How long does it take to burn in the sun?

It depends on two numbers: the UV index where you are, and your Fitzpatrick skin type. The index sets how fast erythemal dose arrives; the phototype sets how much of it reaches a first-reddening reference dose. Both are published, so the calculation is arithmetic rather than opinion — every figure on this page and the 9 pages below is computed, not estimated.

Every level, at a glance

Minutes of continuous unprotected exposure to a first-erythema reference dose. Reference values, not measurements of any individual.
UV index	WHO category	SED per hour	Type I (min)	Type II (min)	Type III (min)	Type VI (min)
UV 3	Moderate	2.7	44	56	78	222
UV 4	Moderate	3.6	33	42	58	167
UV 5	Moderate	4.5	27	33	47	133
UV 6	High	5.4	22	28	39	111
UV 7	High	6.3	19	24	33	95
UV 8	Very high	7.2	17	21	29	83
UV 9	Very high	8.1	15	19	26	74
UV 10	Very high	9	13	17	23	67
UV 11	Extreme	9.9	12	15	21	61

Each level has its own page with the full type I–VI table, the conditions that actually produce that reading, and the published guidance for its band.

“Reporting burn times is not recommended.”

— WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide, WHO/SDE/OEH/02.2 (2002)

The guide gives its reasoning directly: “people tend to interpret burn times to mean that there is a safe level of unprotected sun exposure”, and relating index values to a time to burn therefore “sends out the wrong message”.

We publish the number anyway, for one reason: the question is being asked, and a page that poses it and withholds the answer sends the reader back to a source that will state a number with none of this context. What we will not do is dress the number up as permission. The figure is the output of an arithmetic model applied to reference values, it says nothing about any individual’s skin, and dose accumulates below the threshold exactly as it does above it — the WHO guide notes that while avoiding sunburn is the priority goal, “cumulative UV radiation exposure plays a major role in developing skin cancer and promotes damage to the eyes and immune system”.

These pages answer the generic question. The app answers yours.

Safe Sun Window takes your skin type and the UV index where you actually are, and runs the same calculation on those inputs instead of a whole-number example. Free, no account, no ads. Return visits are counted with a random first-party id that goes to no one else. If you turn on location, your device rounds your position to about 10 km before it reaches our weather provider for the UV reading, with no id attached.

Open Safe Sun Window →

How the calculation works

The UV index is not a scale someone invented for weather bulletins. WHO defines it in Annex C of its practical guide as 40 m²/W multiplied by solar spectral irradiance integrated against the CIE reference action spectrum for UV-induced erythema (ISO 17166:1999 / CIE S 007/E-1998). One index point is therefore 25 mW/m² of erythemally weighted irradiance.

One standard erythema dose is 100 J/m². So an hour of continuous exposure at index U delivers U × 0.025 × 3600 ÷ 100 = U × 0.9 SED. That step is exact given the definition.

The second step is not exact. Standard photobiology reference tables assign each Fitzpatrick phototype a minimal erythema dose — the dose producing the first perceptible reddening. Dividing that reference dose by the hourly rate gives the minutes in the tables above. Real thresholds vary by roughly ±30–50% within a single phototype, and self-assigned phototype adds its own error on top, so these are reference figures for a category and never a measurement of a person.

The six factors that set the index

From WHO’s practical guide, with its own figures:

Sun elevation. “The higher the sun in the sky, the higher the UV radiation level.” Outside the tropics the maximum occurs around solar noon in the summer months, and about 60% of a day’s UV is received between 10 a.m. and 2 p.m.
Latitude. “The closer to equatorial regions, the higher the UV radiation levels.”
Cloud cover. Levels are highest under cloudless skies, but over 90% of UV can penetrate light cloud, and scattering can raise total levels.
Altitude. Levels rise 10% to 12% for every 1000 metres, or about 4% per 300 metres, because a thinner atmosphere absorbs less.
Ozone. Ozone absorbs UV that would otherwise reach the surface, and levels “vary over the year and even across the day”.
Ground reflection. Fresh snow can reflect as much as 80% of UV, sea foam about 25%, dry beach sand about 15%. This raises the dose a person receives without raising the reported index, which is defined for a horizontal surface.

WHO’s exposure categories

Table 1 of the practical guide groups index values into five categories, reported worldwide with a standard colour code (Table 4): Low, below 2 (Green); Moderate, 3 to 5 (Yellow); High, 6 to 7 (Orange); Very high, 8 to 10 (Red); Extreme, 11 and above (Purple). Pages exist here for 3 upward because that is where the guide puts its own threshold: “Above the threshold value of 3, protection is necessary, and this message should be reinforced at UVI values of 8 and above.”

The published protection measures

WHO’s guide lists these as its basic sun protection messages. They are reproduced here verbatim, and they are WHO’s, not ours:

Limit exposure during midday hours.
Seek shade.
Wear protective clothing.
Wear a broad-brimmed hat to protect the eyes, face and neck.
Protect the eyes with wrap-around-design sunglasses or sunglasses with side panels.
Use and reapply broad-spectrum sunscreen of sun protection factor (SPF) 15+ liberally.
Avoid tanning beds.
Protect babies and young children: this is particularly important.

The published scheme is binary rather than graded. The WHO guide recommends protection above a threshold value of 3 and states that of the two proposed concepts — a threshold beyond which protection is recommended, or a graded response adding measures as the index climbs — “there is little scientific basis to support the latter: if sun protection is required, this should include all protective means”. Both the WHO and the EPA do mark a second point at 8, but they mark it by reinforcing the message, not by adding measures. So the measures a reader will find published for UV 4 and for UV 10 are the same measures; what changes with the index is the rate at which erythemal dose accumulates, which is what the table on this page quantifies.

Sources

World Health Organization, World Meteorological Organization, United Nations Environment Programme and International Commission on Non-Ionizing Radiation Protection. “Global Solar UV Index: A Practical Guide.” WHO/SDE/OEH/02.2, 2002.
Source for the exposure-category table (Table 1), the international colour codes (Table 4), the six factors that set the index, the definition of the index in Annex C, and the guide’s explicit position on reporting burn times.
ISO 17166:1999 / CIE S 007/E-1998. Erythema reference action spectrum and standard erythema dose.
The action spectrum the UV index is weighted against, named in Annex C of the WHO guide. One standard erythema dose (SED) is 100 J/m² of erythemally weighted radiant exposure.
United States Environmental Protection Agency. “UV Index Scale.” epa.gov, last updated 15 June 2026; verified 3 August 2026.
Source for the quoted protective-action text and the shadow rule reproduced on these pages. The EPA groups UV 3 to 7 under a single “Moderate to High” label with one block of action text and changes wording only at 8 — which is why the guidance quoted on the UV 3 page is word-for-word the guidance quoted on the UV 7 page.
Fitzpatrick TB. “The validity and practicality of sun-reactive skin types I through VI.” Archives of Dermatology, 1988;124(6):869–871.
The origin of the six-point phototype scale used in the table on every page. WHO’s guide reproduces the classification as its Table 2.
Minimal-erythema-dose values per Fitzpatrick phototype — working reference values, range-checked against ISO 17166:1999; the six-value progression itself is not traced to a named primary document.
These six figures set every number on this page. ISO 17166:1999 states that MEDs for phototypes I to IV would be expected to lie between 150 and 600 J·m⁻², equivalent to 1.5 to 6 SED, and our type I–IV values (2.0, 2.5, 3.5 and 4.5 SED) all fall inside that range — so the figures are consistent with the standard, which is not the same as being taken from it. We have not traced the specific six-value progression to a single named primary document and will not imply that we have; a citation goes here when we can name one. They are category reference values in any case, never a measurement of an individual — minimal erythema dose varies by roughly ±30–50% within a single phototype, and self-assigned phototype adds error of its own.