UV Exposure Time Calculator
Calculate safe sun exposure from UV index and Fitzpatrick skin type (I-VI).
Returns time to sunburn without protection and with SPF 30 and SPF 50.
UV Index is a standardized international scale developed by the WHO that measures the intensity of ultraviolet radiation reaching the Earth’s surface. It predicts the rate at which skin damage can occur and directly informs safe sun exposure time for different skin types.
Safe exposure time formula:
Minutes to Minimum Erythemal Dose (MED) = MED ÷ (UV Index × 0.025)
Where MED is the dose of UV radiation that produces the first perceptible redness for a given skin type, and 0.025 is the standard UV irradiance factor (W/m²) per UV Index unit.
Fitzpatrick Skin Type MED values:
| Skin Type | Description | MED (J/m²) | Approx. minutes at UV Index 6 |
|---|---|---|---|
| Type I | Always burns, never tans | 200 | ~22 min |
| Type II | Usually burns, sometimes tans | 250 | ~28 min |
| Type III | Sometimes burns, always tans | 350 | ~39 min |
| Type IV | Rarely burns, tans easily | 450 | ~50 min |
| Type V | Brown skin, very rarely burns | 600 | ~67 min |
| Type VI | Dark brown/black, almost never burns | 800 | ~89 min |
UV Index scale:
| UV Index | Risk Level | Recommended Action |
|---|---|---|
| 0–2 | Low | No protection needed |
| 3–5 | Moderate | Sunscreen recommended |
| 6–7 | High | SPF 30+ required, limit midday exposure |
| 8–10 | Very high | SPF 50+, protective clothing |
| 11+ | Extreme | Stay indoors 10am–4pm |
SPF and UV protection:
Protected Time = Unprotected Time × SPF
SPF 30 blocks ~97% of UVB. SPF 50 blocks ~98%. No sunscreen blocks 100%.
Worked example: A Type II person at the beach with UV Index 9, using SPF 50:
- Unprotected MED time: ~19 minutes
- With SPF 50: 19 × 50 = 950 minutes theoretical: but reapplication every 2 hours is still essential as sunscreen degrades with sweat and water.