UV 4, Type II — 41.7 min
Index 4 is 0.1 W/m², and this skin takes 250 J/m² to redden. Divide, and you have 41.7 minutes.
Type II — burns easily, tans a little
With sunscreen on
SPF 30 means thirty times longer before the skin reddens. That number comes from a lab applying 2 mg per square centimetre evenly, and most people put on less than half of that. The times below are therefore generous — reapplying every couple of hours matters more than the number on the bottle.
- SPF 15625 min
- SPF 301250 min
- SPF 502083.3 min
- UV index
- 4
- Forecast band
- moderate
- Strength on the skin
- 0.1 W/m²
- Dose that reddens
- 250 J/m²
- Time until it reddens
- 41.7 min
- To last two hours
- SPF 3
- The shadow rule
- Your shadow is longer than you
Nearby cells
Same index, other skin types
Same skin, other index values
How to read this
- The time is the reddening dose divided by the strength the index stands for. Double the index, half the time.
- Sunscreen times are laboratory figures. Real coverage is thinner, so the real time is shorter.
- Snow, water and altitude add ultraviolet. None of that is in this table.
- Redness shows up about eight hours later — by then the exposure has already happened.
Frequently asked questions
Q. How long can I stay out at UV 4?
About 41.7 minutes before the skin starts to redden. It shows up some eight hours later, by which time the exposure is done.
Q. How much does sunscreen add?
SPF 15 gives 625 minutes, SPF 30 gives 1250, SPF 50 gives 2083.3. These are lab figures; real life is shorter.
Q. What should I wear for two hours outside?
SPF 3 or higher works on paper. Reapplying every couple of hours matters more.
Q. How strong is this index?
On the forecast scale it counts as moderate.