DermUV

SPF

SPF (Sun Protection Factor) measures how much UVB a sunscreen blocks: SPF 30 = ~97% of UVB blocked; SPF 50 = ~98%.

SPF explained

SPF is a multiplier on burn time, but only when 2 mg/cm² is applied — most users apply 25–50% of that, so real-world SPF is much lower than labeled. AAD recommends a minimum of SPF 30 broad-spectrum daily, applied 15 minutes before exposure and reapplied every 2 hours.

Related terms

Terms related to SPF connect the UV and sunscreen topic cluster.

Related DermUV pages

Continue with reference material and live city UV forecasts.

What to do with this SPF data

Sunscreen labels are tested at 2 mg/cm². Typical real-world application is closer to half that, so DermUV assumes about half the labelled SPF when it estimates protected time.

The UV index is a measure of erythemal (skin-reddening) irradiance, where each whole point equals 25 mW/m² of UV energy reaching the surface. That is why burn-time estimates fall roughly by half when the index doubles.

Cumulative dose drives photoaging and long-term skin damage more than any single day. Small daily exposures on commutes, walks, and driving add up across a season.

Reapplication matters more than the SPF number. Reapply roughly every two hours outdoors and immediately after swimming, heavy sweating, or towelling, regardless of what the bottle claims.

Cloud cover is not protection. Thin or broken cloud can still pass most incoming UV, and light cloud edges can briefly scatter more UV toward the ground than a clear sky.

UV scale reference

The WHO scale groups UV 1-2 as low, 3-5 as moderate, 6-7 as high, 8-10 as very high, and 11 or above as extreme. Protection is recommended from UV 3 upward, and shade during the middle of the day becomes the priority from UV 8 upward.

A practical routine at moderate-to-high UV is roughly one ounce of broad-spectrum sunscreen for full-body coverage, applied 15 minutes before going out, then reapplied on schedule. Clothing, hats, and shade carry the rest of the load, especially for children and anyone with a history of easy burning.

Keep exploring DermUV

These pages give more depth around spf, from UV-scale reference material to definitions and city forecasts.

DermUV forecast values come from Open-Meteo UV data (CC BY 4.0) and are refreshed throughout the day.