UV Index Exposure Calculator

The UV Index measures the intensity of ultraviolet radiation reaching the Earth’s surface. Developed by the World Health Organization (WHO) and various national weather services, it helps people understand when sun protection is needed and how quickly unprotected skin can burn.

This page adds the things the forecast number leaves out: what snow, water or altitude do to the UV actually reaching you, how much of your sunscreen you really get, and how little sun you need for a day’s vitamin D. For the bare index-and-skin-type figure with any SPF you like, the UV exposure time calculator is the shorter road. Both pages run the same formula and return the same minutes.

Sunburn Time Formula. Time to burn is a dose divided by a rate:

Minutes to burn = MED ÷ (UV Index × 0.025) ÷ 60

MED is the Minimum Erythemal Dose, the joules per square metre that make skin visibly pink some hours later, and it is a property of your skin. The 0.025 is the definition of the index itself: one UV Index unit is 25 milliwatts per square metre of erythemally weighted ultraviolet. So the sum is joules divided by watts, which gives seconds, and the 60 turns those into minutes.

UV Index Scale. These burn times are for type II, and they come from the formula above, so the table and the tool cannot drift apart:

Type I skin burns in about four fifths of those times; type IV takes nearly twice as long.

SPF Protection, and why the label multiplication is optimistic.

Protected Time = Unprotected Time × SPF

Burn in 28 minutes and apply SPF 30, and that arithmetic promises 14 hours. Nobody gets 14 hours. SPF is measured in a lab at 2 milligrams per square centimetre, which for an adult body is close to a shot glass of sunscreen, applied before going out and again every couple of hours. Study after study finds people put on a quarter to a half of that.

The part that catches people is that protection does not fall off in proportion. It falls off roughly exponentially with thickness, so applying half the tested amount does not halve your SPF, it takes you to somewhere near its square root. Labelled SPF 50 behaves more like SPF 7, and SPF 30 more like SPF 5. The calculator shows both numbers and you should plan around the smaller one.

Practical SPF effectiveness:

• SPF 15: blocks ~93% of UVB rays
• SPF 30: blocks ~97% of UVB rays
• SPF 50: blocks ~98% of UVB rays
• SPF 100: blocks ~99% of UVB rays

The difference between SPF 30 and SPF 50 is marginal. SPF 30 applied generously is better than SPF 50 applied thinly, and that is not a slogan: it is the square-root rule read backwards.

UV Index by Time of Day: UV intensity peaks between 10 AM and 4 PM. The shadow rule is a quick guide: if your shadow is shorter than you, the UV is intense and you need protection.

Factors That Increase UV Exposure:

• Altitude: UV increases ~10% per 1,000 meters (3,280 feet) of elevation
• Snow reflection: reflects up to 80% of UV rays (doubles exposure)
• Water reflection: reflects 10–30% of UV rays
• Sand reflection: reflects 15–25% of UV rays
• Cloud cover: thin clouds block only 20–30% of UV; you can still burn on cloudy days
• Latitude: UV is strongest near the equator
• Season: UV peaks in summer months

Reapplication Rule: Reapply sunscreen every 2 hours, or immediately after swimming, heavy sweating, or towel drying. No sunscreen lasts all day regardless of SPF. Apply 15–30 minutes before going outside for best absorption.

Vitamin D Balance: Brief, unprotected exposure (10–15 minutes for fair skin) before applying sunscreen helps vitamin D production. After that, apply sunscreen or seek shade.