The Unit Price Guide: How to Actually Compare Food Costs
The headline price on a food product tells you almost nothing useful. Unit price - cost per 100g, per litre, per serving - is the only number that lets you make meaningful comparisons. Here's how to use it.
Supermarkets are required by law to display unit prices - the cost per 100g, per litre, or per standard unit - alongside the headline price. Most shoppers ignore this number. This is a significant financial error, because the relationship between pack size and unit price is often counter-intuitive and deliberately obscured by promotional pricing strategies.
Why Headline Price Misleads
A 400g tin of chickpeas priced at €0.95 looks cheaper than a 660g tin at €1.45. Unit price tells a different story: the 400g tin costs €0.24 per 100g; the 660g tin costs €0.22 per 100g. The apparently cheaper tin is actually more expensive per unit of product. This type of comparison - where the larger format is better value but single-price framing makes it appear more expensive - is extremely common in supermarket pricing.
Research on consumer price perception found that shoppers consistently struggled to identify better-value options when comparing products of different pack sizes, even with access to price information. Shoppers who used unit price as their primary comparison tool made more accurate value assessments and spent less on equivalent nutritional content than those using headline prices. [Chandon & Wansink, Nutrition Reviews, 2012]
Common Unit Price Surprises
In practice, unit price comparisons regularly reveal non-intuitive value relationships. Bulk rice and legumes almost always offer dramatically better unit prices than smaller packets, even accounting for limited storage. Multipack products are not reliably cheaper per unit than single items - check before assuming. Supermarket "on promotion" products often have an elevated regular price that makes the promotional price appear better than it is relative to own-brand equivalents.
Fresh herbs are one of the most dramatic examples of unit price disparity: a 20g packet of supermarket fresh herbs typically costs €1–1.50, equating to €50–75 per kilogram. A pot of living herbs - basil, parsley, coriander - costs €2–3 and, with basic care, continues producing for weeks. The unit price of a pot versus a cut packet differs by a factor of 10 or more over the plant's productive life.
How to Build the Unit Price Habit
The habit requires only a few seconds of additional attention per purchase decision. When choosing between two products, glance at the unit price displayed on the shelf label - it's typically in smaller text beneath the headline price. Over a monthly food budget, consistent unit-price shopping versus headline-price shopping typically saves 10–15% on the same basket of goods.