Refractometer

A refractometer is a small optical device that measures how much dissolved coffee is in your brew. You place a drop of filtered, cooled liquid on the sensor, and it reads the tds (total dissolved solids) as a percentage. Digital coffee refractometers do this by reading how light bends through the sample.

Why it matters: TDS tells you strength, but combined with your brew weights it lets you calculate extraction yield, the percentage of the coffee’s mass that ended up in the cup. That single number turns vague impressions into something measurable, so you can tell whether a flat cup is under-extracted or over-extracted rather than just guessing.

In practice you taste first and measure second; the refractometer confirms and helps you repeat good results. Most filter coffee lands roughly in the 18 to 22 percent extraction range with TDS around 1.15 to 1.45 percent, though targets vary by taste. It is a tool for dialing in and consistency, not a substitute for your palate. See refractometers-measuring-ey and extraction-theory.