SunkatBurn time calculator
Put in the UV index from your weather app and your skin type. You get the minutes until skin starts to redden without sunscreen, and the shorter limit the Sunkat app uses for a tanning session.
reference model, without sunscreen
Sunkat tanning limit
How it works
Sunburn starts when your skin collects one minimal erythema dose (MED), the amount of sunburn-weighted UV that leaves it visibly red a day later. The UV index measures how fast that dose arrives: one index point is 25 mW/m², or 1.5 J/m² every minute (WHO). So:
minutes to first redness = MED ÷ (1.5 × UV index)
We use the lower end of the published MED range for each skin type (in J/m²: type I 200, type II 250, type III 300, type IV 450, type V 600, type VI 1,000; 100 J/m² is one standard erythema dose, CIE). That makes the result the earliest likely redness, not the average. As a check, fair skin at UV 12 gives 11 minutes, in line with ARPANSA's guidance that fair skin can burn in about 10 minutes on an extreme summer day.
The Sunkat limit is the number the Sunkat app uses for a session. It aims for a fraction of the burn dose, is capped between 5 and 60 minutes per side, and with sunscreen grows only modestly, because real-world sunscreen protection is far below the label.
Minutes to first redness by UV index
Without sunscreen, at a steady UV level. Tap a UV value for skin-type advice, tanning limits and SPF numbers at that level.
| UV index | Type I | Type II | Type III | Type IV | Type V | Type VI |
|---|---|---|---|---|---|---|
| UV 1 | 2 h 13 min | 2 h 46 min | 3 h 20 min | 5 h+ | 5 h+ | 5 h+ |
| UV 2 | 66 min | 83 min | 100 min | 2 h 30 min | 3 h 20 min | 5 h+ |
| UV 3 | 44 min | 55 min | 66 min | 100 min | 2 h 13 min | 3 h 42 min |
| UV 4 | 33 min | 41 min | 50 min | 75 min | 100 min | 2 h 46 min |
| UV 5 | 26 min | 33 min | 40 min | 60 min | 80 min | 2 h 13 min |
| UV 6 | 22 min | 27 min | 33 min | 50 min | 66 min | 111 min |
| UV 7 | 19 min | 23 min | 28 min | 42 min | 57 min | 95 min |
| UV 8 | 16 min | 20 min | 25 min | 37 min | 50 min | 83 min |
| UV 9 | 14 min | 18 min | 22 min | 33 min | 44 min | 74 min |
| UV 10 | 13 min | 16 min | 20 min | 30 min | 40 min | 66 min |
| UV 11 | 12 min | 15 min | 18 min | 27 min | 36 min | 60 min |
| UV 12 | 11 min | 13 min | 16 min | 25 min | 33 min | 55 min |
What changes your burn time
- Time of day. UV peaks around solar noon. In summer, roughly half the day's UV arrives between 11:00 and 15:00 (WHO), so the same skin burns twice as fast at 13:00 as at 10:00.
- Reflection. Fresh snow reflects up to 80% of UV, sea foam about 25% and dry sand about 15% (WHO), so you receive more than the index says.
- Altitude. UV rises about 10% for every 1,000 m you climb (WHO).
- Clouds. Thin or broken cloud lets most UV through. More on clouds, windows and water.
- Sunscreen. Helps, but not by a fixed multiple of minutes. SPF 30 and SPF 50 guides.
Frequently asked
How is burn time calculated?
Burn time is the minutes it takes to collect one minimal erythema dose (MED), the UV dose that makes skin visibly red. One UV index point equals 25 mW/m² of sunburn-weighted UV, which is 1.5 J/m² every minute, so minutes = MED ÷ (1.5 × UV index). Fair type II skin has an MED of about 250 J/m², so at UV 8 it reddens after about 20 minutes.
Why are there two different numbers?
The first is the reference time to first redness, which is the point at which damage is already visible. The second is the Sunkat app's limit per side, which stops well before redness and is deliberately conservative. If you want a tan without burning, the lower number is the one to plan around.
Does sunscreen change my burn time?
It reduces how much UV reaches your skin, but not by a predictable number of minutes. SPF is tested with 2 mg/cm² of sunscreen and most people apply a quarter to half of that. The US FDA states that SPF relates to the amount of sun exposure, not the time spent in it. See how long you can stay in the sun with SPF 50.
Which skin type am I?
Type I always burns and never tans, type II burns easily and tans a little, type III sometimes burns and tans gradually, type IV rarely burns and tans easily, type V very rarely burns, type VI almost never burns. If you are between two types, pick the fairer one.
Where do I find the UV index?
Most phone weather apps show it (on iPhone it is in the Weather app), and national agencies publish forecasts: the Met Office in the UK, ARPANSA and the Bureau of Meteorology in Australia, the DWD in Germany. Use the peak value for the time you will be outside, not the daily average.
Your burn time, live, wherever you are
Sunkat reads the UV index for your location and times each side for your skin type. Free to download on iPhone and Android.
