wo numbers describe any brewed coffee: how strong it is, and how much of the ground dose it gave up. A refractometer reads the first directly and lets you compute the second. Together they turn "this tastes a bit thin" into a coordinate you can aim for again.
This guide is the measurement companion to the extraction foundation. It states the expert target numbers as targets and walks the measure-and-compute procedure, then shows where a cup lands on the Brewing Control Chart.
Before you reach for a refractometer
A refractometer measures one thing and infers the rest. It reads how much dissolved coffee is in the liquid, and from that one reading plus your dose and the mass of coffee in the cup you calculate how far the brew extracted. Strength is measured. Yield is computed. Keep the two ideas separate, because they answer different questions.
The two numbers: strength and yield
TDS, total dissolved solids, is the percent of the cup that is dissolved coffee rather than water. It is strength. A cup at 1.35% TDS holds about 1.35 grams of dissolved coffee in every 100 grams of cup, and that is what the refractometer reports.
Extraction yield is the percent of the dry dose that dissolved into the cup. It is not strength. You can hit the same strength with a small dose extracted hard or a large dose extracted gently, so yield is the number that tells you whether the grounds gave up too little, too much, or about the right amount of their soluble mass.
- TDS%
- Refractometer reading(strength of the finished cup, as a percent)
- Beverage mass (g)
- Weight of liquid in the cup(what you actually poured out and can drink, not the water added)
- Dose (g)
- Dry ground coffee weight(the same dose your brew started from)
- EY%
- (beverage g * TDS%) / dose g(the percent of the dry dose now dissolved in the cup)
The home extraction-yield formula, term by term
The Brewing Control Chart
The SCA Brewing Control Chart plots the two numbers against each other as a map. Strength, the TDS percent, runs up the vertical axis. Extraction yield runs across the horizontal axis. Any brew is a single point on that plane, and the ideal box is the intersection of the two target bands.
| Region | Extraction yield | Strength (TDS) | Tastes like |
|---|---|---|---|
| Ideal box | 18 to 22% | 1.25 to 1.35% | sweet and balanced |
| Under-extracted | Below 18% | Any strength | sour, thin, salty |
| Over-extracted | Above 22% | Any strength | bitter, harsh, drying |
| Weak but in spec | 18 to 22% | Below 1.15% | correct flavor, watery body |
Read the two axes independently. Yield tells you to grind finer or coarser, or to brew hotter or longer, because those levers move how far the coffee extracted. Strength tells you to change the ratio of coffee to water, because that sets how concentrated the cup is. A cup can be correctly extracted yet too weak. That means the vertical axis needs adjusting, not the horizontal one.
Measuring a cup, step by step
A refractometer reading is only as honest as the sample you give it. Hot liquid and suspended fines both throw the reading off, so the procedure exists to remove those two sources of error before you place a drop on the sensor.
Cool the sample
Bring it to room temperature, or use an ATC refractometer that compensates for temperature.
Filter out fines
Optional but steadier: draw the sample through a syringe filter to strip suspended grounds.
Zero on distilled water
Wipe the sensor, place distilled water, set the baseline to zero.
Read TDS%
Wipe dry, place one drop of the cooled sample, read the strength percent.
Compute EY%
Combine TDS% with your dose and beverage mass in the home formula.
Common questions
- What is the difference between TDS and extraction yield?
- TDS, total dissolved solids, is strength: the percent of the cup that is dissolved coffee, read directly by a refractometer. Extraction yield is the percent of the dry dose that dissolved into the cup, computed from TDS, dose, and beverage mass. Strength is measured; yield is calculated. They move on separate axes, which is why the Brewing Control Chart plots one against the other.
- Why do I cool the sample before reading it on a refractometer?
- Optical refractometers are calibrated at a reference temperature, and a hot sample reads differently from a cool one, which skews the TDS number. Cooling the sample to room temperature removes that error. An ATC, automatic temperature compensating, refractometer corrects for it internally, but cooling is still the safest habit for a stable reading.
- My cup is inside the 18 to 22% band but tastes flat. What now?
- Being in the extraction band means the grounds gave up about the right amount of soluble mass; it says nothing about strength or about the coffee itself. A flat, watery cup in the yield band is usually too weak, so raise strength by using a tighter coffee-to-water ratio. If it still tastes flat at correct strength, the cause is upstream of brewing: stale beans, poor water, or an uneven extraction. Taste is the final judge.