Unit Price: The One Number That Makes Prices Comparable

Package prices can't be compared

A 500g jar at $4.29 versus a 750g jar at $5.99: which is cheaper? The sticker prices cannot answer that question — only the unit price can. $4.29 ÷ 0.5kg is $8.58 per kilogram; $5.99 ÷ 0.75kg is $7.99 per kilogram. The bigger jar is about 7% cheaper per unit. Reverse the numbers slightly and the "value size" loses — which happens more often than shoppers assume, because retailers know the big package is bought on faith.

Shelf labels in many jurisdictions are required to display unit prices, but they are printed small, and the units are not always consistent between neighbouring products — one label per 100g, the next per kilogram. When the shelf makes comparison awkward, thirty seconds of division settles it.

Shrinkflation hides in the denominator

Package sizes change more often than prices. A cereal box that quietly drops from 500g to 460g at the same price has raised its unit price 8.7% without touching the number on the sticker. Unit price is the only metric that catches this automatically, because the shrink shows up in the denominator whether or not the front of the box announces it.

The practical habit: for the ten or so staples you buy every week, know the unit price you consider normal. You will spot both the genuine promotions and the silent increases within one shopping trip.

When bulk is a false economy

Bulk pricing usually lowers the unit price, but the saving is only real if you use all of it. A 5kg bag of rice at $2.20/kg genuinely beats five 1kg bags at $3.00/kg — rice keeps. The same logic fails for a 2-litre tub of yoghurt that is 20% cheaper per litre if a third of it is thrown away: a 20% discount with 33% waste is a 7% loss, not a saving.

Bulk also has a cash-flow cost: money spent on eight months of detergent is money not available this month. The bulk-buy calculator prices both effects — the per-unit saving and your realistic usage rate — instead of letting the big number on the shelf make the decision.

Discounts measure against unit price too

"25% off" is only meaningful relative to a real base price. A common pattern around sale events is a raised base price shortly before the discount is applied to it; the advertised percentage is accurate while the saving is not. Unit price cuts through here as well: if you know the product normally sells at $7.99 per kilogram, you can price any promotion in one division, whatever the banner says.

None of this requires spreadsheets at the shelf — it requires one habit (compare per unit, not per package) and a calculator on your phone. That is the entire trick.

Try it yourself

Sources

Figures can change after publication — check the source for current numbers. This guide is general information, not financial or medical advice.