Vitamin D Sunlight Calculator
Enter your latitude, month and skin type. The calculator uses the sun's angle at midday to estimate whether UVB is strong enough for your skin to make vitamin D.
Limited vitamin D from sunlight this month
Midday sun reaches about 40° above the horizon. Synthesis is possible around midday but slower. Darker skin needs roughly 2× longer.
Your "vitamin D winter"
At this latitude, sunlight makes little vitamin D in: January, November, December.
This is a simplified model based on sun angle. Cloud, pollution, altitude, sunscreen, age and time outdoors also matter. Avoid sunburn; it raises skin cancer risk. Most adults who supplement take 600–2,000 IU a day.
Frequently asked questions
What is a vitamin D winter?
The months when the sun is too low for UVB to reach the ground in useful amounts, so skin makes little or no vitamin D. In Boston that's roughly November to February.
How much sun do I need for vitamin D?
It varies with latitude, season, time of day and skin tone. Because sun exposure also raises skin cancer risk, many people rely on food and supplements instead.
Sources
Free tool for education only. It doesn't replace advice from your doctor or pharmacist.