UV Exposure Calculator

The calculator divides an assumed minimal erythemal dose by UV Index-derived irradiance to estimate time to visible reddening. That estimate is not a safe-exposure time. Skin response, spectrum, altitude, reflection, medication, application amount, and changing UV conditions can make real exposure differ substantially.

The Fitzpatrick scale describes a typical tanning and burning response, but self-classification is imprecise and every skin type can sustain UV damage. The calculator assigns a reference erythemal dose to each type for its model. A longer modeled time to redness is not a longer safe exposure time.

The calculator multiplies by SPF as an idealized model. Real protection is not a reliable linear timer because application thickness, missed areas, water, sweat, friction, product spectrum, and reapplication matter. Do not stay in the sun until the displayed time.

The standard public-health rule is to reapply broad-spectrum sunscreen at least every two hours, and sooner after swimming, sweating heavily, or toweling off. The calculator caps its reapply-after time at two hours, but if your skin would burn sooner than that even with sunscreen on, it shows the shorter burn time as the binding limit instead.

The model assumes a flat reading and average conditions. Altitude, time of day, and reflective surfaces like snow, water, and sand all raise your real UV dose, while heavy clouds and shade lower it. Some medications and skin conditions also increase sun sensitivity. Use the result as a planning guide, then watch your skin and seek shade at the first sign of pinkness.

No. It is for education and general sun-safety planning only, and it is not a substitute for professional medical advice. If you have a history of skin cancer, take photosensitizing medication, or have any concern about a mole or sun reaction, talk to a dermatologist or doctor.