The amount of vitamin D you get from exposing your bare skin to the sun depends on:
◾The time of day – your skin produces more vitamin D if you expose it during the middle of the day.
◾Where you live – the closer to the equator you live, the easier it is for you to produce vitamin D from sunlight all year round.
◾The color of your skin – pale skins make vitamin D more quickly than darker skins.
◾The amount of skin you expose – the more skin your expose the more vitamin D your body will produce.
You can read more detail at http://www.vitamindcouncil.org/about-vitamin-d/how-do-i-get-the-vitamin-d-my-body-needs/.