SPF Recommendation: A baseline sun protection factor tailored to your skin type and current UV index. Higher SPF blocks more UVB rays, though the difference diminishes above SPF 30.
Reapplication Timing: How often you should reapply sunscreen; water exposure and sweating shorten this window, so beach and sports activities require more frequent reapplication.
Application Amount: The dermatology standard ensures adequate coverage; undershooting this amount significantly reduces protection effectiveness.
Protection Window: The estimated maximum duration sunscreen remains effective before degradation from UV exposure and water loss requires reapplication.
Individual Variation: This is a general guideline. Consult a dermatologist if you have very sensitive skin, a history of skin cancer, or specific medical conditions.
How This Calculator Works
This tool applies lookup tables based on skin type and UV index to recommend an SPF level, following guidelines from dermatology organizations. Activity level determines the reapplication interval: indoor activities allow longer windows (120 min), casual outdoor extends to 90 minutes, while beach swimming and sports shorten it to 60 minutes due to water loss. The protection window is estimated using minutes = (SPF × 10) / UV index, reflecting how the sun's intensity (UV index) reduces the effective duration of protection. The application amount uses the standard dermatology metric: 1 teaspoon for the face and 1 shot glass (approximately 1.5 oz) for the full body to ensure thorough coverage.
Quick Questions
Does higher SPF really mean better protection?
SPF 30 blocks ~97% of UVB rays; SPF 50 blocks ~98%. The difference is small, but reapplication frequency matters far more than a higher single SPF number.
Can I skip reapplication if I use SPF 50?
No. All sunscreens degrade over time, especially with water exposure or sweating. Reapply every 60–120 minutes depending on activity, regardless of SPF.
What if I have sensitive or very dark skin?
Always consult a dermatologist, as sun sensitivity varies widely. This calculator is a starting point, not medical advice.
Is "waterproof" sunscreen real?
Water-resistant formulations exist, but no sunscreen is truly waterproof. Most last 40–80 minutes in water; reapply after swimming or excessive sweating.
Sources
CDC Sun Safety — Evidence-based sun protection guidance and UV index information.