TDS and Refractometers: Measuring Coffee Strength
Judging by taste alone leaves you at the mercy of your mood and memory. TDS is close to the only objective figure available to a home brewer. That said, measuring for the sake of measuring helps nobody, so it is worth understanding what the number actually tells you.
What TDS measures
TDS stands for total dissolved solids: the proportion of the drink that is dissolved coffee rather than water. It is expressed as a percentage, and filter coffee usually lands between 1.15% and 1.45%. The other 98-odd percent is water.
The catch is that TDS tells you only about strength. It says how concentrated the drink is, not how much you extracted from the grounds. Pouring less water raises TDS; brewing longer also raises TDS.
So TDS is only meaningful alongside the beverage weight and the dose. Combine all three and you get extraction yield.
Yield from three numbers
You need exactly three values: TDS (%), beverage weight (g) and dose (g).
Yield (%) = TDS (%) × beverage weight (g) ÷ dose (g)
The value people get wrong is beverage weight. It is not the water you poured; it is what actually ended up in the server. The grounds hold back roughly 2g of water per gram of coffee, so the figures always differ.
How to take a reading
A coffee refractometer estimates dissolved solids from how much light bends passing through the liquid. Taking the reading takes seconds, but skipping the procedure makes the numbers wander badly.
The largest source of error is temperature. Hot samples tend to read low, so draw off a small amount and let it come close to room temperature. The second is homogeneity: concentration is layered inside the server straight after brewing, so stir before you sample.
Errors and how to read around them
A number on a screen invites trust, but refractometer readings move easily with conditions. Knowing which way each factor pushes stops you panicking at an odd result.
Starting without one
Refractometers are not cheap, and there is a lot you can do first. It is worth exhausting the alternatives before deciding.
Ratio alone controls much of the strength axis: weighing the coffee and the water transforms consistency on its own. Brew time serves as an indirect read on yield: finishing early suggests a low yield, running long suggests a high one.
A refractometer earns its keep at the point where two cups taste different and you genuinely cannot work out why. Until then, a scale and a timer buy you more improvement.
One reading versus many
A single TDS figure tells you that this cup was 1.35% and nothing else. It becomes meaningful once enough readings exist to form your own baseline.
Record TDS and yield across many brews and the range you consistently enjoy emerges. Some people land around 19%, others nearer 21%. The textbook 18–22% is a wide safe zone; the point of measuring is to find your spot inside it.
Log TDS, beverage weight and dose together and the yield falls out automatically, and over time your own distribution appears on the brewing control chart.