Sunbathing Calculator
Calculate safe sun exposure time based on skin type, UV index, and SPF to avoid sunburn. Enter values for instant results with step-by-step formulas.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Safe Time = (MED / UV Index / Environmental Factors) x SPF x 0.75
MED (Minimal Erythemal Dose) is the UV exposure time to cause reddening for your skin type at UV index 1. This is divided by the actual UV index and environmental factors (altitude, reflection), then multiplied by SPF. A 75% safety margin is applied since real-world conditions are variable.
Worked Examples
Example 1: Beach Day with SPF 30
Problem:Fair-skinned person (Type II) at the beach with UV index 8, SPF 30 sunscreen, sea level, near water.
Solution:Base MED for Type II = 100 minutes at UV 1\nMinutes to burn at UV 8 = 100 / 8 = 12.5 minutes\nWater reflection factor = 1.10\nAdjusted burn time = 12.5 / 1.10 = 11.4 minutes\nWith SPF 30 = 11.4 x 30 = 341 minutes\nSafe time (75%) = 256 minutes = 4h 16m\nReapply every 2 hours
Result:Without SPF: ~11 min to burn | With SPF 30: ~4.3 hours safe | Reapply every 2 hours
Example 2: High Altitude Hiking
Problem:Medium-skinned person (Type III) hiking at 2,400m elevation, UV index 9, SPF 50, no water/snow.
Solution:Base MED for Type III = 200 minutes at UV 1\nMinutes to burn at UV 9 = 200 / 9 = 22.2 minutes\nAltitude factor = 1 + (2400 x 0.04 / 300) = 1.32\nAdjusted burn time = 22.2 / 1.32 = 16.8 minutes\nWith SPF 50 = 16.8 x 50 = 842 minutes\nSafe time (75%) = 631 minutes = 10.5 hours
Result:Without SPF: ~17 min to burn | With SPF 50: ~10.5 hours safe | UV 32% stronger at altitude
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy