UV Index Exposure Time Calculator

How it works

time to burn = MED (J/m²) ÷ (UVI × 0.025 W/m²) ; MED by skin type ≈ 200 / 250 / 350 / 450 / 600 / 1000 J/m²; protected time ≈ time × effective SPF

The UV index is erythemal irradiance in units of 25 mW/m², so dividing your skin’s minimal erythemal dose by it gives the minutes to the first sign of burn. Fair skin (type II) at UV 8 burns in about 20 minutes. SPF multiplies that time only at the tested dose of 2 mg/cm² — a shot-glass for the body — and most people apply a third of that, which reduces an SPF 30 to roughly SPF 3. Reapplication and shade matter more than the number on the bottle.

Worked example

UV 8, skin type II (MED 250 J/m²): 250 ÷ (8 × 0.025 × 60) = 21 minutes unprotected; SPF 30 applied thinly (effective ~3) gives about an hour, applied properly about 10 hours.

Frequently asked questions