SPF, dose, actives
The SPF number is a ratio, not a duration: SPF 15 means one fifteenth of the burning radiation reaches the skin when the product is applied evenly at 2 milligrams per square centimetre. The same dose and the same reapplication interval apply to every bottle on the shelf, so the useful questions are how much to put on and how strong the active ingredient is.
| Labelled SPF | Fraction reaching skin | Approx. blocked | Reapply |
|---|---|---|---|
| SPF 15 | 1/15 | about 93% | every 2 hours |
| SPF 30 | 1/30 | about 97% | every 2 hours |
| SPF 50 | 1/50 | about 98% | every 2 hours |
| Test dose | 2 mg/cm² | fixed | — |
| Per-area volume | 25 ml (1 tablespoon) | per limb, face, chest, back | — |
| Benzoyl peroxide | 2.5–10% | concentration band | — |
| Tannic acid (1932) | 10% | UV filter concentration | — |
SPF is the fraction of burning radiation that gets through the film, so a higher number means a smaller fraction.
SPF 15 means 1/15 of the burning radiation reaches the skin when the product is applied evenly at 2 milligrams per square centimetre. Read it as a throughput: the label tells you how much of the burning radiation passes the film, not how many minutes of sun you have left.
The number is measured at one fixed dose. Below that dose the film is thinner and the fraction that gets through rises; above it the film is thicker and the fraction falls. The label stays the same either way, which is why the dose and not the SPF number is the part that changes the outcome.
The arithmetic of the fraction is worth doing once: 1/15 is about 6.7% of the burning radiation, 1/30 is about 3.3%, and 1/50 is 2%. The jump from SPF 15 to SPF 30 halves the throughput; the jump from 30 to 50 removes only another percentage point or so.
The 2 mg/cm² dose translates into roughly a tablespoon, 25 ml, for each area that needs covering.
A tablespoon (25 ml) of sunscreen should cover each limb, the face, the chest and the back for thorough coverage. That is four limbs, plus face, chest and back — seven areas, each getting its own tablespoon, which is why a bottle runs out faster than most people expect.
The dose is not a preference. It is the amount at which the SPF number was measured, so applying less means the number on the label no longer describes the film on the skin.
Areas that are hard to cover evenly — ears, back of the neck, the parting in hair — take the same dose per square centimetre as the easier areas. Uneven application leaves thin patches where the throughput rises above the labelled figure.
Higher-SPF products do not stay effective longer and must be reapplied, usually every two hours.
The most common misreading of the SPF number treats it as a clock. It is not: higher-SPF products do not stay effective longer and must be reapplied, usually every two hours. The film wears, rubs and washes away on the same schedule whether it started at SPF 15 or SPF 50.
The reapplication interval is counted from the moment of application, not from the moment of first sun exposure. Setting a reminder at the time of application is the simplest way to keep the interval honest.
Swimming and sweating interrupt the film sooner than the two-hour mark, so those events trigger a fresh application regardless of how much time has passed.
The only calendar date here is the 1932 tannic acid sunscreen; the remaining figures are current formulation and test values.
Six cards covering the ratio, the dose, the volume, the interval, the active band and the case count.
Figures cited on this page: SPF throughput at 2 mg/cm², the 25 ml tablespoon coverage figure, the two-hour reapplication interval, benzoyl peroxide 2.5-10%, the 1932 tannic acid 10% formulation, and the American Academy of Dermatology acne case count.