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

From abstract percentage to tangible flavour map

The first time specialty barista training programmes introduce refractometers, there is typically a moment of calibration surprise: the instructor brews a cup that tastes genuinely good, measures it at 1.35% TDS and 19.5% EY, and then brews a cup that tastes flat, measures it at 1.20% TDS and 17% EY. The numbers make the difference visible, but the cups already made the point by taste. Refractometer training is not about teaching baristas to trust instruments over palates — it is about giving them a language to describe and reproduce what their palates already detect.

TDS measurement requires more care than simply applying water to a prism and reading a number. Temperature is critical: a 1°C variation in sample temperature shifts the reading by approximately 0.05% TDS on a standard Brix refractometer — enough to meaningfully misplace a shot on the brew chart. Professional protocols call for either temperature-compensated refractometers (most specialty models above €80 include this), or for careful cooling to a reference temperature of 25°C before measurement. The DiFluid R2 Extract, popular in the 2025–2026 specialty community, uses temperature compensation plus a Bluetooth connection to a smartphone app that automatically calculates EY from the TDS reading and your brew parameters.

Going deeper

Cleaning protocol matters too. Coffee residue on the prism from a previous measurement will skew readings. Standard practice is to wipe with a lint-free cloth after each measurement and apply two drops of distilled water to re-zero before the next sample. In a busy bar environment, some baristas keep two refractometers — one for espresso, one for filter — to avoid cross-contamination of residue. The investment in measurement discipline pays compound returns: within two weeks of consistent TDS logging, most baristas identify a repeatable 'signature' range for their specific machine, grinder and water combination, and can predict cup quality from brew parameters alone without tasting every shot.