Sunscreen SPF Reapplication Calculator
See how often to reapply sunscreen and a theoretical unprotected burn time based on SPF, your skin type and the UV index.
How it works
Reapply every 2 hours (80 min if swimming/sweating) · Approx. unprotected burn time = (200 ÷ UV index) × skin-type factor
SPF measures how much longer protected skin resists burning, but real-world protection fades as sunscreen rubs, sweats and washes off — which is why dermatologists say reapply every two hours regardless of the SPF number, and sooner after swimming or towelling. The unprotected burn time here is a rough guide to how quickly bare skin of your type reddens at a given UV index; it is not the time SPF buys you, because people apply far less sunscreen than the SPF test assumes. Reapplication and shade matter more than chasing a higher SPF.
Worked example
For skin type III at UV index 6, unprotected burn time is (200 ÷ 6) × 1 ≈ 33 minutes. Wearing SPF 30 for a normal day, reapply every 120 minutes; if swimming, drop that to 80 minutes and reapply after each swim.
Frequently asked questions
Does higher SPF mean I can reapply less often?
No. SPF 100 still needs reapplying every two hours because the film degrades with time, sweat and friction. Higher SPF blocks slightly more UVB but does not extend reapplication timing.
How much sunscreen should I use?
About one ounce (a shot-glass, ~30 mL) covers an adult body, and roughly a nickel-sized amount for the face. Most people apply only a quarter to half of that, which sharply lowers real protection.
Is the burn time how long my sunscreen protects me?
No — it is only how fast bare skin would burn. Because people underapply, do not multiply it by the SPF and treat that as safe time; reapply on the clock and seek shade instead.
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Educational estimate only, not medical advice — talk to an eye doctor, dentist, dermatologist or other qualified professional about your own care.