True unit cost: the cheapest sticker is usually not the cheapest
Sort a list of toilet paper by price and the cheapest thing on the page is a single roll. Sort by what you actually consume — a sheet — and the cheapest is often the largest box on the page, at a sticker price ten times higher. Sticker price is actively misleading for anything sold in packs.
The arithmetic is trivial. The inputs are the problem.
Cost per unit is pack price divided by units in the pack. For paper that means rolls multiplied by sheets per roll. We checked ten toilet-paper listings on a large public catalogue: the roll count was stated on three of them, the sheets per roll on one, and both on none. The maths cannot be done on most listings because the seller never wrote down the numbers.
So when we rank by unit cost, we rank only the listings that state their units, and we list the rest separately with the exact field each is missing. Filling the gap with a guess would mean inventing the very number we are ranking on.
Watch for the title and the specifications disagreeing
One listing we checked said one roll count in its title and a different one in its specifications. Neither is obviously right. A shopper sees the title; the arithmetic needs the specification. When the two disagree we do not pick one — we flag the conflict, because a ranking built on a coin flip is worse than no ranking.
Do this for everything sold by the pack
- Paper goods: sheets, and mind the “mega roll” multipliers.
- Bags: count per box, and the stated capacity.
- Detergent: loads, not ounces — concentration varies.
- Filters and consumables: months of use per unit, from the maker’s stated interval.
The rule
Never compare pack prices. Compare unit prices, and only where the seller has stated the units. Where they have not, the honest answer is that the listing cannot be ranked — and that is worth knowing before you buy it.
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