Free UV index calculator. Estimate time to sunburn, safe exposure with SPF, and the WHO UV category from your Fitzpatrick skin type.
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The UV Index tells you how strong the sun's sunburning radiation is right now. This calculator combines the WHO/WMO UV Index scale with the Fitzpatrick skin type classification to estimate how many minutes of unprotected sun your skin can take before it starts to burn, how much longer sunscreen buys you, and which SPF to reach for today.
The UV Index is an international standard developed by the WHO and WMO that rates the intensity of sunburn-causing ultraviolet radiation from 0 (night) to 11+ (extreme). It is linear, so UV 10 is twice as intense as UV 5 and burns unprotected skin roughly twice as fast. One UV Index unit corresponds to 0.025 watts per square metre of erythemal (sunburning) irradiance, which is why higher numbers mean shorter safe-exposure times.
Time-to-Sunburn Formula
Judge how long children and fair-skinned adults can stay out, and when to reapply sunscreen.
Plan training and trail time around the safe-exposure window on high-UV summer days.
Decide whether a quick midday walk or commute really needs sun protection today.
Prepare for the stronger UV of tropical, high-altitude, or southern-hemisphere destinations.
See how many minutes unprotected skin can take today's UV before redness and DNA damage begin.
Match sunscreen strength to the current UV level and your skin type instead of guessing.
Schedule beach trips, sport, and errands around the day's safe-exposure window and peak-UV hours.
It depends on the UV Index and your skin. Time to sunburn equals your skin's minimal erythemal dose divided by 1.5 times the UV Index. For fair (Type II) skin at UV 8, that is roughly 250 ÷ (1.5 × 8) ≈ 21 minutes of unprotected sun before redness starts. Darker skin and lower UV give longer times; SPF multiplies them.
The Fitzpatrick scale sorts skin into six types by how it reacts to sun: Type I always burns and never tans (very fair), through Type VI which almost never burns (deeply pigmented). Your type sets your minimal erythemal dose (MED) - the UV energy that causes the first faint redness - which this calculator uses to estimate burn time.
No. SPF 15 blocks about 93% of UVB, SPF 30 about 97%, and SPF 50 about 98%, so the jumps shrink quickly. In real life the bigger problem is under-application: people typically apply a quarter to half the tested amount, which can cut an SPF 50 down to the low teens. Apply generously and reapply every two hours.
UV 0-2 is Low and needs little protection. UV 3-5 (Moderate) and 6-7 (High) call for shade, clothing, and SPF 30+. UV 8-10 (Very High) and 11+ (Extreme) are hazardous - unprotected skin can burn in well under half an hour, so limit midday sun and use SPF 50.
Yes. Up to 80% of UV passes through light cloud, and water, sand, and snow reflect extra UV onto your skin. Always use the UV Index reported for your location rather than judging by how bright or warm it feels, because cloud and cool air hide the real burning risk.