch for perishables: the cheapest unit price on milk you won't finish is the most expensive option of all.
Frequently asked questions
Divide the total price by the quantity in a consistent unit: unit price = price ÷ quantity. A $4.80, 600 g jar costs $4.80 ÷ 6 = $0.80 per 100 g. The lower unit price wins, all else equal.
No. Stores sometimes price mid-size packs lower per unit than the largest size, and shrinkflation quietly shrinks the big pack while keeping the price. Always compute the unit price rather than trusting the package size.
Most US and EU retailers print it on the shelf tag — a small line like "$0.80 / 100 g" or "$2.15 / lb" under the main price. It is legally required in many jurisdictions, but online stores often omit it.
Convert to a common unit first, then divide. A 500 g pack at $4.80 versus an 18 oz (≈510 g) pack at $5.10: compute each per-100 g price and compare — $0.96 vs $1.00 per 100 g here.
Disclaimer: Calculator content is for education only — examples are illustrative and rounded; double-check figures used in financial or contractual decisions.