UV Index Skin Damage Calculator
The UV Index tells you how strong the ultraviolet radiation from the sun is on a given day. Combined with your Fitzpatrick skin type, it estimates how long you can stay in the sun before unprotected skin begins to redden (sunburn). The time-to-sunburn calculation is based on the Minimum Erythemal Dose (MED) for each skin type, divided by the UV irradiance at the stated index. These MED values are sourced from WHO Global Solar UV Index guidelines and the EPA's SunWise program. This is an educational estimate; actual burn time varies with altitude, cloud cover, surface reflection and skin acclimatisation.
UV Index sunburn time formula
Time to burn (min) = MED / (UV irradiance)
UV irradiance (J/m2/min) = UVI × 0.025 × 60 / 40
Simplified: Time = MED × 40 / (UVI × 0.025 × 60)
Where MED is the Minimum Erythemal Dose in J/m2 for each skin type (Type I: 200; Type II: 250; Type III: 300; Type IV: 450; Type V: 600; Type VI: 1,000). A UVI of 1 corresponds to 25 mW/m2 of erythemal UV radiation per WHO/ICNIRP definition.
WHO UV Index exposure categories
- 0 to 2 (Low): Minimal risk. Wear sunglasses on bright days.
- 3 to 5 (Moderate): Apply SPF 30+ sunscreen; seek shade during midday hours.
- 6 to 7 (High): Reduce time in the sun between 10am and 4pm; use SPF 30+ and protective clothing.
- 8 to 10 (Very High): Take extra precautions. Unprotected skin can burn rapidly. Avoid the sun between 10am and 4pm.
- 11+ (Extreme): Take all precautions. Avoid going out during midday hours. Unprotected skin burns in minutes.
UV index skin damage: frequently asked questions
What is the UV Index?
The UV Index (UVI) is an international standard measurement of ultraviolet radiation intensity from the sun, developed jointly by the WHO, WMO, UNEP and ICNIRP. It ranges from 0 (no UV) to 11+ (extreme UV). Each unit of UVI represents approximately 25 mW per square metre of erythemal (skin-reddening) UV radiation. The EPA and NWS publish daily UVI forecasts across the US.
What are the UV Index exposure categories?
WHO categories: 0 to 2 is Low (minimal protection needed); 3 to 5 is Moderate (sun protection recommended); 6 to 7 is High (sun protection essential); 8 to 10 is Very High (extra protection, avoid midday sun); 11+ is Extreme (maximum protection, stay indoors midday). These are the standard categories used by the EPA, WHO and most national health agencies.
What is the Fitzpatrick skin type scale?
The Fitzpatrick scale, published by Thomas B. Fitzpatrick in 1975 in the Archives of Dermatology, classifies human skin into six types based on colour and UV sensitivity: Type I (very fair, always burns, never tans); Type II (fair, usually burns, sometimes tans); Type III (medium, sometimes burns, gradually tans); Type IV (olive, rarely burns, easily tans); Type V (brown, very rarely burns); Type VI (dark brown to black, never burns). Types I and II are most vulnerable to UV damage.
Does SPF sunscreen affect sunburn time?
Yes. SPF (Sun Protection Factor) multiplies your skin's natural protection time. SPF 30 theoretically extends the time to sunburn by a factor of 30, though real-world protection is lower due to incomplete application. Reapplication every 2 hours is recommended by the American Academy of Dermatology (AAD), regardless of SPF rating. This calculator shows unprotected skin exposure time.
How much UV radiation causes DNA damage to skin cells?
According to WHO, UV radiation damages DNA in skin cells at exposure doses as low as 1 Standard Erythemal Dose (SED), which equals 100 J/m2 of erythemal UV. The minimum erythemal dose (MED) for Fitzpatrick Type I skin is approximately 200 to 300 J/m2. At UVI 10, this dose can be reached in as little as 10 to 15 minutes for Type I skin.
Official sources
- WHO Global Solar UV Index: A Practical Guide: who.int.
- EPA SunWise UV Index: epa.gov.
- Fitzpatrick, T.B. (1975). Soleil et peau. Journal of Medical Esthetics. Archives of Dermatology.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.