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Sun Exposure Calculator
UV index risk, safe exposure time by skin type, required SPF, and vitamin D synthesis — based on WHO guidelines and published dermatology research.
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Educational Use Only — Not Medical Advice
Results are estimates based on generalised models (WHO UV guidelines, Fitzpatrick scale). Actual safe exposure depends on cloud cover, altitude, reflective surfaces, medication, and individual variation. Consult a dermatologist for personal guidance. People with skin conditions, history of skin cancer, or photosensitivity should seek medical advice before sun exposure.
WHO UV Index Scale
Low (0–2)Wear sunglasses on bright days. If you burn easily, cover up and apply SPF 15+.
Moderate (3–5)Take precautions — cover up, wear SPF 15–30, and seek shade near solar noon.
High (6–7)Protection required. Wear SPF 30+, wide-brim hat, and UV-blocking sunglasses. Seek shade midday.
Very High (8–10)Extra protection needed. Minimise outdoor activity 10 AM – 4 PM. SPF 50+ essential.
Extreme (11+)Stay indoors. Full protection at all times.
Frequently Asked Questions
What is the UV Index and where can I find it?▼
The UV Index (UVI) is a standardised measure of ultraviolet radiation at the Earth's surface, developed jointly by the WHO, UNEP, and WMO. It ranges from 0 (no UV) to 11+ (Extreme). Check your local weather app, the EPA UV Index forecast (USA), or the WHO Global UV Index report for your region.
What is the Fitzpatrick skin type scale?▼
The Fitzpatrick scale (1975) classifies human skin into six phototypes based on melanin content and response to UV radiation. Type I (very fair) has the lowest melanin and burns fastest; Type VI (dark) has the most melanin and the highest natural protection. The scale is used widely in dermatology to assess sunburn risk and UV sensitivity.
What SPF sunscreen should I use for my skin type?▼
The American Academy of Dermatology recommends a minimum of SPF 30 for most people, applied 15 minutes before going outside and reapplied every 2 hours. For Skin Types I–II or extended outdoor activity, SPF 50+ is advised. Note that SPF 30 blocks about 97% of UVB and SPF 50 blocks about 98% — the difference is small but matters for very sensitive skin.
How does sun exposure produce vitamin D?▼
UVB radiation (290–315 nm) converts 7-dehydrocholesterol in the skin to pre-vitamin D₃, which is further converted to vitamin D₃ (cholecalciferol) by body heat. Vitamin D₃ is then processed in the liver and kidneys to its active hormonal form. Sunscreen effectively blocks UVB, reducing vitamin D synthesis. People at higher latitudes, with darker skin, or who work indoors are at higher risk of deficiency (Holick MF, NEJM 2007).
Can I get vitamin D through a window?▼
Standard glass blocks nearly all UVB (290–315 nm) while transmitting UVA. Since UVB drives vitamin D synthesis, sitting by a closed glass window provides minimal vitamin D production. Specialised UV-transmitting glass exists but is rare in ordinary buildings.
Does cloud cover reduce UV exposure?▼
Clouds reduce UV radiation but not by as much as people assume. Thin to moderate cloud cover can still allow 50–80% of UV to pass through, while heavy overcast may reduce UV to 25–30% of clear-sky levels. Snow, sand, and water can also increase UV exposure by reflecting radiation — snow reflects up to 80% of UV.
Scientific References
- World Health Organization. Global Solar UV Index: A Practical Guide. WHO/SDE/OEH/02.2. Geneva: WHO; 2002.
- Holick MF. “Vitamin D deficiency.” New England Journal of Medicine 2007;357(3):266–281.
- Webb AR, Kline L, Holick MF. “Influence of season and latitude on the cutaneous synthesis of vitamin D₃.” J Clin Endocrinol Metab 1988;67(2):373–378.
- Schalka S, Steiner D, et al. “SPF: An important distinction between in vitro and in vivo.” Journal of Cosmetic Dermatology 2012;11(1):1–7.
- Fitzpatrick TB. “The validity and practicality of sun-reactive skin types I through VI.” Archives of Dermatology 1988;124(6):869–871.
- ICNIRP. “Guidelines on limits of exposure to ultraviolet radiation of wavelengths between 180 nm and 400 nm.” Health Physics 2004;87(2):171–186.