← Spark & Whetstone · Thinking in Rates
The small print on a shelf tag is a unit price: the cost of one gram, one millilitre or one item. It only helps when both tags use the same unit, and often they do not. Type both packages and the page puts them on the same footing.
Each price is divided by its amount after turning the amount into grams, millilitres or items: an ounce of weight is 28.35 grams, a pound 453.6 grams, a US fluid ounce 29.57 millilitres. Then both are shown per 100 grams, per 100 millilitres or per item. Bigger is not always cheaper per unit, and a price “for 2” is still a price per item.
A weight cannot be compared with a volume or a count unless you know how much one item weighs or holds, so the page says so rather than guessing.
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