UV Index Calculator
Illustrative UV estimate from latitude, date, altitude and cloud cover; it is not a local observation or forecast.
Last updated · View algorithm, constants and default inputs (JSON)
Results
About this calculator
This educational approximation estimates UV from latitude, date, hours from solar noon, a broad cloud factor and altitude. It does not model ozone, aerosols, surface reflection or local weather, and is not a measurement or forecast. Use an authoritative local UV forecast for exposure decisions.
Worked example
calculated with the default values below
Latitude 40 degrees · Date 2026-10-31 · Sky conditions Clear sky · Altitude 0 m · Hours from solar noon 0 hours UV index 3.2 · Exposure category Moderate · Solar elevation at noon 34.9 degrees · Clear-sky UV index 3.2 · Day of the year 304
General background references
Disclaimer
These calculators give general estimates. Sizing, pricing, availability and standards vary by brand, retailer and country, so treat the output as a starting point and confirm it against the seller's or authority's own figures.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
δ = 23.45π÷180×sin(2π(284+d)÷365); cos z = max(0,sinφ sinδ+cosφ cosδ cos(15πt÷180)); UVI = 12.5(cos z)^2.42×c×(1+0.0001a)
- φ
- Latitude (rad)
- d
- Day of year
- t
- Solar hour
- a
- Altitude (m)
- c
- Cloud factor
- UVI
- UV index
Educational approximation using solar geometry and broad cloud/altitude factors. It omits ozone, aerosols, surface reflection and local weather; it is not a measurement or official forecast. Use a trusted local forecast for decisions.
Frequently asked questions
Is this an official UV forecast?
No. It is a simplified educational model using solar geometry and broad cloud and altitude factors. It omits ozone, aerosols, surface reflection and local weather, so use a trusted local forecast for sun-exposure decisions. Its category bands follow common UV Index ranges; see the [US EPA UV Index overview](https://www.epa.gov/sunsafety/calculating-uv-index-0).