Coffee Refractometers: Measuring TDS and Calculating Extraction Yield
A coffee refractometer is an optical instrument calibrated to estimate the concentration of dissolved coffee solids—TDS—from a beverage's refractive index. Baristas and roasters use it to quantify beverage strength and calculate estimated extraction yield.
What the instrument measures
Light bends differently as it passes through solutions with different concentrations. A refractometer measures this refractive behavior and applies a coffee-specific calibration to report a TDS percentage. A reading of 1.35% means the instrument estimates about 1.35% of the beverage mass is dissolved coffee material.
TDS is strength, not extraction yield
TDS describes beverage concentration. Extraction yield estimates what percentage of the dry coffee dose dissolved into the beverage. A simplified calculation uses beverage mass × TDS ÷ dry coffee dose, with the units expressed consistently.
For example, 300 g of beverage at 1.35% TDS contains an estimated 4.05 g of dissolved solids. From a 20 g dose, that corresponds to about 20.25% estimated extraction yield.
Sampling matters
Espresso contains crema, suspended fines, oils, and strong concentration gradients immediately after brewing. Stir or otherwise homogenize the beverage according to the instrument's recommended protocol, and use any required sample filtering or settling procedure. A dirty prism, hot sample outside the device's compensation range, or poorly mixed cup can distort results.
What a refractometer cannot tell you
A target extraction percentage is not a taste score. Two coffees with the same TDS and extraction yield can taste completely different because of roast, origin, water, grinder distribution, and extraction evenness. Use measurements to explain and repeat good flavor, not to override sensory judgment.
Watch: Refractometers, TDS, and Measurement Technique
Lance Hedrick compares coffee refractometers while also explaining sampling, TDS, extraction yield, and best practices for getting repeatable measurements.
Our published guide to coffee extraction covers the fundamentals. More measurement terms are in the coffee glossary.
