The Unit-Price Check

Seconds

Compare products by price per unit (per 100 grams, per litre, per sheet, per dose) instead of by pack price, pack size, or the size of the discount sticker, which is the one number that package design and promotions cannot disguise.

How to do it

  1. 1Find the unit price on the shelf label; it is usually in small print in a corner. Many jurisdictions require it (the EU for most goods; some US states), and most large retailers show it even where they need not.
  2. 2Check that the units match. One label per 100 grams and the next per item, or per ounce against per pound, is common; convert before comparing.
  3. 3Where there is no label, divide price by quantity on your phone. For concentrated products use the number of uses (washes, doses, servings), not the volume.
  4. 4Do not assume bigger is cheaper. Check the large pack, the multi-buy, and the "value" size against the ordinary one; a quantity surcharge is found on a meaningful minority of products.
  5. 5Check the promotion against the regular unit price of the alternatives. "Three for ten" is only a saving if the unit price beats what you would otherwise have bought and you will use all three.
  6. 6For products you buy regularly, note the unit price and the net weight once. That is how you catch a pack that shrinks while the price stays the same.

What to say

  • “What is that per kilo? The big box is actually dearer than two small ones.”
  • “Same price as last month, but it was 500 grams and now it is 450.”

When to use it

  • •Groceries, toiletries, cleaning products, pet food, and anything sold in several pack sizes.
  • •Multi-buy, bundle, "family size", "bonus pack", and "new look" offers.
  • •Subscription and delivery boxes whose per-portion price is never stated.
  • •Comparing a branded product with a store brand or a refill.

Counters

ShrinkflationBundle Pricing PsychologyDecoy PricingDecoy EffectCharm PricingPrice Anchoring DisplayAnchoringUp-To ComparativesLoss Leader StrategyCompromise EffectPrestige PricingFree Shipping ThresholdPrice PartitioningDenomination EffectMental Accounting ManipulationMisleading Averages

Evidence and how strong it is

Russo (1977) ran a field experiment in a supermarket chain and found that displaying unit prices, especially in an organized list, shifted purchases toward cheaper-per-unit items and saved shoppers money, which is the classic evidence that the information changes behaviour when it is easy to use. Sprott, Manning & Miyazaki (2003) document quantity surcharges, in which the larger package costs more per unit, across grocery categories and show that they persist because most shoppers assume a volume discount and do not check. The US National Institute of Standards and Technology published a best-practice guide to unit pricing (2014) because inconsistent units and tiny type were undermining its usefulness. Evidence strength: moderate (a field experiment, observational pricing studies, and regulatory consensus); the size of the saving depends on the shopper and the store.

Cautions
  • Cheapest per unit is not cheapest for you if you throw part of it away. For perishables and things you rarely use, the smaller pack can cost less in practice.
  • Unit price ignores quality, concentration, and what you actually want. Use it to compare like with like, then decide whether a difference in quality is worth the difference per unit.
  • Labels contain errors and are sometimes missing for promotional prices. If the numbers look odd, do the division yourself.
  1. Russo, J. E. (1977). The Value of Unit Price Information. Journal of Marketing Research, 14(2), 193-201
    Field experiment showing that clearly displayed unit prices shift purchases toward lower unit-price products.
  2. Sprott, D. E., Manning, K. C., & Miyazaki, A. D. (2003). Grocery Price Setting and Quantity Surcharges. Journal of Marketing, 67(3), 34-46
    Documentation of larger packages priced higher per unit and of shoppers' untested assumption that bigger is cheaper.
  3. National Institute of Standards and Technology (2014). Unit Pricing Guide: A Best Practice Approach to Unit Pricing. NIST Special Publication 1181
    Regulatory best-practice guidance on consistent units and legible presentation of unit prices.
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