How to measure the TDS of your coffee cup?

To measure coffee TDS, use a coffee refractometer. Take a few millilitres of coffee cooled to room temperature (~20°C), place 1-2 drops on the prism, and read the result in °Brix. Convert to % TDS using a standard correction factor (typically ×0.85 for filter, depending on the instrument). The measurement takes under 30 seconds.

TDS measurement has become standard practice in specialty cafés and among competition baristas. It allows objective quantification of beverage concentration, and combined with coffee and water weights, enables calculation of extraction yield — a key indicator of quality and reproducibility.

The refractometer principle is based on the refractive index: light bends differently depending on liquid density. The more dissolved solids (sugars, acids, oils) water contains, the higher the index. The instrument measures this index and expresses it in degrees Brix (°Brix), a unit originally developed for sugar solutions in the wine and food industry. For coffee, empirical conversion factors have been developed: generally, espresso TDS (%) = °Brix × 0.85-0.87, and filter TDS = °Brix × 0.87-0.90, depending on instruments calibrated specifically for coffee.

The most widely used coffee refractometers: the VST Coffee Tool is the professional reference (precision ±0.03% TDS), but its price (~€300-400) limits it to professionals. The DiFluid R2 Extract is a popular alternative at ~€120-150 with a mobile app. The Atago PAL-COFFEE sits in the middle on price and precision. Analogue wine refractometers (€15-30) can approximate the measurement but with less accuracy.

Rigorous measurement protocol: 1) Prepare the coffee normally. 2) Let a 5-10 mL sample cool to room temperature (~20°C) — temperature significantly affects the reading. Some users put the sample in a small container in the freezer for 2-3 minutes. 3) Clean and dry the prism with a microfibre cloth. 4) Place 1-2 drops on the prism, close the lid. 5) Read the °Brix value. 6) Rinse immediately with distilled water and dry. 7) Apply the conversion factor to get % TDS.

A common problem: prism contamination from the previous coffee, which can skew the reading by up to +0.5 °Brix. Always rinse and dry thoroughly between measurements. Also: measure espresso undiluted (no milk or sugar before measurement), and always from the same volume at the same point in the cup for reproducibility.

TDS measurement protocol — key steps

• Step 1: Prepare coffee according to normal recipe
• Step 2: Take 5-10 mL sample and cool to ~20°C
• Step 3: Clean and dry the refractometer prism
• Step 4: Place 1-2 drops on the prism, close the lid
• Step 5: Read °Brix — apply temperature compensation if needed
• Step 6: Convert to % TDS (factor × 0.85-0.90 depending on instrument)
• Step 7: Rinse prism with distilled water and dry thoroughly
• Option: use TDS + coffee/water ratio to calculate extraction yield