How this is calculated
unit price = price ÷ quantity; better value = lower unit price (same units on both sides)
Unit price is total price divided by quantity — dollars per gram, per milliliter, per diaper. Once both options speak the same unit, the comparison is instant and immune to packaging psychology: the 'family size' banner, the taller-but-narrower bottle, the 12-pack that outprices the 10-pack.
The counterintuitive part is how often the big pack loses. Retail pricing analysts call it quantity surcharging — larger sizes priced higher per unit than smaller ones — and studies find it in a meaningful share of supermarket items, precisely because shoppers assume bigger is always cheaper and stop checking. Sales flip the ranking constantly: a discounted small pack regularly beats the full-price jumbo.
Two honest caveats. First, a cheaper unit price only saves money if you use it all — food that expires in the bin has an infinite unit price. Second, make sure the quantities are in the same unit before comparing; 0.5 kg vs 500 g is fine, but liters vs kilograms needs a conversion first.
Frequently asked questions
Isn't the bigger package always cheaper per unit?
No — 'quantity surcharge' (big packs costing MORE per unit) shows up regularly in supermarkets, especially during small-size promotions. That assumption is exactly what makes it profitable.
What quantities should I enter?
Anything, as long as both options use the same unit: grams, milliliters, sheets, capsules, wash-loads. The math only needs consistency, not a specific unit.
Shelf labels already show unit prices — why calculate?
Where mandated, shelf labels help but often mix units (per 100g on one, per kg on another), omit promo prices, or are simply wrong. Ten seconds with your own numbers settles it.