How this calculator works
The UV index is a weighted measure of how much sunburning ultraviolet reaches the ground, with one index unit equal to 25 milliwatts per square metre. Each skin type is assigned a minimal erythema dose, the weighted UV energy that produces a faint redness. Dividing that dose by the local irradiance gives an estimated time to redness. Sunscreen multiplies the tolerated dose by its sun protection factor in the ideal case of a thick, even application. Altitude and reflection raise the irradiance: UV rises about 11 percent per 1,000 m, and fresh snow can reflect up to 80 percent of it.
Formula
effective UV index = UV index x (1 + 0.11 x altitude in km) x (1 + surface reflectance). Irradiance = effective UV index x 0.025 W/m2. Time to redness in minutes = MED / irradiance / 60, where MED is 200, 250, 400, 600, 850 and 1,000 J/m2 for skin types I to VI. Time with sunscreen = time without sunscreen x SPF.
Worked example
At UV index 6, sea level and no reflection, the irradiance is 6 x 0.025 = 0.15 W/m2. For skin type II with a 250 J/m2 MED, redness is estimated after 250 / 0.15 = 1,667 seconds, or about 28 minutes. An SPF 30 sunscreen would extend the calculated time to 30 times that, but real protection is lower and wears off, so reapplying every two hours and staying in the shade remain necessary.
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Frequently asked questions
Is the calculated time a safe tanning time?
No. It is an estimate of when redness may begin, which is already a sign of skin damage. The safe approach is to stay well below it, use shade and clothing, and never use sunscreen to prolong deliberate sunbathing. Repeated redness raises skin cancer risk.
Does SPF 30 really let me stay out 30 times longer?
Only in a laboratory with a thick, even layer that is not rubbed off, washed away or broken down by sweat. Real-world protection is much lower, sunscreen should be reapplied at least every two hours and after swimming or heavy sweating, and SPF ratings mainly describe UVB protection, not UVA.
Why do my results change with altitude and snow?
The thinner atmosphere at altitude absorbs less UV, adding about 11 percent per 1,000 metres, and bright surfaces reflect more UV onto the skin. Fresh snow is the strongest common surface, reflecting up to 80 percent, which is why skiing and mountaineering produce rapid burns.
Are the skin type doses exact for me?
No. The six Fitzpatrick types are broad categories, and the minimal erythema dose varies between people within a type. Medications, skin conditions, age and prior exposure can change sensitivity. The estimate is a general guide, not a medical assessment, and anyone with a photosensitivity condition should follow medical advice.