UV Index 2.5 Explained: Why It Is the Line for Vitamin D
The UV Index is a single number for how strong the sunburning UV is at one moment. On this site vitamin D counts only when it reaches 2.5, the same rule the Sundi app uses. Here is what that means in practice.
Updated 2026-10-07. Education and wellness only, not medical advice.
What the UV Index is
The UV Index is a scale from 0 to 11 or more that describes how strongly the sun's ultraviolet light can damage unprotected skin. It depends on how high the sun is, the thickness of the ozone layer, clouds, altitude and the ground (sand, water and snow reflect UV). The UVB part of it is also the part that makes vitamin D in your skin.
- 1 to 2: low. The sun is too weak to bother with for vitamin D.
- 3 to 5: moderate. Useful, with a longer time needed.
- 6 to 7: high. A full dose takes only minutes, and so does a burn.
- 8 and above: very high to extreme. Keep sessions very short.
Why 2.5 is the line
Below a UV Index of about 2.5 the sun makes so little vitamin D that the time needed becomes impractical, and you would burn or tire long before it adds up. The Sundi app and this site use 2.5 as a gate: hours below it do not count as a vitamin D window. The map's "sun window" is the number of hours a day at or above it.
That is also why one place can be "Excellent" in July and "None" in January: in winter the UV Index never reaches 2.5 in most of northern Europe.
Minutes you need at each level
This is the model behind the map: minutes of midday sun for a full dose (about 1,000 IU, a quarter of the body bare, no sunscreen) for skin type III, and the time to reach one minimal erythemal dose (the start of a burn), which does not depend on age.
| UV Index | Full dose, age 30 | Full dose, age 65 | Time to burn |
|---|---|---|---|
| 2.5 | 21 min | 40 min | 80 min |
| 3 | 18 min | 33 min | 67 min |
| 4 | 13 min | 25 min | 50 min |
| 5 | 11 min | 20 min | 40 min |
| 6 | 9 min | 17 min | 33 min |
| 8 | 7 min | 13 min | 25 min |
| 10 | 5 min | 10 min | 20 min |
Planning estimates, not medical advice. Darker skin needs longer and fair skin burns sooner. See how long in the sun for all six skin types.
Clouds, time of day and glass
- Time of day. The index peaks around solar noon. Early morning and late afternoon often stay under 2.5 even in summer, so the useful window is the middle of the day.
- Clouds. They cut UV, sometimes by half or more, but thin cloud lets a lot through. The map uses real monthly cloud data.
- Glass. It blocks UVB. A sunny window does not make vitamin D.
- Reflection. Water, sand and snow reflect UV, so at the beach you get a little more than the number suggests, for the good and the bad.
Use it
Pick a month on the Sun Atlas map to see the typical noon UV Index and the sun window for every sea-holiday destination. Or start with January, April, July or October.
More guides
- Can You Get Vitamin D From the Sun in Winter in Europe?
- How Long in the Sun for Vitamin D? By Skin Type and Age
- Vitamin D After 60: How Age Changes What the Sun Gives You
- Best Winter Sun for Vitamin D: Where to Go From November to March
- Best Places for Vitamin D in Spring, Summer, Autumn and Winter
- Sun Safety on a Vitamin D Holiday: Get the Dose, Skip the Burn
- Sunny Holiday or Vitamin D Supplement? What a Trip Can and Cannot Do