A refractometer measures how much light bends passing through a liquid, which correlates with dissolved solids. For coffee it gives you TDS — the strength of the brew as a percentage.
What you do with the number
Combine TDS with the beverage weight and the dry coffee weight and you get extraction yield: what percentage of the coffee actually dissolved.
- Under about 18 per cent — under-extracted
- 18 to 22 per cent — the useful window
- Over 22 per cent — over-extracted
Why it is useful
It separates two things that taste similar and need opposite fixes.
A brew can be weak but well extracted (too much water — change the ratio) or strong but under-extracted (too coarse — change the grind). Taste alone confuses these regularly; the numbers do not.
For someone learning systematically, that is genuinely valuable.
Why it is not necessary
Nobody needs it to make excellent coffee. Time, weight and taste get you to the same place, more slowly, for nothing.
Cafés use them for consistency across staff and shifts, which is a different problem from a home brewer’s.
The cost
Two hundred euros and up for one accurate enough to be useful. Cheap ones have too much variance to be worth the trouble.
Spend it on a grinder first.
Last reviewed 8 August 2026