Measuring Your Brew: Scales, Refractometers and Logs

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.

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 think.

Refractometer

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

Using one well:

1. Stir the finished brew thoroughly; concentration varies through the vessel.
2. Draw a sample and let it cool to near room temperature — readings drift with temperature.
3. 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).
4. Clean the lens between readings with distilled water and a lint-free cloth.

Then compute extraction:

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

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 enough.

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 a roast or water problem, not an over-extraction problem.
• 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 itself. The numbers are a map, not the territory — taste remains the final judge.

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.