Are all liquids equal?

You would want to start with an equal weight, not volume, of the buttermilk but be prepared to adjust while you are kneading.

Liquids very definitely do not have the same density. Last time we discussed this I referenced an essay from a Navy helicoper pilot who made a quick assumption that equal volumes of jet fuel and milk would have the same weight. Lucikily he survived, but when he look up the densities in his loadmaster handbook that night he found out that his assumption was off quite a bit.

sPh

You're correct, you'll need more.  When I use buttermilk I add extra 10% of it.

When you do make changes like this you must always keep in mind that hydration is determined by the amount of water in your overall dough. That water can come from plain water, but also from water content in other ingredients.

Milk and buttermilk consist largely of water and milk solids, suspended in the water. Thus equal amounts (weight) of water and buttermilk do NOT contain equal amounts of water. Therefore, if you want to keep hydration the same, you must figure out what part of (butter)milk is water. You may look on the Internet and find different answers, but generally speaking:

Whole milk is 88% water, nonfat milk is 91% water, condensed milk is 66% water, honey has around 20% water (variable). 

Example: The original recipe calls for 150g water (say), but you'd like to use whole milk for its additional properties.  Since the whole milk is only 88% water and you still need 150g water (to keep hydration the same), you'll need 150/0.88 = 170.5g whole milk. If the baker's percentage of the water was 66%, the percentage of the whole milk would need to be 66/0.88 = 75%.

A similar situation occurs if a recipe calls for milk and you have only milk powder. You must then assume that the powder represents the remaining 12% (assuming whole milk powder). Thus if you needed 150g whole milk you make that by using 150*0.12 = 18g powder, and 150-18=132g water. (Typically milk powder does not come in the whole milk variety and if you truly wanted to approach whole milk, you'd also add some butter) A quick rule would be to always assume a 90/10 split for milk, which is close enough for many purposes and also holds true for buttermilk. Thus, in your original question, you were right in thinking you need more, and the answer is that you need to divide the original amount of water by 0.9 to get the amount of buttermilk you should use.

Now, let's say you have a recipe for Challah that uses 150g eggs (that would be three eggs), but you want to make the bread more yellow by adding additional yolks. Two to be precise.

An average-sized egg weighs approximately 57 grams (about 2 ounces). Of this weight, the shell constitutes 11 percent; the white, 58 percent; and the yolk, 31 percent. Normally, these proportions do not vary appreciably for small or large eggs. Thus 1 whole egg is 50g without shell (75% water content), 33g is egg white (87% water content), and 17g is the yolk (52% water content). Therefore 2 yolks means adding 2*17=34g of weight, 52% of which is water: 17.7g. You will therefore need to use 17.7g less water than you otherwise would. (We are ignoring the fat contribution here, or we may be don't care about the extra fat)

The above examples only address hydration effects and assume you want to keep hydration the same. Milk and eggs contribute other properties to the dough and final product besides hydration, but that's for another discussion.  Find the calculations daunting? The Dough Calculator knows all these percentages and will do all kinds of conversions for you. 

--dolf


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