What you are measuring
TDS (total dissolved solids) is the share of the liquid in your cup, by weight, that is dissolved coffee. A coffee refractometer reads it from how much the liquid bends light. Extraction yield (EY) turns that into a share of your dry coffee: how much of the dose ended up in the cup.
They answer different questions. TDS is strength; EY is how far the coffee was extracted. With the same EY, a 1:1.5 shot reads twice the TDS of a 1:3 shot.
The formula
EY (%) = beverage weight (g) × TDS (%) ÷ dose (g)
For example, 18.5 g of coffee, 37 g in the cup and a TDS of 9.4% gives 37 × 9.4 ÷ 18.5 = 18.8%. Use the weight of the liquid in the cup, not the water the machine used, because the grounds hold some of it back. Brew ratio here is beverage weight divided by dose, so 18 g in and 36 g out is 1:2.
Where the numbers usually sit
- The Specialty Coffee Association advises that coffee most often tastes best when 17% to 23% of the dry coffee is extracted. Above about 23% tends to taste bitter and below about 17% sour [1].
- Another reference gives 18% to 22% as the desirable range for most coffee and says espresso yields are generally quoted at 15% to 25% [2].
- Cafés commonly use 15 to 22 g of coffee for 30 to 60 g of beverage [1].
- Rearranged, the formula says that at a 1:2 ratio, 17% to 23% EY is a cup reading of 8.5% to 11.5% TDS. That is arithmetic, not a standard.
Those ranges describe where coffee tends to taste balanced. They are a map, and your palate decides where on it you want to be.
Getting a reading you can trust
- Weigh the beverage on a scale. Crema makes volume misleading.
- Refractive index changes with temperature [1], so let the sample reach the temperature your refractometer asks for.
- Filter the sample through a syringe filter. Espresso carries fine particles that a paper filter lets through and that skew the reading [3].
- Remember that EY is an estimate: it rests on a TDS reading and two weights, and each has some error.