I normally don't keep unsalted butter on hand since I use it so seldom. So when a recipe calls for salt and unsalted butter among its ingredients would it be alright to use salted butter and reduce the amount of salt? And if so, by how much should the salt be reduced? In other words, how much salt is typically contained in a tablespoon, ounce or gram of salted butter?
Depending on brand, it is approximately 1 1/4 tsp per pound (US), or a little more than 1/4 tsp per stick (4 oz).
For most applications, yes it is fine to substitute and adjust; you can just adjust the "salt to taste" step of your recipe in many cases.
There are a very few uses (such as yeast raised dough) where you want to be more precise. I would not use salted butter for a yeast dough by preference, but if I had to, I would calculate the amount of salt to remove from the other ingredients based on this ratio:
1 1/4 tsp salt / 16 oz butter
If you're in the US, labeling laws actually make it pretty easy to know exactly how much salt is in your butter, and yes, it varies by brand.
Salt is sodium chloride, it's 40% sodium by weight. Land O Lakes salted butter (my go-to brand) has 90mg of sodium per tablespoon. That means it has 225mg of salt per tablespoon, or 1.8 grams per stick, 7.2 grams per pound. Table salt weighs 5.7 grams per teaspoon, so Land O Lakes salted butter contains 1.26 teaspoon of salt per pound of butter.
I always bake with unsalted butter, but just now I am making a brioche according to a recipe that will take as much as 6 ounces of butter in one loaf. That’s a stick and a half, a BOATLOAD of butter. I wanted to make it special and use a highly rated, European style cultured butter. I’ve got the butter, but it was only available salted. That’s ok. This brand has 55mg sodium per tablespoon. That’s 660mg sodium for all of the butter in the recipe. 660mg sodium = 1650mg salt (NaCl), or 1.65 grams. The recipe calls for 3.3grams of salt to be added with the (unsalted) butter, so I’ll add (strangely enough) half of that, 1.65 grams.
According to the folks at America's Test Kitchen, the salt content in butter varies and may increase or decrease the amount of water in the butter. If you use unsalted butter, there are few if any variations. I've baked muffins and cookies with both salted and unsalted butter and personally prefer unsalted butter in baking but salted butter when adding to a finished dish (vegetables) or making things like grilled cheese.
If you're substituting the unsalted butter you have into a recipe that calls for salted butter:
- If using metric units, add about 1.5% salt. That means for 100g of butter, add 1.5g of salt.
- If using English measure, add 5/16 tsp per 4 oz stick of butter.
Similarly, if you're substituting salted butter into a recipe that calls for unsalted butter, remove an equivalent amount of salt elsewhere. Be more cautious going this direction, though; for example, if you're making a dish where butter is added separately from the salt, you should think about why the salt is being added separately before using salted butter.