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As others have pointed out, it depends on time of year and where you live. Far north, e.g. Canada, the U.K, Scandinavia the sun won't give you any Vitamin D at all for roughly six months of the year.

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I’ve been using the DMinder app (not affiliated in any way and it’s totally free) to estimate how much vitamin D I’ve been getting. You input how overcast it is and what percent of your skin is exposed and then it uses your skin tone (input at setup), latitude, altitude, and time of day and it’ll track a rough estimate of how much vitamin D you’ve produced while you’re outside.

There’s also “Quick and Easy Screening for Vitamin D Deficiency in Adults”[1] which has a quick and dirty formula in Table 2 to help assess your risk of deficiency without a blood test.

[1]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4998626/pdf/med...

is exposed amount of skin an input?

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