Vitamin D Dose Calculator — evidence-based tool to check your vitamin d dose. Includes formula, tips, and interpretation guide.
Estimated need (IU/day) = Base requirement × Skin tone multiplier × Sun exposure multiplier × Season multiplier
The base requirement follows Institute of Medicine (IOM) recommendations: 600 IU/day for ages 1–70, 800 IU/day for ages 71+. Multipliers increase the recommendation when sun synthesis is reduced: dark skin (Fitzpatrick V–VI) multiplies by up to 1.5×; minimal sun exposure adds up to 1.4×; winter season (in northern/southern latitudes) adds 1.3×. The 25(OH)D blood serum threshold for adequacy is ≥50 nmol/L (≥20 ng/mL); optimal is debated but often cited as 75–125 nmol/L.
Worked example: Age 45, skin tone IV (medium-dark), minimal sun, winter in UK → 600 IU × 1.4 (skin) × 1.4 (sun) × 1.3 (season) ≈ 1,530 IU/day. This falls within NICE's safe supplementation range. Food equivalents at this dose: ~15 portions of fortified milk, or ~6 portions of oily fish — reinforcing why supplementation is practical and food sources alone are rarely sufficient.