The scale · WHO standard
The UV index scale, explained
The UV index scale runs from 0 to 11+ and measures the sunburn-producing power of sunlight at the surface — the same standard worldwide, defined by the WHO. Here is what every band means, what to do at each level, and how to see where today’s number falls for your location.
The full scale, 0 to 11+
Three things people get wrong about the scale
First: it isn’t about temperature. The UV index measures radiation, not heat — a crisp 18°C June day regularly scores higher than a hot August afternoon. Second: clouds barely help. Up to 80% of UV passes through light cloud, so “overcast” does not mean “safe”. Third: the scale is open-ended. 11+ is a floor for “extreme”, not a ceiling — high-altitude and tropical locations regularly exceed it.
Frequently asked questions
What is the UV index scale?
The UV index scale is the WHO’s international measure of sunburn-producing ultraviolet radiation, running from 0 (minimal) to 11+ (extreme). It was designed so the same number means the same risk anywhere on Earth — a UV of 7 in Berlin equals a UV of 7 in Miami.
What is the range of the UV index?
The scale starts at 0 and is open-ended: 11+ is labelled “extreme”, but readings of 13–14 occur in the tropics and at altitude. Values above 16 have been measured near the equator in the Andes.
What does each UV index number mean?
0–2 low, 3–5 moderate, 6–7 high, 8–10 very high, 11+ extreme. Protection is recommended from 3 upward; from 8, midday shade becomes essential. The number measures burning power, not heat — a cool day can have a high UV index.
What is the UV index scale today where I am?
The scale itself never changes — what changes is where today’s reading falls on it. Check your live number on the uvindex.now homepage: it auto-detects your location and shows today’s value, the hourly curve and your personal burn time.