What is the difference between strength and extraction?

Understanding the distinction between strength and extraction is the key to mastering brewing science. Strength, or TDS, is a measure of the ratio of coffee solids to water in your final beverage [6, 7]. If you have a high TDS, your coffee will feel heavy, intense, and concentrated on the palate. If the TDS is low, the coffee will feel light, thin, or 'watery' [6]. You can easily adjust strength by changing your brew ratio—simply using more coffee or less water will increase the strength of the final cup.

Extraction, on the other hand, is a measure of how much of the coffee bean's potential flavor you have successfully pulled into the water [7]. It is expressed as a percentage of the dry coffee weight. If you extract too little (under-extraction), you leave behind the sugars and heavier compounds, resulting in a sour, salty, or thin cup [6, 10]. If you extract too much (over-extraction), you pull out too many bitter, woody, or astringent compounds [2, 10].

These two concepts are independent but related. You can have a very strong cup of coffee that is under-extracted (a 'strong' but sour shot) or a very weak cup of coffee that is over-extracted (a 'watery' but bitter cup) [6]. The goal of a skilled brewer is to find the 'sweet spot' where the extraction is balanced—typically between 18% and 22%—and the strength is at a level that is pleasant to the drinker [6].

In practice, you manage these variables using different tools. You control strength primarily through your brew ratio (the amount of coffee to water). You control extraction primarily through grind size, water temperature, and contact time [1, 7]. By adjusting these variables, you can move your brew toward that ideal balance where the coffee is neither too strong nor too weak, and neither sour nor bitter [6].