How Sunscreen Protection & Reapplication Times are Calculated
Sunscreen provides vital protection against ultraviolet radiation, but perspiration, water immersion, and photodecomposition diminish efficacy over time.
The calculation applies: Protection Window (min) = (Fitzpatrick Skin Factor × SPF × 200) / (UV Index × 25).
What SPF means: Broad-spectrum SPF 30 blocks 97% of UVB rays; SPF 50 blocks 98%. Reapplication every 80–120 minutes during direct exposure is universally advised by dermatologists.
Use the integrated timer to track when you applied sunscreen and get a real-time reapplication reminder. In England, from May to September, UV index can reach 6–8 particularly during midday hours.