UV Exposure Time Calculator

UV Index is a standardized international scale developed by the WHO that measures the intensity of ultraviolet radiation reaching the Earth’s surface. It predicts the rate at which skin damage can occur and directly informs safe sun exposure time for different skin types.

Two numbers in, one answer out. If you are on snow, on water or up a mountain, the forecast index understates what is reaching you, and the UV index exposure calculator applies that reflection and also works out the short unprotected window that covers your vitamin D. Both pages use the same MED table and the same square-root sunscreen rule, so they agree on any input you can give to both.

Safe exposure time formula:

Minutes to Minimum Erythemal Dose (MED) = MED ÷ (UV Index × 0.025)

Where MED is the dose of UV radiation that produces the first perceptible redness for a given skin type, and 0.025 is the standard UV irradiance factor (W/m²) per UV Index unit.

Fitzpatrick Skin Type MED values:

UV Index scale:

SPF and UV protection:

Protected Time = Unprotected Time × SPF

SPF 30 blocks about 97% of UVB, SPF 50 about 98%. Nothing blocks all of it.

Why that multiplication is optimistic, and by how much. The SPF on the bottle is measured in a lab at 2 milligrams per square centimetre. For an adult that works out to roughly a shot glass of sunscreen for the whole body, applied before you go out. Study after study finds people apply a quarter to a half of that, and because protection does not fall off linearly with thickness, applying half gives you nowhere near half the SPF. A reasonable rule is that real-world protection lands near the square root of the labelled number, so SPF 50 behaves more like SPF 7 and SPF 30 more like SPF 5. The calculator shows both figures. Plan around the smaller one.

Worked example: A Type II person at the beach with UV index 9, using SPF 50:

• Bare skin: 250 ÷ (9 × 0.025) ÷ 60 = 18.5 minutes
• SPF 50 on the label: 18.5 × 50 = 926 minutes, or 15 hours 26 minutes, which is longer than the sun is up
• SPF 50 realistically: 18.5 × 7.1 = about 2 hours 11 minutes

That last figure is roughly the reapplication interval anyway, which is not a coincidence. Two hours is how long a real application lasts on real skin.