Two numbers that get confused constantly
TDS is a property of the liquid: 1.35 percent means 1.35 grams of dissolved solids in every hundred grams of coffee. Extraction yield is a property of the grounds: 19.39 percent means 19.39 percent of the dry coffee ended up dissolved. They are not the same and they do not move together unless the ratio is held constant.
Pour half the brew away and the TDS is unchanged while the extraction the arithmetic reports is halved, which is nonsense — nothing happened to the grounds. That is the sanity check for whether you have the two straight.
The bed keeps a fifth of the water
Twenty-two grams of coffee and 360 grams of water, with the bed holding two grams per gram of dry coffee, leaves 44 grams in the grounds. The cup gets 316 grams, not 360. Twelve percent of the water never came out.
Run the yield off 360 and you get 22.09 percent. Run it off 316 and you get 19.39. Nearly three points, always in the same direction, and it is the single most common reason two people with the same refractometer get different numbers off the same brew.
The correction gets larger the weaker the ratio, because the bed holds the same amount whether you poured 300 grams or 500 over it. At 1:12 the bed holds a sixth of the water; at 1:20 it holds a tenth. Same brewer, different error.
Weighing beats estimating
The retention ratio is the only soft number in the chain. Everything else is a scale reading or a refractometer reading. If you weigh the brewed coffee instead — put the carafe on a scale, tare it, pour — the retention drops out entirely and the answer is as good as your scale.
The reason to have the ratio at all is that most people do not weigh the output, and having an approximate correction is a great deal better than having none. Two grams per gram is a starting placeholder to replace with your own, and replacing it takes one brew: weigh the wet bed, subtract the dry dose, divide by the dose.
What the page will not tell you
Whether 19.39 percent is good. There is no answer to that which survives contact with a different coffee, a different roast or a different drinker, and any page that gives you a target range is telling you about its author rather than about your cup. The target field is there so the arithmetic can measure against your number. Where your number should be is a cupping question.
Questions people ask
How do I calculate extraction yield from a TDS reading?
Multiply the TDS percentage by the mass of the brewed coffee to get the grams of dissolved solids, then divide by the dry dose. The trap is the second term: it is the coffee that came out, not the water that went in. With 22 grams of coffee, 360 of water and a bed holding 44, the beverage is 316 grams, and 316 times 1.35 percent divided by 22 is 19.39 percent.
Why does using the brew water instead of the beverage weight matter?
Because the water still in the spent bed contains no coffee you can drink, and treating it as beverage credits the brew with solids that are in the bin. With the defaults it overstates extraction by 2.70 points, every time, always upward. The error grows as the ratio gets tighter, since the bed holds roughly the same amount however much water you poured.
How much water do coffee grounds hold?
Yours is worth measuring rather than borrowing, because it moves with the brewer and with the grind. Weigh the spent bed wet, subtract the dry dose, divide by the dose. A rinsed paper cone, a metal basket and a plunged press all come out differently, and the page uses whatever you put in rather than assuming a figure.
What is a good extraction percentage for coffee?
This page does not say, on purpose. It depends on the coffee, on how it was roasted and on who is drinking it, and a published range tells you about whoever published it. The target field exists so the arithmetic can compare against a number from your own cupping notes and tell you the TDS that would get you there.
Is TDS the same as strength?
TDS is strength — dissolved solids as a share of the liquid. Extraction yield is a different thing: the share of the dry coffee that dissolved. Adding water lowers the strength and does not change the extraction. Grinding finer can raise the extraction with the strength barely moving. Keeping the two apart is most of the value of measuring at all.