Sunscreen Calculator

How long you can stay outdoors before your skin starts to burn depends on three things working together: your skin type, the strength of the sun at that moment, and the product you put on. Pairing those variables gives you a reapplication window rather than a guess, which matters for avoiding sunburn and lowering long-term skin cancer risk. Knowing your own window helps you plan exposure instead of reacting to redness after the fact.

The estimate starts from your skin's natural burn time, known as the Minimal Erythemal Dose (MED), which is read from your Fitzpatrick skin type. The calculator stores that baseline as minutes to burn at a UV index of 1 (from 67 minutes for Type I to 500 for Type VI) and divides it by the current UV index, since stronger sun burns skin faster. Multiplying the result by your SPF gives a "theoretical" protection time, but the FDA warns that SPF measures how much UV exposure a sunscreen withstands, not how long you can stay out, so the effective protection the calculator reports is capped at 120 minutes, in line with the FDA's advice to reapply at least every two hours. The reapplication schedule then adds one reapplication at each two-hour mark before your time outdoors ends.

No product blocks 100% of UV, so the number you get is a ceiling, not a promise. Reapply after swimming or heavy sweating, and cover the spots people routinely skip, such as the ears, the back of the neck, and the tops of the feet. SPF 30 already blocks about 97% of UVB rays and higher numbers block only slightly more, so applying enough and reapplying on time do more for your skin than chasing an ultra-high number on the bottle.