THE NUMORIX GUIDE
How to use the Vitamin D Calculator
Last reviewed September 14, 2026
What this calculator does
The engine applies a skin-type factor, a time-of-day factor, and a latitude band factor to an estimated body surface area.
Formula and method
The engine applies a skin-type factor, a time-of-day factor, and a latitude band factor to an estimated body surface area. Its body-surface term is 0.007184 x weight^0.425 x 175^0.725, so height is fixed at 175 cm rather than entered. It multiplies the exposed fraction of that area by 250 IU/min, the selected duration, and the three factors, rounds to IU, and compares the result with a fixed 600 IU reference.
Variables and inputs
Enter skin type 1-6, weight in kg, latitude in degrees, time 10am/12pm/2pm, sun duration in minutes, and exposed body percentage of 25, 50, 75, or 100. Outputs are estimated IU, a fixed 600 IU daily reference, and its percentage. Unknown time or skin keys fall back to a factor of 1.0 or 0.7 in the engine.
Worked example
With the defaults of skin type 3, 70 kg, latitude 40, noon, 15 minutes, and 25% exposed skin, estimated surface area is 0.007184 x 70^0.425 x 175^0.725 = 1.848 m2 and exposed area is 0.462 m2. Production is round(250 x 0.462 x 0.70 x 1.00 x 0.70 x 15) = 849 IU, or 142% of the engine's 600 IU reference.
How to interpret the result
The number is a simplified sunlight-production scenario, not a measured vitamin D level and not a supplement dose. A result above 600 IU does not prove that blood vitamin D is adequate; a result below it does not prove deficiency.
Common mistakes to avoid
Enter weight in kilograms and latitude as a signed or unsigned degree value; the engine uses absolute latitude. Do not treat 12pm as a universal UV maximum, and do not assume the fixed 175 cm term means height was measured. Keep exposed percentage separate from duration.
Assumptions and limitations
Real vitamin D production depends on season, cloud, altitude, ozone, UV index, clothing, sunscreen, skin response, age, and behavior, none of which this model measures. Unprotected UV exposure raises skin-cancer risk, and vitamin D status is assessed clinically with a blood 25-hydroxyvitamin D test. Do not extend sun exposure or set a supplement dose from this estimate.