The 3-litre bottle can cost more than the 1.5-litre one

RECEIPT 0451 · 18:42 Liquid detergent, 1.5 L — €4.20 — €2.80/L Liquid detergent, 3 L — €8.97 — €2.99/L

The two bottles on that receipt are the same brand, and the big one comes out nineteen cents dearer per litre. The phenomenon has had a name since the 1970s: quantity surcharge. The researcher Stanley Widrick measured it in United States supermarkets in 1979, and it has been found since in chains all over the world, in detergent, in rice, in nappies and in coffee.

It works because the shopper takes the discount for granted. The large format is associated with saving, that association saves you the check, and the margin can be parked there without anyone complaining. Nobody is cheating anybody: the price is on display and it is correct.

Against that there is one printed line almost nobody looks at. In Spain, since Royal Decree 3423/2000, the shelf label must carry, next to the selling price, the price per unit of measure: euros per litre, per kilo or per item. It is set in tiny type, almost always under the big number, and it is the only figure on the label that lets you compare two different bottles.

Tomorrow, in the detergent aisle, put your thumb over the big price on both bottles and read only the one underneath.