Sunburn Time Calculator
Estimate how long until unprotected skin burns at a given UV index and skin type.
About the Sunburn Time Calculator
The UV index is erythemally weighted UV irradiance ÷ 0.025 W/m², so UV index 8 delivers 0.2 W/m² of sunburn-causing UV. Time to burn is the skin type's minimal erythemal dose (roughly 200 J/m² for very fair skin to over 1,000 J/m² for the darkest) divided by that irradiance. Individual sensitivity varies a lot, so use this only as a rough guide.
The formula
Minutes = MED (J/m²) ÷ (UVI × 0.025 W/m²) ÷ 60
Worked example
Example: Time to sunburn (unprotected) = about 19 minutes
- Inputs: UV index: 9; Skin type (Fitzpatrick): II: usually burns, tans minimally; Sunscreen SPF (1 = none): 1.
- Formula: Minutes = MED (J/m²) ÷ (UVI × 0.025 W/m²) ÷ 60
- Result: Time to sunburn (unprotected) = about 19 minutes
Things to keep in mind
- Approximate; reflection from snow, sand and water, altitude and medications increase exposure. Sun damage builds up before visible burning.
Questions people ask
When does UV peak?
Around solar noon, roughly 10 a.m. to 4 p.m.
Do darker skin types need sunscreen?
Yes, UV still damages skin and eyes.