Minutes of sun for adequate vitamin D — by season, latitude, skin tone and the winter problem
Vitamin D is the vitamin your skin manufactures — given UVB of sufficient intensity, which depends on latitude, season, time of day, skin tone and exposed area in ways most advice glosses over. This estimator computes realistic synthesis minutes for your situation, states the winter truth plainly (above ~37°N, midwinter sun makes zero vitamin D no matter how long you stand in it), and weighs the whole exercise against the sunburn clock.
| Factor | Effect |
|---|---|
| Season + latitude | The gatekeeper: winter sun above ~37°N (a line through Richmond–St. Louis–Fresno) delivers no synthesis-grade UVB at all |
| Skin tone | Melanin is natural sunscreen: deep skin tones need 2–4× the minutes of fair skin — and show the highest deficiency rates at northern latitudes |
| Time of day | Midday (10–3) is 2–3× more efficient than morning/evening — the shadow rule: shorter than you = synthesis possible |
| Skin exposed | Arms + legs ≈ baseline; face-and-hands-only stretches times 60%+ |
| Glass & sunscreen | Both block UVB — office windows make zero D; SPF 30 cuts synthesis ~95% |
The same UVB that synthesizes D damages DNA — dermatology and endocrinology have negotiated an uneasy consensus: brief, sub-burn exposure (the estimator's minutes, well under the burn clock) or supplements, never deliberate tanning. Practical synthesis: get the short midday exposure when feasible, sunscreen after, and supplement through winter. D3 at 1,000–2,000 IU/day is the standard, cheap, well-tolerated dose; a 25(OH)D blood test (~$40, target ≥30 ng/mL) converts guessing to knowing — worth it once, especially for darker-skinned northerners, the deficiency demographic hiding in plain sight.
Salmon (~500–1,000 IU), canned tuna (~150), fortified milk (~120/cup), egg yolks (~40), UV-exposed mushrooms (variable) — a normal diet supplies 100–300 IU/day against a ~600–800 IU requirement. Food is a supplement to the sun/supplement decision, not a substitute.
No — glass blocks UVB entirely (while passing the UVA that ages skin: the worst of both). The sunny desk is a placebo; open air is the requirement.
SPF 30 blocks ~95-97% of it, yes. The compromise dermatologists live with: your brief calculated minutes bare, then protection — or skip the ritual and supplement. Never trade sunburn for vitamin D; synthesis saturates long before damage does.
Stores fade over weeks-to-a-couple-months — summer levels decay by midwinter, which is exactly what population studies show (D nadirs in March). Northern winters need a plan: supplement, or accept the trough.
The consensus adult dose: 1,000-2,000 IU/day of D3, more only under testing (toxicity is real above ~10,000 IU/day chronic). With a deficiency test result, clinicians dose higher temporarily. D3 beats D2; take it with a meal containing fat.
Mostly no — beds emit primarily UVA (tanning, aging, cancer) with little synthesis-grade UVB. The WHO classifies them as carcinogenic; as a D strategy they're all risk, no mechanism.
Darker-skinned people at northern latitudes, adults 65+ (skin synthesis declines ~50%), the housebound, people with malabsorption or on certain meds, and anyone with symptoms (bone pain, fatigue, frequent illness). One $40 test replaces years of guessing.
Yes — everything computes locally in your browser.
Summer: minutes, not sessions. Winter above the line: a cheap D3 capsule and no illusions. And once, a blood test — the only sentence in the vitamin D debate that ends it.