Free UV index calculator
Turn any UV index reading into its WHO risk category and an estimate of minutes until fair skin burns — updated live, as you type.
On this page13 sections
WHO/WMO standard UV index. Sunburn times are for unprotected fair skin (Type II) and vary by skin type, altitude, and cloud cover.
Results are estimates. Consult a professional.
What is the UV index?
The UV index is a standardized scale, running from 0 upward, that summarizes the strength of the sun's ultraviolet radiation at ground level around midday. The higher the number, the faster unprotected skin and eyes can be damaged. This UV index calculator takes the reading your weather app reports, sorts it into the World Health Organization risk category, and estimates how many minutes of midday sun it takes fair skin to start burning.
The scale was designed to be the same everywhere: a UV index of 7 means the same hazard in Sydney, Madrid, or Denver. Values typically run from 0 on a dark winter morning to around 11 in the summer tropics, with readings above 11 — labelled "extreme" — possible at high altitude or near the equator. The index is deliberately simple so that anyone can act on it without knowing the underlying radiation physics.
UV index risk categories and sun-protection actions
The UV index is split into five risk categories defined by the World Health Organization. Each band carries a recommended level of sun protection. The calculator places your reading into the matching band; the table below shows the boundaries it uses and what each level means in practice.
| UV index | Category | What to do |
|---|---|---|
| 0–2 | Low | Minimal risk. No protection needed for most people on a normal day. |
| 3–5 | Moderate | Seek shade near midday; wear a hat and sunglasses; apply SPF 30+ sunscreen. |
| 6–7 | High | Protection required. Cover up, use sunscreen, and reduce time in the sun between 10 a.m. and 4 p.m. |
| 8–10 | Very high | Take extra care. Unprotected skin burns quickly; minimise midday sun and reapply sunscreen often. |
| 11+ | Extreme | Maximum risk. Avoid the sun during midday hours; shade, clothing, hat, sunglasses, and sunscreen are all essential. |
Bands and the recommended actions follow the World Health Organization's "Global Solar UV Index: A Practical Guide" (2002), the standard adopted by the US EPA and national weather services. These are the exact thresholds the calculator uses to label a reading.
World Health Organization, "Global Solar UV Index: A Practical Guide" (2002) — the source of the 0–2 / 3–5 / 6–7 / 8–10 / 11+ category scheme and its recommended protection actions.How the sunburn-time estimate works
Alongside the risk category, the calculator estimates how long fair skin can stay in midday sun before it starts to burn. Sunburn happens once skin absorbs a threshold dose of UV energy — the minimal erythemal dose. The stronger the UV index, the faster that dose builds up, so the time to burn falls as the index rises.
Fitzpatrick skin types
The estimate is anchored to Fitzpatrick skin type II — typical fair, easily burning skin. Other skin types reach the sunburn dose at different speeds, so read the minutes as a benchmark and adjust for your own skin:
- Type I (very fair, always burns): burns in roughly half the time shown.
- Type II (fair, the calculator's basis): the time the calculator returns.
- Types III–IV (medium to olive): tolerate roughly two to three times longer before burning.
- Types V–VI (brown to deep brown): three to five times longer, but UV still causes damage — sunscreen and shade still matter.
The UV index reading explained
Only one number goes in: the UV index value itself. Almost every weather app, forecast site, and smartwatch reports it, usually as the forecast peak for the day. Enter that reading and the calculator handles the category and sunburn-time estimate.
One thing worth knowing: the reported UV index is the midday peak, when the sun is highest. Early morning and late afternoon UV is far lower, because the sunlight passes through more atmosphere at a low angle. A forecast of UVI 8 does not mean 8 all day — it means 8 around solar noon, tapering off toward sunrise and sunset.
A worked example using the UV index calculator
Priya's weather app shows a peak UV index of 7 for a clear afternoon and she plans to be outside at lunchtime. She wants to know how serious that is and how long she can stay out before her fair skin starts to burn.
Step 1 — Enter the reading
UV index 7. That single value is all the calculator needs.
Step 2 — Find the risk category
A reading of 7 falls in the 6–7 band, so the category is High. The WHO guidance for High is clear: protection is required — cover up, use sunscreen, and limit midday exposure.
Step 3 — Estimate the sunburn time
Plugging 7 into the formula gives 250,000 / (7 × 25 × 60) = 23.8, which the calculator rounds to 24 minutes for fair (type II) skin.
What changes the UV index: cloud, altitude, and reflection
The UV index is not fixed by the season alone. Several factors push it up or down on any given day, and a few of them surprise people — clouds protect less than expected, and reflective surfaces add UV from below.
- Altitude raises it. UV climbs by roughly 10–12% for every 1,000 m of elevation, because there is less atmosphere overhead to absorb it. A mountain hike carries far more UV than the same latitude at sea level.
- Clouds lower it — but less than you think. Thick overcast can cut UV by 30–50%, yet thin or broken cloud often blocks little, and scattering off cloud edges can briefly push ground-level UV above a clear-sky value. A hazy or partly cloudy day is not a safe day.
- Reflective surfaces add to it. Fresh snow reflects up to 80% of UV, sand around 15%, and water and sea foam a meaningful share. Reflection means skin and eyes are hit from below as well as above — the reason snow and beach glare burns so readily.
- Sun angle and time of day drive it. The index peaks when the sun is highest, near solar noon, and drops sharply toward morning and evening when sunlight travels through more atmosphere.
Why winter and high-altitude UV still matters
It is easy to assume cold weather means safe sun, but UV strength depends on the sun's angle and the atmosphere, not the air temperature. Winter sun is weaker than summer sun at the same place, yet two conditions can push the winter UV index back into dangerous territory: altitude and snow.
On a clear winter day in the mountains, thinner air and highly reflective fresh snow combine. Snow can reflect up to 80% of UV back upward, effectively doubling the dose reaching the face and eyes, while every 1,000 m of altitude adds another 10–12%. The result is that ski slopes and high-altitude winter conditions can reach a "very high" UV index of 8–10 even at noon — strong enough to cause snow blindness and facial sunburn in a few hours.
Common UV index mistakes to avoid
- Trusting clouds to protect you. Thin and broken cloud blocks little UV, and scattering can briefly raise it. Judge the day by the UV index forecast, not by how bright it looks.
- Ignoring UV in cold weather. Temperature has nothing to do with UV strength. Snow and altitude can produce a "very high" reading on a freezing day.
- Treating the sunburn time as a hard limit. It is an estimate for fair (type II) skin under clear midday sun. Reflection, altitude, medication, and your own skin type all shift it — use it as a warning, not a countdown to stay out for.
- Assuming a low daily UV means a safe day. The reported value is the midday peak. A modest forecast can still hide an hour around noon when most of the day's UV arrives.
For related "feels-like" weather readings, see the heat index calculator for how hot it feels in humidity and the apparent temperature calculator for the all-conditions feels-like figure.
How accurate is the sunburn-time estimate?
The risk category is exact — it is the WHO's published banding applied directly to your reading. The sunburn-time figure is a reasoned estimate, not a guarantee. It assumes Fitzpatrick skin type II, a 250 J/m² erythemal dose, and clear midday sun, and it treats one UV index unit as 25 mW/m² of erythemally weighted UV. Those are standard, defensible values, but real skin varies.
Treat the minutes as a benchmark for fair skin and scale to your own type: very fair skin (type I) burns in about half the time, darker types in several times longer. Reflection off snow, sand, or water, higher altitude, certain medications, and prior sun exposure all shorten the safe time further. When in doubt, protect earlier than the number suggests — sunscreen, shade, and cover-up cost nothing and the estimate only describes the onset of visible burning, not the slower cumulative damage UV causes.
Frequently asked questions about the free UV index calculator
About this UV index calculator
This UV index calculator sorts a UV reading into the World Health Organization's five risk categories and estimates how long unprotected fair (Fitzpatrick type II) skin can stay in midday sun before it starts to burn. It runs entirely in your browser — nothing you enter is sent to a server or stored — so your reading stays private and the result updates instantly as you type.