A coffee refractometer measures the refractive index of the brew, which correlates with total dissolved solids — the strength.
What you do with the number
TDS plus the brew ratio gives you extraction yield — the percentage of the coffee that dissolved.
Extraction % = (TDS × beverage weight) ÷ dose
Plot it against strength on the Brewing Control Chart and you can see whether a brew is under-extracted, over-extracted or in the target 18 to 22% window.
Why it is genuinely useful
It removes guesswork from diagnosis. “Sour” could be under-extraction or a bright coffee. The number settles it.
It makes changes measurable. You can see that a grind change moved extraction from 17.5 to 19.8%, rather than hoping.
It is essential in a commercial setting, where consistency across staff and shifts matters.
Why most people do not need one
They cost real money — a decent one is more than a good grinder for many people — and the same information is available by tasting.
Sour means under-extracted. Bitter and drying means over-extracted. That is what the number would tell you.
If you buy one
Filter the sample and let it cool to a consistent temperature, or the readings wander. Measure the same way every time.
Weak or under-extracted?
The number’s real value is separating two faults that taste alike and need opposite fixes. A brew can be weak but well extracted — too much water, so change the ratio — or strong but under-extracted — too coarse, so change the grind. Taste confuses these regularly; the refractometer does not. Below about 18 percent extraction is under-extracted, 18 to 22 percent is the useful window, above 22 is over-extracted.
Spend it elsewhere first
A better grinder improves your coffee. A refractometer measures it.
Last reviewed 30 October 2026