What Is the UV Index? A Plain-English Guide
The UV index is a 0–11+ scale for how strong the sun’s burning ultraviolet rays are. Here’s what each level means and exactly what to do at each.
The quick answer
The UV index is an international scale, from 0 upward, that tells you how strong the sun’s skin-damaging ultraviolet radiation is at a given place and time. The higher the number, the faster your skin burns. It was developed by the World Health Organization, the WMO and others so the advice is the same everywhere.
What each level means
| UV index | Band | What it means |
|---|---|---|
| 0–2 | Low | Minimal risk. Safe to be outside; sunglasses on bright days. |
| 3–5 | Moderate | Seek shade near midday. SPF 30, hat, sunglasses. |
| 6–7 | High | Protection needed. Shade 10am–4pm; cover up; SPF 30+. |
| 8–10 | Very High | Extra care. Avoid the midday sun; SPF 50; unprotected skin burns fast. |
| 11+ | Extreme | All precautions. Stay out of the sun in the middle of the day. |
Check the live UV index for your location →
How it’s measured
The index is the sun’s erythemal (skin-reddening) irradiance, weighted for how different UV wavelengths affect skin, then divided into easy whole numbers. Each unit of UV index equals 25 milliwatts of burning UV per square metre. That fixed relationship is what lets us turn the index into a personal burn time in the tanning time calculator.
What pushes the UV index up
- Time of day — highest around solar noon (roughly 10am–4pm).
- Season & latitude — stronger in summer and nearer the equator.
- Altitude — rises about 10% per 1,000 m of elevation.
- Reflection — water, sand and snow bounce UV back onto you.
- Ozone & cloud — thin cloud still lets through up to 80% of UV.
What to do with the number
Look up the index before you head out, then match it to your skin. Pale skin at UV 8 can redden in under 20 minutes, while olive skin lasts roughly twice as long. Use the skin type quiz to find your Fitzpatrick type, then the tanning time calculator to turn the index into a safe time — and read the best time of day to tan for timing.