How long does today's UV take to deliver a sunburn dose?

Two inputs — your skin type and today's UV index — run through published photobiology values. It computes the time to an erythemal (sunburn) reference dose, and the vitamin-D synthesis window. 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.

How this works

No camera, no AI guess — just two facts (your skin type and today's UV index) run through published photobiology reference values. One UV Index point equals 25 mW/m² of erythemal (sunburn-causing) irradiance, the World Health Organization's own definition. From that, standard minimal-erythema-dose reference tables for each Fitzpatrick skin type give a computed time to that dose — and, above UV Index 3 (below that, vitamin-D-effective UVB is negligible), a rough vitamin-D synthesis window from published models (Webb; Engelsen's FastRT).

What's a Fitzpatrick skin type?

A six-point scale (Fitzpatrick, 1988) describing how skin typically responds to strong sun — from always burns, never tans (Type I) to never burns (Type VI). It's the standard reference dermatology and photobiology have used for decades. Self-rating it is approximate, which is exactly why the figure shown is a reference value for a category rather than a countdown for you.

Where does the UV Index come from?

Two ways, and it tries the easier one first. When you open the check it asks your browser for your location; allow it and your device rounds the position to about 10 km, our proxy looks up the current index for that area, and the number is filled in for you. If you decline, or the lookup fails, the manual field stays put and you enter the figure your weather app already shows. Either way it is the same quantity, defined by the WHO as the ground-level intensity of sunburn-causing UV radiation. A number you type yourself always wins over a late automatic result.

Burn times for a specific UV index

If you already know the number and just want the table, there's a reference page for every UV index from 3 to 11 — the full Fitzpatrick type I–VI figures at that level, where and when that reading actually occurs, and the arithmetic: how long it takes to burn, UV 3 to 11.

Is this medical advice?
No. It is computed output from public photobiology reference values, not a diagnosis, a prescription, or a substitute for a doctor or dermatologist, and it never states that a duration of exposure is safe. If you have a skin condition, sun sensitivity, or are on a photosensitizing medication, this tool isn't calibrated for you.
How accurate is the window?
Minimal-erythema-dose reference values vary roughly ±30-50% within a single Fitzpatrick type, and self-rating your own type is imprecise. The figure the app reports is a fixed 0.6 multiple of the computed threshold — an arithmetic margin for that spread, not a measurement of your skin and not a built-in guarantee. The WHO guide notes that erythemal dose accumulates below the threshold as well as above it.
Why not just check the UV Index directly?
The UV Index alone is the same number for everyone in a location — it doesn't account for how your specific skin type responds. This turns that one shared number into a personal estimate.