What is the difference between TDS and extraction yield?

In the world of brewing science, TDS and extraction yield are the two primary metrics used to quantify a cup of coffee. TDS is a measure of strength; it is typically measured using a refractometer and expressed as a percentage [7]. It tells you how much 'stuff' from the coffee bean is currently in your water. A higher TDS means a stronger, more intense cup, while a lower TDS means a weaker, more diluted cup [6, 7].

Extraction yield, however, is a calculation that relates the strength of the brew back to the amount of dry coffee you started with [7]. It answers the question: 'Of the 20 grams of coffee I put in the basket, what percentage actually ended up in my cup?' This is the true measure of how well you have managed your brewing variables like grind size, temperature, and time [7].

These two metrics are linked by your brew ratio. If you keep your extraction yield constant but add more water, your TDS will drop because you have diluted the same amount of dissolved solids into a larger volume of liquid [7]. Conversely, if you keep your brew ratio the same but change your grind size, you will likely change your extraction yield, which will in turn change your TDS [7].

In practice, using a refractometer to track these numbers allows you to move beyond guesswork. By measuring your TDS and calculating your extraction yield, you can identify exactly why a cup tastes the way it does. If your extraction yield is low (under 18%), you know you need to grind finer or increase your contact time. If your TDS is too high for your preference, you know you need to adjust your brew ratio to dilute the cup to your liking [7].