The one calculation that makes packages comparable
Unit price is the package price divided by the quantity in the package. The useful denominator depends on the product: per 100 g or per kilogram for solids, per 100 mL or litre for liquids, and per item for countable goods such as eggs or garbage bags.
Compare only like with like. A price per 100 g cannot be compared directly with a price per 100 mL unless you know the product’s density. A price per item cannot be converted to weight without knowing how much each item weighs.
Example: two bags of rice
Bag A: $8.49 for 4 kg → $8.49 ÷ 4 = $2.1225/kg
Bag B: $13.99 for 8 kg → $13.99 ÷ 8 = $1.74875/kg
Bag B is about $1.75/kg versus $2.12/kg. It has the lower unit price, although it requires $5.50 more at checkout.
Choose a denominator you can read quickly
Per 100 g is often easier for small packages; per kilogram works well for produce, rice, meat, and larger packages. Per pound remains common in Canadian advertising for produce and meat even when the checkout calculation uses metric weight.
| Display | Calculation | Result |
|---|---|---|
| Per 100 g | $6.45 ÷ 500 × 100 | $1.29/100 g |
| Per kg | $6.45 ÷ 0.5 | $12.90/kg |
| Per lb | $12.90/kg × 0.45359237 | $5.85/lb |
A three-pass shelf check
- Match the product category: compare rice with rice, not a dry product with a prepared version that includes water.
- Match the denominator: convert both offers to the same mass, volume, or count unit.
- Check the conditions: membership, multi-buy quantity, package variety, limit, and the amount your household will actually use.
Common unit-price mistakes
- Dividing quantity by price instead of price by quantity.
- Comparing $/100 g with $/kg without multiplying or dividing by 10.
- Treating ounces as fluid ounces. Ounces measure mass; fluid ounces measure volume.
- Using the regular package price when the sale requires two or three packages.
- Ignoring drained weight, concentrated products, or a package that contains several smaller units.
- Assuming a shelf label is current when the package size or selected variety differs.
Sources
Official sources were retrieved on August 27, 2026. Retailer and government guidance can change.