126 burn times — at UV 8, fair skin reddens in twenty minutes
A time to sunburn for every meeting of 21 UV index values and 6 skin types. Under the same sun, type I and type VI are five times apart.
Divide what falls by what you can take
A UV index of 1 means 0.025 W/m² of erythemally weighted ultraviolet. At index 8 that is 0.2 W/m² landing on skin. The minimal erythema dose is how much has to accumulate before the skin reddens some eight hours later, measured in J/m² — so dividing dose by strength gives time. Type I sits at 200 J/m², type VI at 1000.
SPF is a multiplier
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.
Where the forecast draws its lines
These bands come from the World Health Organization: moderate from 3, high from 6, very high from 8, extreme from 11. A forecast saying "high" means somewhere between 6 and 7.5 — and within that band the time to burn still differs threefold by skin type.
Reading it off your shadow
The higher the sun, the stronger the ultraviolet and the shorter your shadow. If your shadow is shorter than you are, the index is usually past 6 — a rough check when you have not seen a forecast. A shadow longer than you means a low sun and weak ultraviolet.
These figures are a starting point
Snow and water bounce more ultraviolet back at you, and altitude adds a few per cent every couple of hundred metres. Some medicines and sensitive skin will redden well before these times.
Skin type
- Type I — always burns, never tans200 J/m²
- Type II — burns easily, tans a little250 J/m²
- Type III — sometimes burns, tans well350 J/m²
- Type IV — rarely burns, tans easily450 J/m²
- Type V — very rarely burns600 J/m²
- Type VI — never burns1000 J/m²
Find it by index and skin type
Time until it reddens
UV 1low0.025 W/m²
UV 1.5low0.0375 W/m²
UV 2low0.05 W/m²
UV 2.5low0.0625 W/m²
UV 3moderate0.075 W/m²
UV 3.5moderate0.0875 W/m²
UV 4moderate0.1 W/m²
UV 4.5moderate0.1125 W/m²
UV 5moderate0.125 W/m²
UV 5.5moderate0.1375 W/m²
UV 6high0.15 W/m²
UV 6.5high0.1625 W/m²
UV 7high0.175 W/m²
UV 7.5high0.1875 W/m²
UV 8very high0.2 W/m²
UV 8.5very high0.2125 W/m²
UV 9very high0.225 W/m²
UV 9.5very high0.2375 W/m²
UV 10very high0.25 W/m²
UV 10.5very high0.2625 W/m²
UV 11extreme0.275 W/m²
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 8?
It depends on your skin: 16.7 minutes for type I, 20.8 for type II, 83.3 for type VI.
Q. What does the UV index actually mean?
It is the strength of erythemally weighted ultraviolet. Index 1 is 0.025 W/m², so index 8 puts 0.2 W/m² on your skin.
Q. Does SPF 30 really mean thirty times longer?
In the lab, yes. That figure assumes 2 mg per square centimetre applied evenly; most people use less than half, so real protection is much shorter.
Q. Can I burn on a cloudy day?
Yes. Cloud does not block ultraviolet completely, and thin cloud can even scatter extra onto you.
Q. Any way to judge it without a forecast?
Look at your shadow. If it is shorter than you are, the index is usually past 6.