About the Vitamin D Calculator

Why this exists

Nearly one billion people worldwide have insufficient vitamin D levels, yet the amount your skin can synthesise depends on factors most people never think about: latitude, season, time of day, cloud cover, skin pigmentation, age, and how much skin is exposed. This calculator brings those variables together in one place so you can make better-informed decisions about sun exposure and supplementation.

The science behind the calculator

The estimates are built on well-established photobiology and atmospheric science models:

• Beer-Lambert atmospheric model — calculates how UV-B radiation is attenuated as it passes through the atmosphere, accounting for solar zenith angle and altitude.
• Ozone climatology — uses latitude- and season-dependent total column ozone data to determine UV-B absorption in the stratosphere.
• Fitzpatrick skin type classification — accounts for differences in melanin concentration across six skin types, which directly affects the rate of pre-vitamin D₃ photosynthesis.
• UV spectral efficiency (action spectrum) — weights UV wavelengths by their effectiveness at converting 7-dehydrocholesterol to pre-vitamin D₃ in the skin.

Key references

The models and data used in this calculator draw from the following peer-reviewed research:

1. Webb, A. R. & Holick, M. F. (1988). Influence of season and latitude on the cutaneous synthesis of vitamin D₃. The Journal of Clinical Endocrinology & Metabolism, 67(2), 373–378.
2. Holick, M. F. (2007). Vitamin D deficiency. The New England Journal of Medicine, 357(3), 266–281.
3. Madronich, S. (2007). UV radiation in the natural and perturbed atmosphere. In Environmental Effects of Ozone Depletion and Its Interactions with Climate Change.
4. CIE (Commission Internationale de l'Eclairage). Action spectrum for the production of pre-vitamin D₃ in human skin. CIE 174:2006.
5. Fitzpatrick, T. B. (1988). The validity and practicality of sun-reactive skin types I through VI. Archives of Dermatology, 124(6), 869–871.