What the calculator does, what it doesn't, and how to trust the numbers.
How many minutes of sun exposure it takes to synthesize a target dose of vitamin D, given your location, time, Fitzpatrick skin type, exposed body area, and age. It also shows the time to one MED (minimal erythemal dose — the sunburn threshold) so you can compare "time to target" against "time to burn" at a glance.
Treat absolute IU figures as order-of-magnitude under best conditions — published synthesis models disagree by up to ±20%, and self-reported skin type is the single largest source of error. The relative shape (when synthesis is viable, when it peaks, how a cloudy day compares to a clear one) is much more reliable than the absolute number.
Live UV index is fetched from currentuvindex.com, which serves a TEMIS-class forecast feed driven by satellite ozone and cloud-cover observations. When the live feed isn't reachable, the calculator falls back to a clear-sky estimate computed from solar elevation alone.
Yes, in two places. The live UV index already includes current cloud cover from satellite. The yearly chart applies a separate cloud modification factor (Foster–Lopez) using a three-year monthly climatology from Open-Meteo's historical archive. The clear-sky fallback ignores clouds entirely.
Cutaneous vitamin D synthesis is dominated by UVB, and UVB suffers disproportionate atmospheric attenuation at low sun angles. Below about 20° solar elevation almost no UVB reaches the surface, so cutaneous synthesis is effectively zero — even though the sun is clearly visible and you can still tan or burn from UVA at those angles. The model gates synthesis off below 20° and ramps it in quadratically to full at 45°.
Minutes to target is the time needed to synthesize your daily IU goal (default 1000 IU). Time to sunburn is the minutes to one MED, the dose at which fair skin reddens 24 hours later. Compare them: if time to sunburn is shorter than minutes to target, you can't hit the target in one session safely — shorten the session and repeat across days.
Window glass blocks essentially all UVB, so being indoors near a sunny window gives zero synthesis. Broad-spectrum sunscreen at the labeled application rate reduces UVB by roughly the SPF factor (SPF 30 → ~3% transmitted). Shade (umbrella, tree canopy) sharply reduces direct UVB but indirect scattered UVB still reaches you — typically 30–50% of open-sky values. The calculator assumes open-sky direct exposure.
Fitzpatrick is a six-point scale of skin sun-reactivity, not ancestry. Pick the type that best matches how your skin behaves: I — always burns, never tans; II — usually burns, tans minimally; III — sometimes burns, tans gradually; IV — rarely burns, tans easily; V — very rarely burns, tans darkly; VI — never burns. Ancestry hints in the UI are a rough guide, not a rule.
No. D Fetcher is a calculator for educational and planning purposes only. It does not consider individual health conditions, medications, supplements you may already be taking, or serum vitamin D levels. Consult a clinician for personal medical guidance.
No. Inputs you type into the page are saved only in your browser's localStorage so they persist between visits — they never leave your device. Geocoding queries and the UV index / cloud fetches go directly from your browser to the upstream APIs (zippopotam.us, Open-Meteo, currentuvindex.com); D Fetcher's server doesn't sit in between.