Single-shot. Solar geometry computed in-browser. UV index from a TEMIS-class forecast feed. Synthesis model based on Holick's rule, normalized against published Mediterranean-basin reference times.
Fitzpatrick measures sun reactivity, not ancestry. The hint is just to help you self-identify — pick by how your skin behaves in the sun, not by background.
By default, the calculator uses the current time and auto-refreshes every minute.
Solar geometry: NOAA Solar Position Algorithm (simplified Reda & Andreas / Spencer 1971). Deterministic, no external data. Sub-degree accuracy is sufficient for vitamin D synthesis thresholds.
UV index source: live TEMIS-class forecast feed driven by satellite ozone column and cloud cover. If the API call fails, the calculator falls back to a clear-sky estimate from solar elevation alone, which is the dominant signal anyway.
Synthesis model: reference times to 1000 IU at 25% body exposure are anchored to published Holick's rule values (Type I ≈ 5 min, Type III ≈ 7.6 min, Type VI ≈ 25 min at UVI 7). IU/min then scales linearly with UVI and body-area fraction, with an age multiplier (linear decline after age 20, floor 0.4 at age 70+).
Known weaknesses: linear UVI scaling overstates synthesis at very low elevations where action-spectrum mismatch dominates — synthesis effectively gates off below solar elevation ~20° and ramps in quadratically to full at 45°. Doesn't model altitude, surface albedo (snow, sand, water), or window glass (which blocks UVB entirely). Skin-type self-report is the single largest error source in practice. Treat absolute IU as ±20% under best conditions.