UV Index Calculator (Clear-Sky Estimate)
Estimate the clear-sky UV index from your latitude, date, time and altitude using a standard solar-elevation model. See your risk category and sun-safety advice.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Clear-sky estimate only. Real UV index varies with clouds, ozone, air pollution and ground reflection (snow, sand, water). For sun-safety decisions, check your local forecast UVI.
Show the formula & steps
What this UV Index Calculator does
This tool gives a clear-sky estimate of the UV index for any location, date and time. You enter your latitude, the date, the local solar time and your altitude, and it returns an estimated UV index, a risk category and sun-safety advice.
It is an estimate, not a measurement. The real UV index also depends on clouds, ozone, air pollution and ground reflection, so use it as a guide and check your local forecast for actual decisions.
How the calculation works
The UV index is driven almost entirely by how high the sun sits in the sky — the solar elevation angle. The tool computes it in three steps.
Solar declination (δ) = 23.45° × sin(360° × (284 + day-of-year) / 365) Hour angle (H) = 15° × (local solar time − 12) Solar elevation = arcsin(sin φ · sin δ + cos φ · cos δ · cos H)
where φ is your latitude. The elevation is then turned into a clear-sky UV index with an empirical model:
Clear-sky UVI ≈ 12 × sin(elevation)^1.18 × altitude factor
The altitude factor adds about 6% per 1000 m (capped) because thinner air absorbs less UV. When the sun is below the horizon, the elevation is negative and the UV index is zero.
Worked example
For latitude 34°N, 21 June (summer solstice), local solar noon (12:00), at sea level (0 m):
- Day of year ≈ 172, so declination δ ≈ +23.4°
- Hour angle H = 15° × (12 − 12) = 0°
- Solar elevation = arcsin(sin 34° · sin 23.4° + cos 34° · cos 23.4° · cos 0°) ≈ 79.4°
- Clear-sky UVI ≈ 12 × sin(79.4°)^1.18 × 1.00 ≈ 11.7 → Extreme risk
At that level, unprotected skin can burn quickly, so shade, clothing, a hat and SPF 50+ sunscreen are essential.
UV index risk categories
| UV index | Category | What to do |
|---|---|---|
| 0–2 | Low | Minimal protection needed |
| 3–5 | Moderate | SPF 30+, sunglasses, shade at midday |
| 6–7 | High | Cover up, limit sun 10am–4pm |
| 8–10 | Very High | Minimise midday sun, reapply SPF often |
| 11+ | Extreme | Avoid midday sun, full protection |
Why the UV index matters
The UV index was created by the World Health Organization and meteorological agencies to communicate skin-cancer and sunburn risk in a single, easy number. Tracking it helps you decide when to seek shade, what sunscreen to use and how long you can safely stay outside — especially for children, fair skin and high-altitude or snow-covered environments where UV is intensified.
Frequently asked questions
How is the UV index calculated in this tool?+
It first finds the sun's elevation angle from your latitude, the date (which sets the solar declination) and the local solar time (which sets the hour angle), then converts that elevation into a clear-sky UV index using an empirical model and boosts it slightly for altitude. It is an estimate, not a measurement.
Why is this only a clear-sky estimate?+
The real UV index depends on cloud cover, the thickness of the ozone layer, air pollution and aerosols, and how much UV the ground reflects (snow, sand and water reflect a lot). This calculator assumes a clear sky and average atmosphere, so always check your local forecast UVI for decisions.
What do the UV index numbers and colours mean?+
0–2 is Low (green), 3–5 Moderate (yellow), 6–7 High (orange), 8–10 Very High (red) and 11+ Extreme (violet). The higher the number, the faster unprotected skin and eyes can be damaged, so more protection is needed as the index rises.
Does altitude really increase UV exposure?+
Yes. UV intensity rises roughly 6 percent for every 1000 metres of altitude because there is less atmosphere overhead to absorb UV. That is why sunburn is common when skiing or hiking at altitude even in cool weather, and this tool adds an altitude boost to reflect it.
What time of day is the UV index highest?+
The UV index peaks around solar noon, when the sun is highest in the sky and its rays travel through the least atmosphere. In most places this is roughly 10am to 4pm in summer, which is why sun-safety advice focuses on those midday hours.