Sunburn time.

Worked example

UV index 8, skin type II. The minimal erythema dose is taken as 2.5 standard erythema doses (SED), 1 SED = 100 J/m². UV index 8 is 8 x 0.025 = 0.2 W/m², so 250 J/m² takes 250 / 0.2 = 1,250 seconds, about 21 minutes of unprotected sun. The tool shows half of that, about 10 minutes, as the point to be covered up or in shade. With SPF 50 applied at half the test amount, effective SPF is 50^0.5 = about 7, so the same arithmetic gives about 2.5 hours, and sunscreen still needs reapplying every 2 hours.

How it works

Minutes to first reddening = SED for your type x 100 J/m² / (UV index x 0.025 W/m²) / 60. The UV index is defined as 40 times the erythemally weighted irradiance in W/m², so each UV index point is 25 mW/m². The SED values used here are the low end of commonly published ranges: type I 2, II 2.5, III 3, IV 4.5, V 6, VI 10.

With sunscreen, effective SPF is estimated as SPF raised to (applied mg/cm² / 2), reflecting the roughly exponential loss of protection with thinner application. The protected time is the unprotected time times that effective SPF. This is a teaching estimate, not a safe-exposure allowance. The US FDA is explicit that SPF relates to the amount of exposure, not time, and sunscreen should be reapplied at least every 2 hours and after swimming or sweating.

What this is not. It does not include reflection from water, snow or sand, altitude, cloud gaps, medicines that raise sun sensitivity, or tanning-bed use. Skin can be damaged before it looks red, and tanning is itself a sign of UV damage.

Sources and basis: WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide (2002), UV index = 40 x effective irradiance in W/m²; standard erythema dose, 1 SED = 100 J/m² (CIE / ISO 17166); typical minimal erythema dose ranges by Fitzpatrick type, low end used; Faurschou A and Wulf HC, Photodermatol Photoimmunol Photomed 2007 (SPF falls with thinner application); Health Canada, UV index and sun safety.

FAQ

How long does it take to get sunburned?

It depends on the UV index and your skin. At UV index 8, a type II skin type starts to redden in roughly 20 minutes with no protection by this estimate; at UV index 3 it is about an hour. Damage can start before redness shows.

Does SPF 50 mean I can stay in the sun 50 times longer?

No. SPF describes the amount of UV dose, not time, and it is measured at 2 mg/cm², about two to four times what people usually apply. Use shade, clothing and a hat first, and reapply sunscreen at least every 2 hours.

What UV index is safe?

Health Canada advises sun protection at UV index 3 and above. At 8 and over, minutes matter: seek shade at midday and cover up.

Why does the calculator show half the time?

Individual sensitivity varies and the minimal erythema dose is the point where reddening is already visible. Half is a cautious planning point, not a safe limit.

Can dark skin burn?

Yes. Types V and VI burn less easily but still take UV damage and can develop skin cancer, often diagnosed later. Sun protection applies to every skin type.