Unit price is the simplest idea in retail math and the most frequently botched. The formula is price divided by quantity, expressed in a normalized base unit: dollars per gram, dollars per litre, dollars per sheet, dollars per count. The trap isn't the division — it's the normalization. Stores round differently, use different base units on similar products, and sometimes print a unit price that quietly mixes units (price per millilitre on one bottle, price per gram by weight on another).
Start by converting every package to the same base unit before you compare anything. If one jar of peanut butter is labelled 500 g and another is 454 g, they're already both in grams, but a jar labelled in pounds needs converting (1 lb = 453.59 g) before it can sit in the same comparison. Mixing weight and volume units is the single most common way shoppers get fooled — a 200 g bag of chips is not directly comparable to a 355 mL bottle of oil.
Worked example
Example only: Brand A sells a 900 mL bottle of olive oil for $11.49. Brand B sells a 1 L (1000 mL) bottle for $12.29. Brand A: $11.49 ÷ 900 = $0.0128/mL, or $1.28/100 mL. Brand B: $12.29 ÷ 1000 = $0.0123/mL, or $1.23/100 mL. Once both are converted to the same unit, Brand B is marginally cheaper per 100 mL — about 4% — even though its shelf tag looked pricier because the bottle is bigger and the total is higher.
Same purchase, three unit bases (example figures)
| Package | Price | Size | Price per L | Price per 100 mL |
|---|---|---|---|---|
| Brand A | $11.49 | 900 mL | $12.77 | $1.28 |
| Brand B | $12.29 | 1 L | $12.29 | $1.23 |
Decision rule
Decision rule: never compare shelf unit-price tags across brands unless you've confirmed both use the same base unit and the same measurement type (weight vs. volume vs. count). If they don't match, convert by hand or use a calculator that normalizes for you.
Where rounding hides real differences
Retailers commonly round posted unit prices to two decimal places at the cent level, which can mask a genuine price gap on cheap staples. On a $0.30/kg bag of potatoes, a shelf tag rounds to the same figure for two products that differ by 8–10% at checkout on a large order. If you're buying in bulk, don't trust the rounded tag — carry at least three significant digits through your own math, then round only your final answer.
Count-based products are their own trap
Diapers, garbage bags, coffee pods and paper towels are sold by count, but the count is rarely the unit that matters. A box of 44 diapers isn't equivalent to a box of 44 diapers in a different size — sizes have different absorbency and cost-per-change economics. For paper towels, sheet count ignores sheet size entirely; a "mega roll" with fewer, larger sheets can have a lower cost per usable square metre than a roll with more, smaller sheets. When count is the label unit, ask what the count is a proxy for, and if you can find the actual dimension (area, millilitres, absorbent grams), price against that instead.
- Confirm both products use the same measurement type (weight, volume, or count) before comparing.
- Convert to a shared base unit yourself rather than trusting two different shelf tags.
- For count-based goods, look for the underlying physical unit (area, volume, capacity) if the count doesn't map cleanly to utility.
- Carry extra decimal precision through the calculation and round only the final number.
Try it
Unit Price Calculator →Enter any price and size combination and get a normalized per-unit cost instantly.
All figures in this guide are illustrative examples, not current prices. Cotco does not sell products and is not affiliated with any retailer. Verify prices, sizes, and product details with the seller before buying.