How long does sunscreen really last? The honest answer is not a countdown clock. It depends on how much you applied and what you’re doing, and the practical rule from dermatology bodies is simple: reapply every two hours outdoors, sooner if you swim or sweat. What this calculator shows you instead is something more useful, and more surprising: what your SPF number actually delivers at the amount most people apply.
Your real-world SPF
The halving estimate comes from a BMJ dosage study; it is an approximation, not a lab measurement of your bottle. Whatever the number says, the reapplication rule stays the same.
What the numbers mean
Two facts do most of the work here. First, the differences between labels are smaller than they look: SPF 15 filters out about 93% of UVB rays, SPF 30 about 97%, and SPF 50 about 98% [1]. Second, those numbers assume the lab dose of 2 mg per square centimeter, and a dosage study by Taylor and Diffey in the BMJ found that a realistic thinner application delivers only about half the stated SPF [2]. So an SPF 50 applied the way most people apply it behaves more like an SPF 25. Still good protection, but not an all-day pass.
That’s why no serious body publishes a “you can stay out X minutes” formula. Sunscreen rubs off, sweats off and breaks down with sun exposure, so the working rule is time-based: reapply every two hours outdoors, and immediately after swimming, sweating or towel-drying [3]. Water-resistant labels only promise their SPF for 40 or 80 minutes in the water; we broke down what those labels legally mean in our water resistance guide.
Curious whether SPF 30 or 50 is the better buy in the first place? We broke that down too: SPF 30 vs 50. And since the applied amount decides everything, the how much sunscreen guide is the companion read.
Sources
[1] Sunscreen FAQs and stats, American Academy of Dermatology (SPF 15 about 93% UVB, SPF 30 about 97%, SPF 50 about 98%): link
[2] Taylor and Diffey, BMJ 2002 (2 mg per square centimeter standard; realistic application delivers about half the labeled SPF): link
[3] Advice on sunscreen, Cancer Council Australia (reapply every 2 hours and after swimming, sweating or towel-drying): link