How long in the sun to make your vitamin D?
When the sun is high, a few minutes to half an hour on the arms and legs is enough, depending on skin color. When it is low, in winter above about 40° latitude, the skin makes almost none, even at noon.
Choose a city, a month, a time and your skin type: the animation shows UVB rays getting through (or not) the ozone layer, the skin molecule they transform, then the path of vitamin D to the liver, kidney and gut.
How does the skin make vitamin D?
UVB rays transform a skin molecule, 7-dehydrocholesterol, into previtamin D3. Body heat then changes it into vitamin D3 within a few hours.
Vitamin D3 is not active yet. The liver turns it into 25(OH)D, the storage form, the one measured in a blood test. The kidney then makes calcitriol from it, the real hormone, which helps the gut absorb calcium from food. Only UVB rays from 290 to 315 nanometers do this work (Webb, 1988).
Why almost nothing in winter?
Because the sun is too low: its rays cross more air and the ozone layer stops almost all UVB. In Boston (42° N), skin exposed in full sun from November to February made no previtamin D3 at all; in Edmonton (52° N), from October to March (Webb, 1988).
Our calculation for 3 cities, on the 21st of the month at solar noon, under a cloudless sky (sun height, then UV index estimated with the Madronich formula, 2007):
| City | January | March | June | September | December |
|---|---|---|---|---|---|
| Paris | 21°, UV 1 | 41°, UV 4.4 | 65°, UV 9.8 | 41°, UV 4.5 | 18°, UV 0.7 |
| Madrid | 29°, UV 2.2 | 49°, UV 6.4 | 73°, UV 11.2 | 49°, UV 6.4 | 26°, UV 1.7 |
| Panama City | 61°, UV 9 | 81°, UV 12.1 | 76°, UV 11.6 | 81°, UV 12.1 | 58°, UV 8.3 |
These are theoretical fair-weather values at sea level. Clouds, pollution and shade lower them. Calculated on October 27, 2026.
How is the number of minutes calculated?
We start from an often-cited rule: a quarter of the dose that reddens the skin, on a quarter of the body, gives about 1,000 IU of vitamin D (Holick, 2007). The time therefore depends on the UV index, the skin and the exposed area. Dark skin needs more time: melanin filters UVB.
The doses that redden each skin type are rough averages: each skin reacts in its own way. The result is an order of magnitude, not a prescription. Below a sun height of 40°, we show "almost nothing", based on Webb's measurements (1988).
Frequently asked questions
Can you make too much vitamin D in the sun?
No. When exposure goes on, extra previtamin D3 turns into inactive molecules. Excess vitamin D comes from supplements, not from the sun (Holick, 2007).
Do you make vitamin D behind a window?
Hardly: ordinary window glass stops almost all UVB, the rays that make vitamin D.
Does sunscreen stop you from making vitamin D?
It filters UVB, so in theory it reduces production. In practice, a review of 75 studies concludes that the risk is low, because people rarely apply enough to block everything (Neale, 2019).
How much vitamin D do you need per day?
The European Food Safety Authority (EFSA) sets an intake of 15 micrograms per day (600 IU) for an adult, when the skin makes little (EFSA, 2016).
Why is vitamin D useful for bones?
Without it, the gut absorbs calcium poorly. Bone then lacks the material to rebuild itself: see the page on osteoporosis.
See also
Bone that renews itself Kidney stones A 24-hour day
Sources
- Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab. 1988;67(2):373-8. PMID 2839537
- Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-81. PMID 17634462
- Madronich S. Analytic formula for the clear-sky UV index. Photochem Photobiol. 2007;83(6):1537-8. PMID 18028230
- EFSA NDA Panel. Dietary reference values for vitamin D. EFSA Journal. 2016;14(10):4547. DOI 10.2903/j.efsa.2016.4547
- Neale RE, et al. The effect of sunscreen on vitamin D: a review. Br J Dermatol. 2019;181(5):907-15. DOI 10.1111/bjd.17980