Measuring Your Brew: Scales, Refractometers and Logs

What to measure, what the instruments actually tell you, and how to turn a refractometer reading into an extraction number.

You can brew excellent coffee by taste alone. But the moment you want to repeat a cup, share a recipe someone else can follow, or figure out why Tuesday's brew beat Monday's, you need numbers.

The essential instrument: a scale

Everything downstream depends on mass. A scale that reads to 0.1 g for dosing and to 1 g for water is enough; a single 0.1 g scale that handles both is ideal. Timing built into the scale is a convenience, not a requirement. This is Cup Engineering practice, in line with practitioner guidance¹.

Weigh the coffee, weigh the water going in, and — if you're going to compute extraction — weigh the beverage coming out.

Thermometer / kettle

A variable-temperature kettle is the easy answer. If you're using a stovetop kettle, water 30 seconds off the boil in a cool room is roughly 93–95 °C; pre-heat the brewer or accept that you're brewing several degrees cooler than you think¹. The SCA standard's target at the point of contact is 93 ± 3 °C².

Refractometer

A coffee refractometer measures how much light bends passing through a drop of coffee and converts that refractive index to a TDS percentage¹. Coffee-specific meters (VST, Atago and their clones) are calibrated for coffee's particular mix of solubles; a generic Brix refractometer will be off¹.

Using one well¹³:

Stir the finished brew thoroughly; concentration varies through the vessel.
Draw a sample and let it cool to near room temperature — readings drift with temperature.
Filter the sample through a syringe filter or a folded paper filter if your brew has visible oils or fines (important for espresso and French press, optional for pour over). Gagné's tests found unfiltered espresso samples read noticeably higher than filtered ones¹.
Clean the lens between readings with distilled water and a lint-free cloth.

Then compute extraction:

EY (%) = TDS (%) × beverage mass (g) / coffee dose (g)

This is the SCA definition of extraction yield². For espresso the same formula applies with the beverage weight from the cup. Some people add an adjustment for the water absorbed by the puck; for practical dialling in, the simple form is enough¹.

InteractiveRatio and strength

Ratio sets strength; extraction sets flavour balance. Hold extraction still and slide the ratio to see strength move — then change extraction and see how little the ratio matters to it.

Dose20 g

Ratio1:16.7

Extraction yield20 %

Water to pour

334 g

Beverage

292 g

42 g stays in the grounds

Dissolved coffee

4.00 g

Strength (TDS)

1.37 %

In the window. Strength is right; now let flavour decide whether extraction should move.

How this is computed: water = dose × ratio; beverage = water − dose × 2.1 g/g retained in the spent grounds (typical for paper-filter brews; immersion retains more, espresso less); dissolved = EY × dose; TDS = dissolved ÷ beverage. The 1.15–1.45 % strength window is the SCA filter standard (its 2020s revision widens the acceptable range); 1:15–1:17 is where that window and 18–22 % EY overlap. Sources: [2] [lingle-handbook] [1]

What the numbers are good for

Reproducing a cup. If your best V60 was 1.38 % TDS / 21 % EY, you know where to aim next time.
Diagnosing taste. Bitter at 19 % EY is more likely a roast or water problem than an over-extraction problem, because panels find bitterness rising with extraction only as you push past the ideal range⁴.
Comparing gear. A grinder that lets you reach 22 % EY without astringency is genuinely better than one that tastes harsh at 20 %.
Sharing. A recipe that states its target TDS and EY is far more useful than one that says "medium grind".

What they are not

TDS and EY say nothing about which compounds are in the cup. Two coffees at identical numbers can taste completely different depending on evenness, roast, water and the coffee itself⁴. The numbers are a map, not the territory — taste remains the final judge, which is why the SCA's own quality assessment is built on trained sensory evaluation rather than on instruments⁵.

Keeping a log

Record, at minimum: coffee, roast date, dose, water, ratio, grind setting, temperature, time, and a short taste note. Add TDS and EY when you have them, and a 1–5 rating. Each recipe page in this app has a brew log for exactly this; the data feeds the analytics page so patterns across many brews — and many brewers — become visible.

How the numbers are computed

The brew-log form and the calculator both use the SCA definition², implemented as measuredEy() in lib/coffee.ts:

EY (%) = TDS (%) × beverage mass (g) / dose (g)

Worked example: a 15 g AeroPress dose yields 180 g of beverage that reads 1.42 % on the refractometer. Dissolved coffee is 1.42 % × 180 g = 2.56 g, and 2.56 / 15 = 17.0 % extraction, just under the 18 % floor of the ideal range², which matches a cup that tastes a little sharp. If you did not weigh the beverage, the log estimates it as water minus dose × retention (2.0 g/g for AeroPress in Cup Engineering's model), a working assumption that is close enough for dialling in but not for comparing grinders; weigh the cup when the comparison matters. The chart position follows directly: EY on the horizontal axis, TDS on the vertical, against the 18–22 % and 1.15–1.35 % standard box².