UV Index Calculator
Enter today's UV index (from any weather app) and your skin type to estimate how long until you burn in midday sun — and how much an SPF changes that.
What the UV index numbers mean
| UV index | Category | What to do |
|---|---|---|
| 0–2 | Low | No protection needed for most people. |
| 3–5 | Moderate | Seek shade near midday; hat and sunscreen for extended time outside. |
| 6–7 | High | Protection required. Reduce midday sun exposure. |
| 8–10 | Very High | Extra protection. Unprotected skin burns quickly. |
| 11+ | Extreme | Take all precautions. Avoid the sun midday. |
How the burn-time estimate works
One UV index unit equals about 25 milliwatts per square metre of sunburning (erythemal) ultraviolet energy. Each skin type can absorb a certain dose before it reddens, called the minimal erythemal dose. Dividing that dose by the incoming UV energy gives an estimated time to burn. An SPF value multiplies that time by roughly its number: SPF 30 gives about 30 times longer, in theory.
Treat the result as a ceiling, not a promise. Real sunscreen protection is usually lower than the label because most people apply too little and reapply too rarely.
What raises your real exposure
Several things push the effective UV higher than the reported index. Snow reflects up to 80 percent of UV back at you, sand and water around 15 to 25 percent, so ski slopes and beaches burn faster. UV also climbs roughly 10 to 12 percent for every 1,000 metres of altitude. And thin cloud barely blocks UV — you can burn badly on an overcast day.
Protecting yourself
Shade, a wide-brimmed hat, UV-blocking sunglasses, and tightly woven clothing beat sunscreen alone. When you do use sunscreen, apply a generous layer 15 minutes before going out and reapply every two hours, and more often after swimming or sweating. The sun is strongest from about 10 a.m. to 4 p.m.