How does one calculate extraction percent? I am reading “Everything But Espresso” by Scott Rao and am in the chapter “The Coffee Brewing Control Chart”.  

In the section “How to Calculate Extraction without a Chart”, he says you need three values:


Brew strength (total dissolved solids)
Dry coffee mass
Brewed coffee mass


You plug those values into the following two equations to get extraction percent:

TDS% x Brewed Coffee Mass = Extracted Mass

Extraction% = (Extracted Mass) / (Dry Mass)

So, my question is: how do you get brewed coffee mass?  Do you have to take out your mug before you start everything and then weigh the solution afterwards?  

Also, is it necessary to have a refractometer to calculate the TDS?

Without knowing how to get those two variables, I obviously can’t solve the whole equation. 

Any tips on any of this is greatly appreciated.
You have everything right. If you want Extraction % you need to measure:


Total dissolved solids: You will have to measure this using a refractometer. There are many out there, for commercial settings they start at around 300€. I have no experience with cheaper, "home use" refractometers but I'd assume they lack accuracy. 
Dry coffee mass (in g): You have to weigh the ground coffee before brewing, that's all there is to it.
Brewed coffee mass: As in 2. you will have to weigh your brew. Tar your kitchen scale with the cup on it, brew coffee and put the cup back. Voila.


Now you just all pluck it in the equation and you are good to go. Suppose you have 15g of ground coffee, yield 190g of brewed coffee with a TDS of 1,3%, which is great for a pour over.

That gives you an extracted mass of 2,47g.

2,47g/15g = 0.165. So the extraction % would be 16,5%.