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General estimate for planning only — not medical advice. Consult a doctor about your specific situation.
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How This Tool Works
Calculation Method
Computes Time to burn from UV index and Skin type.
Site methodology
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- These tools provide general estimates for planning and education only.
- They are not medical advice and not a substitute for a qualified healthcare professional.
- Always consult your doctor or another healthcare provider about your specific situation.
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Worked example: the burn-time formula step by step
The core model says: time to burn = base minutes x 10 / UV index, where the base row for your skin type is the minutes it takes to burn at UV 10. The six base rows are 15, 25, 35, 50, 70, and 100 minutes for skin types 1 through 6. Take a medium skin (type 3) on a day with UV index 5. Start with 35 x 10 = 350, then 350 / 5 = 70, so unprotected skin burns in about 70 minutes, shown as 1h 10min.
Compare that to a fair-skinned person (type 2) on the same UV 5 day: 25 x 10 = 250, then 250 / 5 = 50 minutes. The two extra skin-type steps buy 20 more minutes. Now hold skin type constant and move the sun: the same type 3 person at UV 10 gets 350 / 10 = 35 minutes, and at UV 8 gets 350 / 8 = 43.75, which the tool rounds to 1h 3min after converting 43.75 minutes into 43 minutes plus a rounded remainder. Note the inverse relationship: doubling UV halves burn time, so a UV 10 day is exactly twice as fast to burn as a UV 5 day for the same person.
A dark skin (type 6) example shows the top of the range: 100 x 10 = 1,000, divided by UV 5 gives 200 minutes, or 3h 20min, the longest result the formula can produce at that UV. The shortest possible result at the input cap (UV 15, type 1) is 15 x 10 / 15 = 10 minutes.
Where the model breaks: limits, caps, and real conditions
The input ranges are hard gates. UV must be greater than 0 and at most 15; skin type must be 1 to 5 in the picker's index (covering types 1 through 6). At UV 0 the tool rejects the entry instead of returning an infinite burn time, since 10 divided by 0 is undefined. UV 16 or higher is rejected even though real extreme days can exceed 15; the standard UV Index scale tops out at 11+, and the tool treats 11 and above as the extreme band where the note about burning fast applies. Anything outside these bounds returns the invalid-input message rather than a number.
The formula is a first-order estimate and ignores several real effects. It assumes the stated UV index holds for the whole exposure window, but UV peaks near solar noon and can double between morning and midday, so a 2-hour beach session spanning the peak is not one constant-UV problem. It does not model reflection: water adds roughly 10 to 25 percent, snow up to 80 percent or more, and sand about 15 to 25 percent to effective UV. Altitude raises UV about 10 percent per 1,000 m. Cloud cover is already baked into a measured UV index reading, so do not discount further on a hazy day.
The base minutes describe time to the first perceptible redness (minimal erythema dose), not a painless tan threshold, and they vary person to person by a factor of about two either side. Treat the output as a planning floor: with SPF 30 sunscreen properly applied, the effective exposure time is roughly 30 times longer, but real-world application amounts are typically half the tested dose, so most dermatology guidance still says reapply every 2 hours.