Across 19 tracked cities, the national average is ~235 optimal days per year — days when the sun clears the 45° threshold needed for effective UVB synthesis. Honolulu leads with 365 optimal days/year. Explore the month-by-month breakdown and all cities below.
The data above shows averages across cities. Your real window depends on where you are today, current cloud cover, and your skin type. New York is loaded below — type your town to switch.
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If you had to pick one city in United States for vitamin D, Honolulu would be hard to beat — it gets 365 optimal days a year, well above the national average of ~235. That gap can be the difference between a quick 15-minute top-up and planning a much longer outing.
Some do, yes — a handful of cities in United States can still get some synthesis through December, January, and February. It varies quite a bit depending on where exactly you are, so it's worth checking the individual city pages for specifics rather than assuming one answer fits the whole country.
The highest UV in United States comes from Honolulu, where it can peak at 13.03 in summer — high enough to burn quickly. Keep synthesis windows short (10–20 min for medium skin) and apply SPF 30+ the moment you're done. More time outside doesn't mean more vitamin D; past a point, it's just sun damage.
A lot, actually. United States spans roughly 26° of latitude — a huge range. A city near the equator can have good UV year-round, while a northern city might go months in winter with no synthesis at all. That's why the differences between individual city pages here can look so dramatic.
Sadly, no — same answer wherever you are in United States. Glass blocks UVB almost entirely, which is the exact wavelength needed for vitamin D synthesis. Sitting in bright sunshine through a window feels warm, but it won't move your vitamin D levels. You need real, unfiltered sky on your skin.
Yes, but maybe less than you'd expect. Thin cloud cover barely makes a dent — around 90% of UVB still gets through on a lightly overcast day. Heavy overcast (75%+ cloud cover) is more significant: you might need to stay out about twice as long for the same effect. On a genuinely grey day, it's usually not worth it. SunDose uses today's cloud forecast for your exact location so you always get a realistic estimate.