The UV index is an international standard measurement of how strong the ultraviolet (UV) radiation from the sun is at a particular place on a particular day. It is a scale primarily used in daily forecasts aimed at the general public.

Its purpose is to help people to effectively protect themselves from UV light, of which excessive exposure causes sunburns, eye damage such as cataracts, skin aging, and skin cancer. Public-health organizations recommend that people protect themselves (for example, by applying sunscreen to the skin and wearing a hat) when the UV index is 3 or higher; see the table below for complete recommendations.

The U.S. National Weather Service daily forecasts of ultraviolet levels use these numbers for various exposure levels:

"Time to Burn" and "Actions to Take" apply to people with fair skin that sometimes tans but usually burns. People with lighter skin need to be more cautious. People with darker skin may be able to tolerate more exposure.