Across 20 tracked cities, the national average is ~179 optimal days per year — days when the sun clears the 45° threshold needed for effective UVB synthesis. Marseille leads with 195 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. Paris is loaded below — type your town to switch.
Loading live data…
If you had to pick one city in France for vitamin D, Marseille would be hard to beat — it gets 195 optimal days a year, well above the national average of ~179. 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 France 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 France comes from Toulon, where it can peak at 9.66 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.
It makes a real difference. Across France's ~8° of latitude, the more southerly cities get a few extra weeks of good synthesis windows each year. Not a dramatic gulf, but noticeable — worth checking the city pages if you're in the northern part of the country.
Sadly, no — same answer wherever you are in France. 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.