Our Vitamin D Calculator estimates the amount of vitamin D your skin produces from sunlight during the time period you select. It also estimates the earliest time when sunlight may begin to damage your skin. In addition, you will see a UV data graph for your selected day. The calculator is based on the following formula by Miyauchi and Nakajima (2016):
The calculator only needs the following information: UV data retrieved based on your selected location, time spent in sunlight, age, skin type, clothing, and sunscreen use. Two different equations are used depending on the UV index level.
UV index ≥ 1.6
When the UV index is below 1.6, the relationship is no longer linear. In this case, the following second-order polynomial is used to calculate the vitamin D-producing UV radiation:
UV index < 1.6
The equation for lower UV index values provides a less precise estimate, because the relationship between UV radiation and vitamin D production is more complex at low UV levels.
The U.S. National Institutes of Health (NIH) recommends 15 micrograms (600 IU) per day for people aged 1 to 70, and 20 micrograms (800 IU) for those over 70. For infants under 1 year, the recommendation is 10 micrograms (400 IU) per day (National Institutes of Health, 2024). Sources of vitamin D include sunlight, supplements, and food. Foods rich in vitamin D include fish, cod liver oil, beef liver, and egg yolks.
Holick (2010) found that the human body can produce a maximum of 10,000 to 25,000 IU of vitamin D per day. For this reason, the calculator sets an upper limit of 10,000 IU. This maximum is typically reached in summer conditions when the UV index is high enough and a large part of the body is exposed to sunlight.
When the body produces large amounts of vitamin D, sunlight begins to break down the vitamin D that has formed in the skin (Holick et al., 1981). Because of this natural regulation mechanism, it is not possible to get a harmful amount of vitamin D from sunlight.
Skin type has a significant effect on vitamin D production in the skin. The melanin pigment in darker skin acts as a natural sunscreen that filters UVB radiation (Holick et al., 1981). This means that people with darker skin need more time in the sun to produce the same amount of vitamin D as people with lighter skin. Our calculator uses skin type factors based on Miyauchi and Nakajima (2016) and Chen et al. (2007).
Although darker skin produces vitamin D more slowly, this is a protective mechanism shaped by evolution: melanin protects the skin from UV radiation damage, which is especially important in regions with strong sunlight. Lighter skin, on the other hand, produces vitamin D more efficiently, which is beneficial at northern latitudes where UV radiation is weaker.
MacLaughlin and Holick (1985) showed that a 70-year-old person produces only about 37% of the vitamin D that a young adult produces. Our calculator takes this into account by using an age factor that gradually decreases with age.
The calculator estimates the skin area exposed to sunlight based on your selected clothing. The calculation is based on the "rule of nines" model (Moore, Popowicz, and Burns, 2024), which states that the total body surface area of an adult is approximately 18,000 cm². For example, the head and neck make up 9%, one arm makes up 9%, and so on. Then the calculator assigns an exposure factor to each body part that is exposed to sunlight, based on the study by Cheng et al. (2020). This way we can calculate a realistic estimate of your vitamin D production.
Sunscreen filters UVB radiation, which the skin needs for vitamin D synthesis. For example, SPF 15 filters about 93% of UV radiation. SPF 30 filters about 97%, and SPF 50 filters up to 98% of radiation. This means that vitamin D production slows down significantly when using sunscreen. For example, if you would produce 1,000 IU of vitamin D in 30 minutes without sunscreen, it would theoretically take over 15 hours to produce the same amount with SPF 30 sunscreen. On the other hand, sunscreen also extends the time you can spend in the sun without risk of skin damage.
Chen, T.C., Chimeh, F., Lu, Z., Mathieu, J., Person, K.S., Zhang, A., Kohn, N., Martinello, S., Berkowitz, R. and Holick, M.F. (2007). Factors that influence the cutaneous synthesis and dietary sources of vitamin D. Archives of Biochemistry and Biophysics, 460(2), 213–217.
Cheng, W., Brown, R., Vernez, D. and Goldberg, D. (2020). Estimation of Individual Exposure to Erythemal Weighted UVR by Multi-Sensor Measurements and Integral Calculation. Sensors, 20(15), 4068. https://doi.org/10.3390/s20154068.
Holick, M.F. (2010). Vitamin D and Health: Evolution, Biologic Functions, and Recommended Dietary Intakes for Vitamin D. In Vitamin D: Physiology, Molecular Biology, and Clinical Applications (2nd ed.). Springer.Holick, M. F., MacLaughlin, J. A., Clark, M. B., Holick, S. A., Potts Jr, J. T., Anderson, R. R., Blank, I. H., Parrish, J. A., & Elias, P. (1981). Regulation of cutaneous previtamin D₃ photosynthesis in man: skin pigment is not an essential regulator. Science, 211(4482), 590–593.
MacLaughlin, J., & Holick, M. F. (1985). Aging decreases the capacity of human skin to produce vitamin D3. Journal of Clinical Investigation, 76(4), 1536–1538.
Miyauchi, M. and Nakajima, H. (2016). Determining an Effective UV Radiation Exposure Time for Vitamin D Synthesis in the Skin Without Risk to Health: Simplified Estimations from UV Observations. Photochemistry and Photobiology, 92(6), 863–869.
Moore, R.A., Popowicz, P. and Burns, B. (2024) 'Rule of Nines', StatPearls. Treasure Island (FL): StatPearls Publishing. Available at: https://www.ncbi.nlm.nih.gov/books/NBK513287/
National Institutes of Health, Office of Dietary Supplements. (2024). Vitamin D – Consumer Fact Sheet. https://ods.od.nih.gov/factsheets/VitaminD-Consumer/