How the Calculator Works
Content last reviewed: October 2026
This page explains, step by step, how VitaminDcalc turns your answers into an estimated sun exposure time. The goal is transparency: every factor used by the calculator is listed here, together with its limitations.
1. Sun position and solar time
From your latitude and the date, the calculator computes the solar declination and the solar zenith angle (SZA, the angle between the sun and the point directly overhead) for every minute of the exposure. Your clock time is converted to true solar time using the time zone (including daylight saving time), the longitude and the Equation of Time, because the sun's highest point rarely happens at 12:00 on the clock. With “Use my location”, the exact longitude of your device is used; with country and region, a representative longitude and the country's main time zone are used.
2. Estimating the UV Index
Under a clear sky, the UV Index is estimated from the solar zenith angle with the empirical relation UVI ≈ 12.5 × cos(SZA)2.4, and set to zero when the sun is very low. Cloud cover multiplies it by 1.0 (0–20% clear sky), 0.7 (30–60% partly cloudy) or 0.3 (70–100% overcast).
3. From UV Index to UV dose
By definition, a UV Index of 1 equals 25 mW/m² of erythemally weighted irradiance. Over one minute this gives 1.5 J/m² per UV Index point on a horizontal surface. The simulation adds up this dose minute by minute, following the changing height of the sun.
4. The vitamin D target
The target follows the rule of thumb attributed to Dr. Michael Holick: exposing about 25% of the body to one quarter of the minimal erythemal dose (MED) produces roughly 1,000 IU of vitamin D. The MED depends on skin type (Fitzpatrick I to VI, from 200 to 1,000 J/m² in this model). A larger exposed skin area reaches the target proportionally faster. The 1,000 IU figure is a modeling reference, not an intake recommendation.
5. Body geometry
The UV Index is measured on a flat, horizontal sensor, but the body is three-dimensional. For vitamin D, the model assumes that only about half of the exposed skin faces the sun at any time (factor 0.5) and that the lit side receives about 80% of the horizontal dose because of its curvature (factor 0.8), giving a combined factor of 0.4.
6. Age and BMI adjustments
Synthesis efficiency is reduced by about 13% per decade after age 20 (to a minimum of 30%), reflecting the decline of 7-dehydrocholesterol in aging skin. For BMI, the result is adjusted by 0.8 for overweight and 0.6 for obesity, because vitamin D is diluted in body fat. If you choose “I don't know”, the calculator shows the result for each BMI profile.
7. Sunburn safety limit
The simulation stops before the vitamin D target if the accumulated dose reaches 80% of your MED. This limit uses the full ambient dose (without the body-geometry reduction), because sunburn is local: the shoulders, face and other surfaces facing the sun can receive close to the full horizontal dose. If this limit is reached, the result warns you not to stay longer.
8. Limitations
This is a simplified model. It does not measure real-time ozone, aerosols, altitude, reflection from snow, sand or water, shade, or the use of sunscreen, and it cannot account for medications that increase sun sensitivity or for individual differences in vitamin D metabolism. The results are general estimates for educational purposes and are not medical advice. If you are concerned about your vitamin D level, ask a healthcare professional about a blood test (25(OH)D).
The scientific sources used to build the model are listed in the references section of the calculator page.