What a serum costs per use, and why almost nobody can tell you

Cost per use is the number that should decide most skincare purchases. A 15 mL serum at $22 and a 50 mL serum at $32 are not comparable on price, and neither is comparable on size once you know one of them is used twice as heavily.

Computing it needs two facts: how much is in the pack, and how much you use each time. Every product publishes the first. Almost none publishes the second.

What the directions actually say

Across the nineteen products behind this site’s three rankings, the doses read like this:

• “a pea-sized amount”
• “a blueberry-sized amount”
• “a few drops”
• “two pumps”
• “apply an even layer”
• “apply liberally”
• “apply from head to toe once or twice daily”

Only one of them can be converted into a quantity without inventing something. A pump volume is not published by any of the brands that specify pumps. A pea and a blueberry are not units. “Apply liberally”, on a sunscreen, is the one case where a real dose exists in the standard behind the number on the box, and it is never printed on the front.

What we do instead

Cost per use appears in our tables and reads “Not computable” wherever the label does not support it, which is nearly everywhere. We do not fill that column with an assumed pump volume, because a made-up denominator would make the tidiest number on the page the only fabricated one.

What we publish instead is cost per millilitre or per gram, computed from the pack size and the price. It is a weaker figure than cost per use, and it is real.

The unit trap underneath it

Serums are sold by volume and creams by weight. An ounce on a jar of cream is an ounce of mass, not a fluid ounce, and converting between them means assuming a density that nobody publishes. So a cream is quoted per gram, a serum per millilitre, and the two are never ranked against each other as though the units matched.

The one exception

A single product in this set ships a cap intended to measure its own dose. One, out of nineteen. It is a sunscreen, in the one category where dose has a defined meaning, and it is the only place on the shelf where a brand has made the dose its own problem rather than the buyer’s.