UV Index Calculator — Sun Exposure Risk and Protection Guidelines

The UV index was created specifically to communicate sun exposure risk in terms people could act on. A number between 0 and 11+ translates directly into protection requirements — and knowing it before you step outside is genuinely useful for skin safety, especially at altitude or near the equator.

Check UV risk for any conditions at CalcHub.

The UV Index Scale

UV Index	Exposure Level	Time to Burn (Fair Skin)	Protection Needed
0–2	Low	60+ minutes	None required
3–5	Moderate	45–60 minutes	SPF 15+, hat
6–7	High	25–30 minutes	SPF 30+, shade at midday
8–10	Very High	15–20 minutes	SPF 30–50, protective clothing
11+	Extreme	Under 10 minutes	Maximum protection, avoid midday sun

What Affects UV Index

Latitude: Lower latitudes receive more direct sunlight. The equator receives UV nearly vertically year-round. Season and time of day: UV is highest from 10am–4pm and peaks near the summer solstice. At solar noon, UV is roughly 3–5× higher than at 8am or 6pm. Altitude: UV increases about 4–8% per 300 meters (1000 ft) of elevation. At 3000m, UV is 40–50% higher than at sea level for the same latitude and conditions. Ozone: Stratospheric ozone absorbs UV-B radiation. Thinner ozone (seasonal holes over poles) lets more UV through. Cloud cover: Clouds reduce UV, but not as much as you might expect. Thin cloud cover can still let through 75–90% of UV. Surface reflection: Snow reflects 80%+ of UV. Sand reflects 15–20%. Water reflects 5–10%. Beaches and ski slopes amplify UV exposure significantly.

How to Use the Calculator

1. Enter your latitude, date, and time of day
2. Optionally input altitude and cloud cover
3. Get estimated UV index and recommended protection level

The CalcHub calculator also shows vitamin D synthesis windows — when UV levels are high enough for D production but not dangerously high.

SPF and What It Actually Means

SPF (sun protection factor) indicates how much longer you can stay in the sun before UV-B causes sunburn. SPF 30 filters 97% of UV-B. SPF 50 filters 98%. The difference sounds small but matters for prolonged exposure.

SPF ratings assume you apply 2mg/cm² — about twice what most people apply. Under-application (the common mistake) reduces effective protection dramatically. A thin application of SPF 50 sunscreen may provide only SPF 10–15 of actual protection.

Does the UV index measure UV-A or UV-B?

The UV index is weighted toward UV-B, which causes sunburn and drives most UV-related cancer risk. UV-A (which penetrates more deeply) contributes to aging and some cancer risk but doesn't cause immediate sunburn. Broad-spectrum sunscreens protect against both. The index doesn't separately report UV-A.

Can you get sunburned on a cloudy day?

Absolutely. UV passes through clouds quite effectively. The classic scenario: an overcast day at the beach where people stay out longer than they would in direct sun, then burn worse than they would have on a clear day because they skipped sunscreen. UV index forecasts account for expected cloud cover.

Is there a "safe" amount of UV exposure for vitamin D?

The body produces vitamin D when UV-B hits skin. Brief unprotected exposure (5–30 minutes, 2–3 times per week) on arms and legs is generally considered adequate for vitamin D synthesis at moderate latitudes. Darker skin requires longer exposure. This exposure is substantially below what causes sunburn risk for most skin types. The CalcHub calculator shows the "vitamin D window" alongside burn risk thresholds.

Related Calculators

• Sunrise/Sunset Calculator — when UV peaks each day
• Heat Index Calculator — combined heat and sun risk assessment
• Cloud Base Calculator — cloud cover effects on UV and visibility