Sunscreen SPF & Sun Exposure Calculator
Calculate your safe sun exposure time based on SPF, skin type, and the UV index. Find out when to reapply and how much sunscreen you actually need.
Sunscreen and UV Guide
Understanding SPF
SPF (Sun Protection Factor) indicates how much longer you can stay in the sun without burning compared to unprotected skin. SPF 30 theoretically allows 30× longer exposure. However, this assumes a full application of 2mg per cm² of skin — most people apply 25–50% of the required amount, meaning effective protection is closer to SPF 15–20 even when using SPF 50. SPF 30 blocks approximately 97% of UVB rays; SPF 50 blocks 98%; SPF 100 blocks 99%. The difference between SPF 30 and SPF 100 is margina
UV Index and Skin Types
The UV Index ranges from 0 (no UV) to 11+ (extreme). In the UK: UV is typically 1–3 from October to March (minimal risk for most skin types). In summer (May–September): 4–7 at noon, occasionally reaching 8+ on clear days. Australia and the Mediterranean regularly reach 10–12+. Skin type I (very fair) can burn in under 10 minutes at UV Index 8. Skin type VI has a minimum erythemal dose approximately 30–40× higher but is not immune to UV damage or skin cancer.
How Much Sunscreen
The standard is 2mg per cm² of skin. For the face and neck: approximately 1 teaspoon (5ml) or two finger lengths of product. For the full body: 35–40ml — roughly a shot glass. A typical 200ml bottle should last approximately 5 full-body applications. Most people use far less, dramatically reducing effectiveness. Using a spray sunscreen makes it very easy to under-apply — rub in after spraying to ensure even coverage. For lips, use a dedicated SPF lip balm.
Reapplication and Water Resistance
Sunscreen must be reapplied every 2 hours regardless of SPF, or immediately after swimming or heavy sweating (even if the product claims water resistance). 'Water resistant for 40 minutes' means it maintains its SPF rating after 40 minutes of swimming — after which it must be reapplied. 'Very water resistant / 80 minutes' applies up to 80 minutes of water activity. No sunscreen is truly waterproof. The morning application before leaving the house wears off long before your beach day ends.
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