UV index and sunburn time calculator

What the UV index number means

The UV index is a measure of the sun's ultraviolet radiation weighted for how much damage each wavelength does to human skin. It is not a temperature and it is not related to how hot the day feels. A clear, cold day in the mountains in March can carry a higher UV index than a hazy, humid 95 °F afternoon at sea level, which is why so many people come back from spring ski trips with burned faces.

Each point on the scale represents 25 milliwatts per square meter of erythemally weighted irradiance. The scale is linear, so a UV index of 10 delivers twice the burning power of a 5, and your safe exposure time is cut in half. There is no upper bound written into the definition: readings above 11 are labelled "extreme", and high-altitude tropical locations have recorded values above 20.

Four factors drive it. Sun angle matters most, which is why the index peaks within an hour of solar noon and in summer. Altitude adds roughly 10% per 3,000 feet as there is less atmosphere to absorb radiation. Ozone thickness varies seasonally and geographically. Clouds reduce UV, but far less than they reduce visible light and heat — thin overcast may cut UV by only 10 to 20%, which is exactly why overcast days produce so many unexpected burns.

Reflection is what catches most people out

The burn times here assume you are standing on ordinary ground. Surfaces that reflect UV effectively double your dose from below, and the numbers are larger than most people expect. Fresh snow reflects up to 80% of UV, dry sand around 15 to 25%, sea foam and whitecaps up to 25%, and open water roughly 10% — much more when the sun is low and hitting the surface at a shallow angle.

Snow is the extreme case, and it explains snow blindness, which is literally sunburn of the cornea. It also explains why skiers burn under their chin and inside their nostrils, places that never see direct sun at sea level.

Shade helps but is not a wall. Sitting under a beach umbrella still exposes you to scattered UV from the open sky and reflected UV from the sand, which together can be a third of the direct dose. A UPF-rated shirt is a more reliable defense than shade, and unlike sunscreen it does not need reapplying.

Why SPF rarely delivers what it promises

SPF is a laboratory ratio. An SPF 30 product tested at the standard application density of 2 milligrams per square centimeter lets through about one thirtieth of the burning radiation, which is where the "30 times longer" figure comes from. For an average adult that density works out to roughly one ounce — a full shot glass — for the whole body.

Studies of actual use consistently find people apply a quarter to a half of that. Because protection does not fall off linearly with thickness, applying half the recommended amount of SPF 30 leaves you with something closer to SPF 5 or 6 than 15. This calculator multiplies by the full SPF figure, so treat the result as a best case that assumes you applied generously and reapplied every two hours, and after swimming or heavy sweating regardless of what the bottle claims about water resistance.

The jump from SPF 30 to SPF 50 is also smaller than the number suggests: 97% of UVB blocked versus 98%. That last percentage point is worth having if you burn easily, but it is not worth relying on in place of a hat, a shirt and shade. And nothing about the SPF number describes UVA protection — for that you want a product labelled broad spectrum, since UVA drives photoaging and contributes to melanoma without ever causing a visible burn.

The formula

Burn time from erythemal dose

Irradiance (W/m²)  = UV index × 0.025
Burn time (min)    = MED ÷ irradiance ÷ 60 × SPF

MED = minimal erythemal dose by Fitzpatrick skin type (J/m²)
  Type I    200      Type IV    450
  Type II   250      Type V     600
  Type III  300      Type VI   1000

The minimal erythemal dose is the amount of UV that produces just perceptible reddening 24 hours later. Published values vary between studies, and individual variation within a skin type is wide, so this is an estimate of the right order of magnitude rather than a precise countdown. Medications including doxycycline, some diuretics and retinoids can cut your personal threshold dramatically.

Practical tips

• Apply sunscreen 15 to 30 minutes before going out, so the film has time to set.
• The most commonly missed spots are ears, the back of the neck, the tops of the feet, and the scalp along a part line.
• UV peaks between 10 a.m. and 4 p.m. local daylight time. If your shadow is shorter than you are, the sun is strong enough to burn.
• Sunglasses should say they block 99 to 100% of UVA and UVB. Cataracts and eyelid cancers are both linked to cumulative exposure.
• One blistering sunburn in childhood roughly doubles lifetime melanoma risk, which makes shade and shirts for kids a better investment than any sunscreen.

Frequently asked questions