UV Index Burn Time Calculator
Ultraviolet radiation from the sun causes sunburn, premature skin aging, and skin cancer. The UV index, published daily by NOAA and the EPA, measures the intensity of UV radiation at the surface. Combining the UV index with your skin type (using the standardized Fitzpatrick scale) gives an estimate of how long you can be exposed before unprotected skin begins to burn. This calculator uses WHO-referenced minimal erythemal dose (MED) values and the relationship between UV index and erythemally effective irradiance to give you a practical burn time estimate.
UV burn time formula
MED (J/m2) by skin type: Type I: 200, II: 250, III: 350, IV: 450, V: 600, VI: 800
UV Irradiance (mW/m2) = UVI * 25
Burn Time (s) = MED / (UV Irradiance / 1000) * SPF
Burn Time (min) = Burn Time (s) / 60
The WHO/ICNIRP defines erythemally effective irradiance at UV index 1 as 25 mW/m2. The MED values by skin type are referenced from WHO/IARC UV exposure guidelines. SPF multiplies the burn time as a linear factor (practical protection is typically lower due to application variability).
UV protection recommendations (EPA/WHO)
- UV index 1-2 (Low): No protection needed for most people.
- UV index 3-5 (Moderate): Cover up, wear sunglasses, apply SPF 30+ for extended outdoor time.
- UV index 6-7 (High): Protection essential. Reduce midday sun exposure.
- UV index 8-10 (Very High): Extra protection required. Avoid midday sun outdoors.
- UV index 11+ (Extreme): Take all precautions; unprotected skin can burn in minutes.
UV index burn time: frequently asked questions
What is the UV index?
The UV index (UVI) is a dimensionless measure of ultraviolet radiation intensity at the Earth's surface on a scale from 0 (no UV) upward with no upper limit, though values rarely exceed 14 in the continental US. The WHO/WMO/EPA global UV index was standardized in 1994 and is now reported by weather services worldwide. A UVI of 1-2 is low, 3-5 moderate, 6-7 high, 8-10 very high, and 11+ extreme.
How is burn time estimated from the UV index?
Burn time estimates are based on the minimal erythemal dose (MED): the UV energy dose at which skin just begins to redden for a given skin type. For skin type II (average fair skin), MED is approximately 200 J/m2. At UV index 1, the irradiance is approximately 25 mW/m2 in the erythemal action spectrum, so time to MED = 200 / 0.025 = 8,000 seconds. Higher UV index values proportionally reduce burn time.
What are the Fitzpatrick skin types?
The Fitzpatrick scale classifies skin into six types based on UV response. Type I: always burns, never tans (fair, freckled). Type II: usually burns, tans minimally. Type III: sometimes burns, tans uniformly (medium skin). Type IV: burns minimally, tans well (olive skin). Type V: rarely burns, tans profusely (brown skin). Type VI: never burns (dark brown/black skin). Higher types have longer burn times at the same UV index.
Does sunscreen extend burn time?
Yes. SPF (sun protection factor) multiplies burn time. SPF 30 sunscreen applied correctly extends time to burn by a factor of 30. However, most people apply less than the tested amount and miss areas, so practical protection is often lower. Reapplication every 2 hours and after swimming is essential. UV-protective clothing with UPF ratings provides more reliable protection than sunscreen alone.
What time of day has the highest UV index?
UV index peaks around solar noon (typically between 10 am and 2 pm local standard time). UV is minimal before 10 am and after 4 pm in most locations. Cloud cover can reduce UVI but not eliminate it; thin clouds allow 70-90% of UV to penetrate. Snow and sand significantly increase UV exposure by reflecting UV radiation onto the skin from below.
Official sources
- EPA: UV Index Scale.
- NOAA: UV Safety.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.