UV Index Calculator
Convert erythemal UV irradiance to the UV index, with the risk band and the UV dose for a set time.
What it does
Sunburn is not caused by heat or by visible light. It comes from ultraviolet radiation, and the part of it that reddens skin most efficiently is called erythemal UV, weighted to match the way skin responds across the wavelengths. The UV index exists to turn that physical quantity into a number a person can act on.
The scale is deliberately simple. One UV index unit is exactly 25 milliwatts per square meter of erythemal irradiance, so the index is the irradiance in watts per square meter multiplied by 40. The US Environmental Protection Agency publishes the scale and the bands that go with it, and forecasts around the world use the same convention, which is why an index of 8 means the same thing in Sydney and in Seattle.
The bands carry advice rather than measurement. Low, from 0 to 2, needs no protection for most people. Moderate, 3 to 5, means sunscreen and a hat near midday. High, 6 to 7, adds shade and sunglasses. Very high, 8 to 10, means minimising exposure around midday. Extreme, 11 and above, means avoiding the sun and staying inside or in deep shade.
The index also depends on time of day and on the sky. It peaks when the sun is highest, which is solar noon rather than clock noon, and it falls sharply toward morning and evening. Ozone, cloud, altitude and reflection from water, sand or snow all move it. Snow can nearly double the dose because it bounces light back at your face, and altitude adds roughly ten percent per thousand metres.
What the index does not tell you is how long you personally can stay out. That depends on skin type, on sunscreen and on how much of your skin is exposed. The dose in joules per square meter is the physical part of the answer, and it lets you compare two sessions on different days even when the index was the same.
How to use
- Get the erythemal UV irradiance for the moment you care about, in watts per square meter, from a weather service, a UV sensor or a research station.
- Enter it in the first field. The tool converts it to the UV index and names the risk band.
- Set the time you plan to spend outside, in minutes.
- Read the dose, which is irradiance multiplied by time, in joules per square meter.
- Use the band advice as a floor for protection, and remember that sunscreen, clothing and shade change your personal dose.
- Recheck the index later in the day, because it can halve within two hours either side of solar noon.
Formula
- erythemal irradiance
- UV power on a flat surface, weighted for how strongly it reddens skin, in watts per square meter
- UV index
- The unitless risk number, where one unit equals 0.025 W/m2
- UV dose
- Energy delivered to a square meter of surface over the exposure, in joules per square meter
The factor of 40 follows from the definition: 1 index unit = 25 mW/m2 = 0.025 W/m2, and 1 / 0.025 = 40. The index is reported as a whole number, so the tool rounds it, while the dose keeps the unrounded irradiance so a long session is not distorted by rounding.
Worked example
Example: 0.2 W/m2 for 30 minutes
A summer morning with an erythemal irradiance of 0.2 W/m2, and half an hour planned outside.
- UV index = 0.2 x 40 = 8, which falls in the very high band.
- The advice for that band is to minimise sun exposure and stay in shade near midday.
- Dose = 0.2 W/m2 x 30 minutes x 60 seconds = 360 J/m2.
- For comparison, 0.05 W/m2 gives an index of 2, low risk, and 0.15 W/m2 gives an index of 6, high risk.
- Above 2 W/m2 the tool asks for a smaller figure, since the index would pass 80, far beyond anything measured at ground level.
Reference table
| UV index | Band | What to do |
|---|---|---|
| 0 to 2 | Low | No protection needed for most people |
| 3 to 5 | Moderate | Sunscreen and a hat near midday |
| 6 to 7 | High | Sunscreen, hat, sunglasses and shade near midday |
| 8 to 10 | Very High | Minimise exposure and stay in shade near midday |
| 11 and above | Extreme | Avoid the sun and stay indoors or in deep shade |
Limits
- The conversion is exact, but the irradiance you feed it is a snapshot for one place and moment, not a whole day average.
- Personal burn time depends on skin type, sunscreen use and exposed area, and none of that is in the index.
- Cloud does not map neatly to the index: thin cloud can leave it nearly unchanged while thick overcast cuts it sharply.
- Reflection from snow, sand and water can add substantially to the dose on your face and body, and a flat surface reading misses that.
Common mistakes
- Treating the UV index as a temperature or as a measure of how hot it feels.
- Assuming a cloudy day needs no protection, when scattered cloud can still leave a high index.
- Forgetting that the index peaks at solar noon, not at clock noon, especially where daylight saving shifts the clock.
- Reading the index at sea level on a beach and ignoring the extra dose from sand and water reflection.
- Adding the dose across a whole day and comparing it with a single burn threshold, when the skin repairs between exposures.
Try it yourself
Quick checks. Answers sit tucked under each question.
Erythemal irradiance is 0.15 W/m2. What is the UV index and the band?
6, in the high band. The index is 0.15 x 40 = 6, which calls for sunscreen, a hat, sunglasses and shade near midday.
What dose does 0.05 W/m2 deliver over an hour?
180 J/m2. The irradiance times the time in seconds is 0.05 x 3600 = 180, and the index for that irradiance is 2, low.
A sensor reads 0.5 W/m2. What does the index say?
20, extreme. That is 0.5 x 40 = 20, well into the top band, where the advice is to avoid the sun.
Questions
What exactly is one UV index unit?
Twenty five milliwatts per square meter of erythemal UV, which is 0.025 W/m2. The index is the erythemal irradiance divided by that, or multiplied by 40.
Does a high UV index mean it will be hot?
Not necessarily. UV and temperature are different things: a cool clear day at altitude can carry a high index, and a hot hazy day can carry a lower one.
Can the UV index go above 11?
Yes. The scale has no fixed top, and high altitude, ozone thinning and snow reflection can push it into the teens. The extreme band starts at 11 and continues upward.
Why does the index peak at midday?
Because the sun's rays have the shortest path through the atmosphere then, so less UV is absorbed or scattered before it reaches the ground.
Does sunscreen change the index?
No, the index describes the sunlight, not your protection. Sunscreen reduces the dose your skin receives, so apply it and reapply it, but the index itself stays the same.
What is the UV dose used for?
It is the physical energy delivered per square meter, which makes sessions comparable. Weather and health services use dose thresholds in their own guidance, and the tool shows it so you can see the effect of time.
Sources
- US Environmental Protection Agency: UV Index guide for the public
- US Environmental Protection Agency: UV index scale and the risk bands