UV Index Sunburn Timer
How long can you stay in the sun before burning? This calculator provides an approximate estimate based on the UV index, your Fitzpatrick skin type, and sunscreen SPF. It uses the concept of minimal erythemal dose (MED) - the UV exposure that causes first reddening. These are rough estimates only - actual burn time varies significantly based on many factors. Reapply sunscreen regularly.
How We Calculate This
MED by skin type (J/m²): I=200, II=250, III=300, IV=450, V=600, VI=1000. UV irradiance = UVI × 0.025 W/m². Time to MED = MED / (irradiance × 60). SPF multiplier uses 1 + 0.8×(SPF-1), with SPF capped at 50.
Frequently Asked Questions
What is MED (Minimal Erythemal Dose)?
MED is the amount of UV radiation that causes just-noticeable reddening (erythema) of the skin within 24 hours. It's measured in J/m² and varies by skin type.
How does SPF work?
SPF (Sun Protection Factor) indicates how much longer you can stay in the sun before burning compared to unprotected skin. SPF 30 theoretically provides 30× longer protection, but real-world protection is usually less due to application and degradation.
Why cap SPF benefit?
Real-world SPF protection rarely matches lab conditions. Most people under-apply sunscreen, don't reapply, and some UV penetrates regardless. We scale SPF to a realistic effectiveness and cap the input at SPF 50.
What are Fitzpatrick skin types?
A 1-6 scale classifying skin by its response to UV: Type I always burns and never tans, Type VI has deeply pigmented skin that rarely burns. Each type has different baseline sun sensitivity.
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This calculator provides rough estimates only. Actual burn time depends on many factors including altitude, reflection, skin condition, and medication. Always follow sun safety guidelines and consult a dermatologist for personalized advice.