It seems to be common knowledge among bakers that salted butter has a higher water content than unsalted*. How much higher?
And if there's a different amount of water, why does this nutritional data say there's the same amount of fat in salted and unsalted butter? (I'm fairly sure I've seen the same on butter in the store, and I'll try to remember to confirm that next time I'm there.)
*I'm pretty sure I remember seeing the claim several places online, but the one I was able to quickly find is from Cook's Illustrated (click to expand the second-to-last question) via an answer here, saying that "salted butter almost always contains more water than unsalted butter". (It's possible that a lot of other people's knowledge ultimately came from there.) The nutritional information I've found, along with my own baking experience, does not back this up, but Cook's Illustrated is generally pretty trustworthy, hence the question.
Best Answer
The FDA definition for butter specifies only that it be made exclusively from cream or milk, contain a minimum of 80% butter fat by weight, and may include salt and coloring.
FDA Butter Definition
USDA Butter Standards
If you assume the butter in question is at the low end of the spectrum with exactly 80% fat and equal amounts of milk solids, then unsalted would have less water than salted. Since there is no standard that says milk solids must also be a specific percentage, or that fat content can only be 80% and no more, there is really no way to accurately make the claim that salted butter always contains more water than unsalted.