Glossary
Unit price vs pack price
Pack price is the total on the shelf label. Unit price is that total divided by a standard quantity — per 100 g, per kg, per litre, per piece. Only unit price makes two different pack sizes comparable, and it is the basis of every number on this site.
Definitions
- Pack price: what the item costs at checkout. 2.49 for a 500 g bag.
- Unit price: pack price divided by pack quantity, expressed per standard unit. 2.49 / 5 = 0.498 per 100 g.
Most supermarkets are required to print the unit price on the shelf label, usually in small grey type under the big number. It is the only figure on the label that is comparable between products.
Why the big number lies
Three routine mechanisms make pack price useless on its own:
1. Non-round pack sizes. 425 g, 380 g and 500 g cans sitting next to each other at similar pack prices. Only the per-100 g figure ranks them. 2. Shrinkflation. Same pack, same price, 8% less product. The pack price is unchanged; the unit price is the only place the increase shows up. 3. The bulk assumption. Bigger is *usually* cheaper per unit, but not always — promotional small packs frequently beat the family size, and in several of our country data sets the mid-size pack is the cheapest per kg roughly one time in five.
Worked comparison
| Product | Pack price | Pack size | Unit price |
|---|---|---|---|
| Olive oil A | 4.20 | 500 ml | 0.84 / 100 ml |
| Olive oil B | 7.10 | 1 L | 0.71 / 100 ml |
| Olive oil C (promo) | 3.10 | 400 ml | 0.775 / 100 ml |
B wins, C beats A despite being the smallest bottle, and none of that ranking is visible from the pack prices alone.
Unit price and recipe costing
Recipes are written in grams and millilitres, so recipe costing runs entirely on unit prices:
Ingredient cost = unit price × (amount used ÷ unit size)
200 g of spaghetti at $0.30 per 100 g is 0.30 × 2 = $0.60. This is why every ingredient in our database is stored as a price per 100 g, per 100 ml, or per piece rather than as a pack price: pack sizes differ per chain and per country, unit prices do not.
Careful with "per piece"
Eggs, tortillas, buns and limes are priced per piece, and per-piece prices are only comparable when the pieces are the same size. A large egg and a medium egg are about 12% apart in weight. Where it matters we store an average piece weight so per-piece items can still be converted into per-100 g protein and calorie figures.
Practical habit
Photograph the shelf label, not the product. The unit price line is the one that tells you whether the "family size" is actually a saving — and across a full weekly shop, choosing on unit price instead of pack price is worth roughly 5-12% of the bill without changing a single item on your list.
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