UV Index Calculator

The UV index is an international standard measurement of the strength of ultraviolet radiation from the sun at a particular place and time. Developed by the World Health Organization (WHO) in collaboration with the World Meteorological Organization (WMO), the UV index helps people understand when they need to take precautions to protect their skin and eyes from harmful UV radiation.

How the UV Index Scale Works

The UV index is a linear scale that starts at 0 and has no upper limit, though values above 11 are rare except at high altitudes and near the equator. The scale is divided into five exposure categories: Low (0–2), Moderate (3–5), High (6–7), Very High (8–10), and Extreme (11+). Each category corresponds to increasing levels of risk from unprotected sun exposure.

The UV index is calculated based on the predicted amount of UV radiation reaching the Earth's surface at solar noon, which is typically between 11 a.m. and 1 p.m. local time. It accounts for cloud cover, altitude, latitude, ozone layer thickness, and surface reflection. Snow, sand, and water can reflect UV radiation and effectively increase exposure.

Skin Types and Sun Sensitivity

The Fitzpatrick scale classifies skin into six types based on how it reacts to UV exposure. Type I skin is very fair, always burns, and never tans — people with this skin type may burn in as little as 10 minutes at UV index 8. Type II is fair and burns easily. Type III is medium and sometimes burns. Type IV is olive-toned and rarely burns. Type V is brown and very rarely burns. Type VI is dark and essentially never burns from UV exposure.

Regardless of skin type, prolonged UV exposure increases the risk of skin cancer and premature aging. Even people with darker skin types who don't visibly burn should use sun protection, as UV radiation can still cause cellular damage that accumulates over time.

How to Check the UV Index

Most weather services and smartphone weather apps include the UV index in their daily forecasts. The UV index is typically highest between 10 a.m. and 4 p.m. and peaks around solar noon. It tends to be higher at lower latitudes (closer to the equator), at higher altitudes, during summer months, and on clear days. However, up to 80 percent of UV radiation can penetrate light cloud cover, so protection is still important on overcast days.

Certain environmental factors can amplify UV exposure. Fresh snow reflects up to 80 percent of UV radiation, effectively doubling your exposure. Sand reflects about 15 percent, and water reflects about 10 percent. Being near these surfaces means you receive UV from both above and below, increasing the need for comprehensive sun protection.

Sun Protection Recommendations

For UV index values of 3 or above, the WHO recommends taking protective measures. Apply broad-spectrum sunscreen with at least SPF 30, reapplying every two hours and after swimming or sweating. Wear protective clothing including a wide-brimmed hat, UV-blocking sunglasses, and long sleeves when practical. Seek shade during peak UV hours.

For very high and extreme UV (8+), additional precautions are necessary. Try to stay indoors during the peak hours of 10 a.m. to 4 p.m. If you must be outside, use SPF 50+ sunscreen, wear tightly-woven clothing, and limit your time in direct sunlight. Children and fair-skinned individuals should be especially careful, as their skin is more susceptible to UV damage.

UV Radiation and Health

While some UV exposure is necessary for vitamin D production, excessive exposure is the primary cause of skin cancer, the most common cancer worldwide. Ultraviolet radiation damages the DNA in skin cells, which can lead to mutations and uncontrolled cell growth. The three main types of skin cancer — basal cell carcinoma, squamous cell carcinoma, and melanoma — are all linked to UV exposure, with melanoma being the most dangerous.

UV radiation also causes photoaging — premature wrinkling, age spots, and loss of skin elasticity. Up to 90 percent of visible skin aging is caused by UV exposure rather than the natural aging process. Protecting your skin from UV radiation is one of the most effective anti-aging measures you can take. UV radiation can also damage the eyes, contributing to cataracts and other eye conditions.