Across 4 tracked cities, the national average is ~156 optimal days per year — days when the sun clears the 45° threshold needed for effective UVB synthesis. Liège leads with 157 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. Bruxelles is loaded below — type your town to switch.
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If you had to pick one city in Belgium for vitamin D, Liège would be hard to beat — it gets 157 optimal days a year, well above the national average of ~156. That gap can be the difference between a quick 15-minute top-up and planning a much longer outing.
No — during winter the sun simply doesn't climb high enough anywhere in Belgium for meaningful synthesis. The atmosphere filters out most UVB at shallow angles. Supplementation (800–2,000 IU/day) is the practical approach during those months.
The highest UV in Belgium comes from Liège, where it can peak at 8.3 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.
Somewhat. Even a degree or two of latitude shifts the solar elevation angle, and therefore UVB intensity. Within Belgium's relatively compact range, the differences are modest — coastal exposure, local weather patterns, and altitude often matter more than latitude at this scale.
Sadly, no — same answer wherever you are in Belgium. 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.