Sunburn Calculator
How long can you stay in the sun before burning? This sunburn calculator gives an approximate UV burn time based on the UV index, your Fitzpatrick skin type, and your sunscreen SPF, so you can plan safe sun and tanning time. 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.
Reviewed by the SparkCalc editorial team · May 2026
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.
Methodology last reviewed: May 2026. How SparkCalc works
Sources: EPA – UV Index Scale
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.
How do I find the UV index for my location?
Most weather apps and forecast sites publish a daily UV index, and the EPA offers an hourly UV index forecast by ZIP code. Enter that value here to estimate your UV burn time. The higher the UV index, the faster unprotected skin reddens.
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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.
References