How this calculator works
Vitamin D synthesis in the skin depends on UV-B radiation from sunlight. UV-B reaches Earth's surface only when the sun is high enough in the sky — specifically, when the solar elevation angle exceeds approximately 15°. Below this angle, UV-B is absorbed by the atmosphere before reaching ground level, regardless of how sunny it appears.
This calculator uses your latitude to compute the solar declination angle for each month of the year (based on Earth's axial tilt of 23.45°), then estimates daily UV-B exposure and resulting vitamin D synthesis adjusted for your skin type, daily outdoor duration, and the fraction of skin exposed. The output is an estimate — individual results vary based on cloud cover, altitude, pollution, and personal biology.
Why latitude matters so much
The further you live from the equator, the lower the sun sits in the sky during winter — and at angles below 15°, synthesis drops to zero. This creates a "vitamin D winter" — a period each year when supplementation is the only option regardless of outdoor time.
- Below 37° latitude (Los Angeles, Rome, Sydney): year-round synthesis possible, though winter months are reduced.
- 37–51° latitude (London, Edinburgh, northern USA): vitamin D winter of 3–5 months.
- Above 51° latitude (Scotland, Scandinavia, northern Canada): 5–7 month vitamin D winter.
- Above 60° latitude (Helsinki, Oslo, Anchorage): near-zero synthesis for 6+ months.
The skin type effect
Melanin — the pigment responsible for skin colour — acts as a natural UV-B absorber. This is why people with darker skin evolved in low-latitude, high-UV environments where excess vitamin D production was a risk. At high latitudes, the same melanin becomes a significant disadvantage.
A person with Type VI skin (very dark) requires roughly 5–10× more sun exposure than a person with Type I skin (very fair) to produce equivalent vitamin D. This makes vitamin D deficiency disproportionately common in dark-skinned populations living at high latitudes — a major contributor to health disparities in countries like the UK, Canada, and Scandinavia.
What "no data" actually means
When this tool shows zero synthesis, it means the physics are simply impossible — no amount of outdoor time, no amount of skin exposure, and no amount of anything other than supplementation or UVB lamps will produce vitamin D during those months. The UV-B wavelengths required (290–315 nm) are completely absorbed by the atmosphere at low solar angles.
When and how to supplement
If this tool shows you have zero-synthesis months or months below 1,000 IU/day estimated synthesis, supplemental vitamin D3 is clinically appropriate. Key guidance:
- Form: Always use vitamin D3 (cholecalciferol), not D2 (ergocalciferol). D3 is 87% more effective at raising serum 25(OH)D levels.
- Dose: 2,000–4,000 IU/day for most adults; 1,000–2,000 IU/day for maintenance. The NIH upper limit is 4,000 IU/day.
- Pair with K2: Vitamin K2 (MK-7, 90–200 mcg/day) activates proteins that direct calcium to bone and prevent arterial deposition — essential when supplementing D3 above 2,000 IU/day.
- Take with fat: Vitamin D3 is fat-soluble. Take with your largest meal for 32–57% better absorption.
- Test, don't guess: The correct blood test is serum 25-hydroxyvitamin D [25(OH)D]. Target: 40–60 ng/mL (100–150 nmol/L). Test before supplementing and retest after 3 months.
Read our complete Vitamin D dosing and safety guide or the Vitamin D3 ingredient guide for the full clinical picture including toxicity thresholds, drug interactions, and the D3 vs D2 evidence.