Can you get vitamin D through a window?

Bottom lineNo - window glass blocks the UV-B rays your skin needs to make vitamin D, so sun through a window counts as indoors. UV-A does pass through glass and contributes to skin aging, so long hours by bright windows give the downside without the vitamin D. Only open-air exposure at a UV index of roughly 3+ produces meaningful vitamin D.

No. Ordinary glass blocks essentially all UV-B - the specific wavelength band your skin needs to make vitamin D. A sunny seat by the window feels warm because visible light and UV-A come through, but for vitamin D purposes it counts as indoors.

The frustrating asymmetry

Here is the part worth knowing: UV-A does pass through glass, and UV-A contributes to skin aging and, over long cumulative exposure, skin damage. So all-day sun through a car or office window gives you some of sunlight's downside with none of the vitamin D upside. If you spend hours beside bright glass daily, that is a (mild) reason for sun care, not a vitamin D source.

What actually works

• Open air, bare skin, sun reasonably high. Even a balcony or an open window changes the answer - it is the glass that blocks UV-B, not being at home.
• Moderate UV or better. Below a UV index of about 3 (typical of winter at higher latitudes), skin makes very little vitamin D even outdoors - the best time of day matters too.
• Minutes, not hours. At decent UV, 10-30 minutes a few times a week does meaningful work for lighter skin tones; darker skin needs longer.

If your life is mostly indoors - office, car, home - you are in one of the classic risk groups for low vitamin D. The signs are worth knowing, and food or a 400-600 IU supplement fills the gap reliably.

Femora's Sunshine feature only counts real outdoor sessions - which is exactly the point.

Related: how much sun do you need for vitamin D · signs of low vitamin D · vitamin D converter