How Much Sunscreen You Actually Use (And Why It Matters)

There is a gap between the SPF printed on a bottle and the protection most people get from it, and it has nothing to do with the sunscreen being poor. It is about how much goes on.

The number is measured at a specific thickness

SPF is not an abstract rating. It is measured in a laboratory with sunscreen applied at 2 mg per square centimetre of skin, an internationally agreed thickness. Change the thickness and you change the result.

In 2001, Autier and colleagues gave 148 students an SPF 10 or SPF 30 sunscreen to use on their summer holidays and then measured how much they had actually used. The median thickness applied was 0.39 mg per square centimetre, a little under a fifth of the amount the label is based on. That did not vary by sex, skin type, where the study was run, or which SPF they were given.¹

Less product is not proportionally less protection

Schalka and colleagues tested this directly, applying the same sunscreens at 2, 1.5, 1.0 and 0.5 mg per square centimetre on 40 volunteers and measuring the protection each time. The SPF fell as the amount fell, significantly in both the SPF 15 and SPF 30 groups, and the relationship was not a straight line.²

Pissavini and Diffey modelled what that means in practice, combining measured SPF with the average amount people apply and how unevenly they spread it. Their conclusion was that typical use of an SPF 15 in strong sun leads to redness anyway, and that someone spending a long time outdoors is better served by an SPF 30 applied carefully.³

Osterwalder and Herzog have written about how much confusion the SPF number causes worldwide, which is worth knowing if you have ever wondered why the guidance feels inconsistent.⁴

What this looks like at the basin

Roughly a third of a teaspoon for a face and neck is the figure usually quoted, and it is more than it sounds. If a 50 ml sunscreen lasts you a year of daily use, you are almost certainly applying less than the label assumes.

Two habits do more than switching brands:

• Put on more than feels necessary. A layer that disappears instantly is usually too thin.
• Go back over the edges. Ears, hairline, the sides of the neck and the tops of the hands are where application is thinnest, and unevenness was part of what the modelling above found.

None of this makes sunscreen the only thing that matters, and none of it is a substitute for shade or a hat when the UV index is high. It is simply that the number on the bottle describes a thicker layer than most of us put on.

Ask us

If you are not sure whether the sunscreen you own suits your skin, bring it in. We are at 245 Parnell Road seven days, and a therapist will tell you honestly whether it is worth replacing.

References

Each PMID resolves on Europe PMC, so you can read the paper rather than take our word for it.

1. Autier P. Quantity of sunscreen used by European students. British Journal of Dermatology, 2001. PMID 11251560
2. Schalka S. The influence of the amount of sunscreen applied and its sun protection factor (SPF). Photodermatology, Photoimmunology & Photomedicine, 2009. PMID 19614894
3. Pissavini M. The likelihood of sunburn in sunscreen users is disproportionate to the SPF. Photodermatology, Photoimmunology & Photomedicine, 2013. PMID 23651270
4. Osterwalder U. Sun protection factors: world wide confusion. British Journal of Dermatology, 2009. PMID 19775352