UV Index Exposure Calculator
Calculate safe sun exposure time from UV index level 1-11 and Fitzpatrick skin type.
Returns time to first burn, sunscreen need, and clothing recommendations.
UV Index (UVI) is a standardized scale measuring the intensity of ultraviolet radiation reaching the Earth’s surface at a given time and location. It was developed by the World Health Organization (WHO) and Environment Canada in 1992 and is now used globally to guide sun protection decisions.
UV Index calculation (simplified): UVI = (Σ E(λ) × ε(λ) × Δλ) ÷ 0.025
Where:
- E(λ): spectral irradiance (power per unit area per wavelength)
- ε(λ): erythema action spectrum (how effectively each wavelength causes sunburn)
- 0.025: the normalizing constant (W/m² per UVI unit)
In practice, UVI is measured by calibrated radiometers and reported by meteorological agencies. The consumer formula is simpler: UVI = Erythemally weighted irradiance ÷ 0.025 W/m²
UV Index scale and sun protection guide:
| UVI | Exposure Level | Protection Required | Time to Burn (fair skin) |
|---|---|---|---|
| 0–2 | Low | Minimal | 60+ minutes |
| 3–5 | Moderate | SPF 30+, shade midday | 45–60 minutes |
| 6–7 | High | SPF 30+, hat, limit midday | 30 minutes |
| 8–10 | Very High | SPF 50+, seek shade | 15–25 minutes |
| 11+ | Extreme | Maximum protection, avoid midday | Under 10 minutes |
Factors that increase UV Index:
- Altitude: UV increases ~10–12% per 1,000 meters of elevation
- Latitude: Highest at the equator; decreases toward poles
- Ozone depletion: 1% less ozone → ~2% more UV-B reaching surface
- Reflection: Snow reflects 80% of UV; sand/water reflect 15–20%
- Time of day: UV peaks from 10 AM to 4 PM (2 hours each side of solar noon)
- Season: Summer solstice = highest annual UVI in your hemisphere
SPF formula: time protection extension: Protected Burn Time = Natural Burn Time × SPF
At UVI 6 (30-min burn for fair skin) using SPF 30: 30 × 30 = 900 minutes — but note SPF tests use much more sunscreen (2 mg/cm²) than most people apply; realistic protection is often SPF ÷ 2 to SPF ÷ 3.
Worked example: Miami, Florida, USA. June 21 (summer solstice), 11 AM.
- Expected UVI: 10–11 (Very High to Extreme)
- Fair-skin burn time unprotected: ~10–15 minutes
- With SPF 50 applied correctly: ~500–750 minutes (theoretical)
- Practical recommendation: Reapply SPF 50 every 90–120 minutes, seek shade between 11 AM–3 PM, wear UV-protective clothing (UPF 50+)
At UVI 11+, vitamin D synthesis is achieved within 3–5 minutes — there is no need to stay in full sun longer for vitamin D benefits.