How to use the sun exposure estimate
Combine the forecast UV index with skin type and altitude.
1
Find the UV index
Use the forecast for the time and place you will be outside.
2
Choose skin type
Pick the description closest to your untanned skin.
3
Treat it as a ceiling
Use the number to plan shade and cover, not as a target for tanning.
What does this tool do?
The UV index describes erythemally weighted sunlight intensity, and skin type sets the minimal erythema dose. This calculator combines the two to give a rough time to redness, adjusted for altitude.
The Formula
Burn time (min) = MED (J/m²) ÷ (0.025 × UV index × altitude factor × 60); altitude factor = 1 + 0.1 × kmFrequently Asked Questions
Does a higher UV index mean faster burning?
Yes. Erythema time falls roughly in proportion to the UV index, although cloud, altitude and reflective surfaces change the actual dose.
Does sunscreen make the time unlimited?
No. SPF reduces the UV reaching the skin but is not a timer, and most people apply less than the tested amount. Reapply every two hours and after swimming or sweating.