Getting Vitamin D From Sunlight
How your skin makes vitamin D from the sun, how much you actually need, why darker skin and winter make it harder — and how to top up without burning.
This is educational information, not medical advice. If you think you’re deficient, ask your doctor for a blood test rather than relying on the sun.
The quick answer
Your skin makes vitamin D when UVB hits it. For most fair-skinned people a short spell of midday sun — minutes, not hours — on bare arms and legs a few times a week is enough in the warmer months. You never need to burn, and you can’t make it through glass or at a low UV index.
How sunlight makes vitamin D
UVB rays convert a cholesterol-like molecule in your skin into vitamin D, which your body then activates. Because it’s UVB-driven, it only really happens when the UV index is 3 or higher — the same band that also causes burning, which is why timing matters.
What changes how much you need
- Skin type: melanin slows synthesis, so darker skin (higher Fitzpatrick types) needs longer.
- UV index: more UVB means faster synthesis; under UV 3 there’s little point.
- Skin exposed: arms and legs make far more than just face and hands.
- Season & latitude: in winter at high latitudes the sun is too low for UVB.
- Age: older skin makes vitamin D less efficiently.
The vitamin D calculator takes your skin type, the UV index and how much skin is exposed and estimates a sensible, sub-burn exposure.
Doing it without burning
The dose for vitamin D is a fraction of what it takes to redden your skin, so stay well within your burn time. Brief, regular exposure beats one long session — and once you’ve had your few minutes, cover up or apply SPF.
When to choose a supplement
In winter, at high latitudes, for darker skin, or if you cover up or stay indoors, sunlight may not be enough. A vitamin D supplement is a safe, UV-free way to maintain your levels — your doctor can advise on the right dose.