Reference · measuring strength

Coffee TDS and refractometers: what a 1.3% reading means

A coffee refractometer turns one drop of brewed coffee into a single number: total dissolved solids, or TDS. Here is what that number is, what 1.3% means in grams, how the instrument gets it, how to take a reading the way standards and researchers do, and how much the last digit can be trusted.

Updated 2026-10-24 · sources listed at the end · no devices were tested for this page

What TDS is

SCA Standard 310-2021 defines brew strength as “the amount of total dissolved solids (TDS) in the brew, expressed in percentage or in parts per million”, usually estimated with a coffee refractometer. It is a mass fraction: in UC Davis research, TDS is the mass of dissolved coffee per unit mass of liquid. The idea goes back to Ernest Earl Lockhart, director of the Coffee Brewing Institute, whose 1957 paper “The Soluble Solids in Beverage Coffee as an Index to Cup Quality” led to the classic brewing control chart.

TDS is not extraction. Strength says how concentrated the cup is; extraction yield (EY) says how much of the dry coffee dissolved. They are linked through the amount of beverage per gram of coffee, which is why a refractometer reading plus two weights gives you EY in the extraction yield calculator.

What 1.3% means

A reading of 1.30% means 1.30 g of dissolved coffee in every 100 g of drink; the other 98.7 g is water. In parts per million that is 13,000 ppm. A 300 g mug at 1.30% holds about 3.9 g of dissolved coffee. If you brewed 20 g of coffee into 320 g of beverage (16 g of drink per gram of coffee), the same reading means an extraction of 1.30% × 16 = 20.8%.

Reference range	TDS	Is 1.30% inside?	From
Classic control chart “ideal” region (SCA Golden Cup)	1.15–1.35%	Yes, near the top	Batali et al. 2020
SCA home-brewer certification target	1.15–1.55%	Yes	SCA Standard 310-2021
European countries	1.20–1.45%	Yes	Wikipedia, Coffee extraction
Nordic countries	1.30–1.50%	Yes, at the bottom edge	Wikipedia, Coffee extraction
Ristretto espresso	over 5%	No — espresso is a different scale	Wikipedia, Coffee extraction

So 1.3% is an ordinary, middle-of-the-road filter strength by every published range above. It does not say whether the cup tastes good: in the UC Davis study of drip coffee brewed at 1.0, 1.25 and 1.5% TDS, strength was the main driver of perceived acidity, and the team behind the new brewing chart calls the classic chart’s “strong” and “weak” labels potentially misleading, because some attributes, such as sweetness, rise at lower TDS. The brewing control chart explainer covers those findings.

How a coffee refractometer works

A refractometer measures the refractive index of a liquid — how strongly it bends light. A digital refractometer shines an LED through a prism that touches the sample; light striking the boundary beyond the critical angle is totally reflected, the rest partly passes into the sample, and a sensor array finds the edge between the two and calculates the refractive index. A coffee refractometer, in SCA’s wording, is “especially designed to estimate” TDS “from its refraction index”: a US patent for one such device expresses TDS as a function of refractive index and temperature.

Temperature is the catch. The same patent gives water’s refractive index as 1.333 at 20 °C, changing by about 0.0001 per degree, and Wikipedia’s Brix article notes that refractive index “changes dramatically with temperature”. Instruments correct for this automatically, but only within a stated range, and they still need the drop and the prism at a steady temperature.

LED — the light source, aimed up into the prism.
Prism — touches the sample from below.
Sample drop — about 0.3 ml in the well (the volume both manuals in the table below give).
Sensor array — light beyond the critical angle is totally reflected; the edge between bright and darker parts gives the refractive index, which a coffee-specific formula turns into TDS.
Temperature sensor — for automatic temperature compensation, valid only inside the maker’s stated range.
Classic box — 1.15–1.35%, the control chart’s “ideal” region (SCA Golden Cup).
Other bands — SCA 310 home-brewer target 1.15–1.55%, Europe 1.20–1.45%, Nordic 1.30–1.50% (sources in the table above).
A 1.30% reading — inside all four bands, at the edge of the Nordic one.
Published accuracy — ±0.03 and ±0.15 percentage points, the two manufacturer specs in the table below: the wider one spans 1.15–1.45%.

Drawn by pourxlab.com for this page. Panel A follows Wikipedia’s Refractometer description and is a generic schematic, not any brand’s design (not to scale). Panel B uses the ranges and accuracy figures cited on this page; its TDS axis is to scale.

Refractometer specs: what manuals say

Two manufacturers’ published figures, quoted to show what the numbers on a spec sheet mean. This is not a buying guide.

Published figure	VST LAB Coffee III (manual)	ATAGO PAL-COFFEE (TDS) (catalog)
TDS range	0.00–20.00%	0.00–22.00%
Resolution (last digit shown)	0.01%	0.01%
Accuracy	±0.03% (0–4.99%) / ±0.05% (5–20%)	±0.15%
Temperature compensation	15–35 °C	10–100 °C
Sample	0.3 ml	at least 0.3 ml
Zero setting	Distilled or deionized water at 15–35 °C; RO water not recommended; at least daily	—
Espresso	Filter through a syringe filter before cooling and measuring	“Measurement fluctuation by 0.5% is common”

Resolution is not accuracy: both show two decimals, but the stated accuracies differ fivefold. On a brew with 16 g of drink per gram of coffee, ±0.03 in TDS moves the calculated extraction by about ±0.5 points; ±0.15 moves it by ±2.4 points — more than half the width of the 18–22% box.

How to take a reading

Zero the instrument with distilled or deionized water, as the VST manual and the UC Davis immersion study did, then clean and dry the prism.
Stir the brew before sampling, and draw the sample straight into a syringe or covered container to limit evaporation (Barista Hustle).
Filter when particles are present. SCA 310 says to filter so “no coffee particulates are included”; the VST manual asks for a syringe filter for espresso and for coffee from metal filters, cupping, cold brew, Turkish coffee, AeroPress or fine grinds, one filter per measurement.
Cool the sample to room temperature. The UC Davis drip study cooled samples in sealed containers in an ice bath, then used one drop; the VST manual says to cool a small portion in a room-temperature glass and let the drop sit 15–20 seconds in the well. Barista Hustle reports that a 5 °C mismatch between drop and prism can move a calculated extraction by up to 1%.
Read more than once. SCA 310 says to test at least three times and average, then rinse the well with distilled water.
Record the reading with the weights — dose and beverage — so the number can become an extraction yield later.

Water TDS meters are a different tool

Water TDS is often estimated from electrical conductivity rather than with a refractometer. Conductivity responds only to dissolved ionized solids, so it suits water whose solids are mostly inorganic salts, and even then it is converted with a factor of 0.55–0.8 and correlates with lab TDS to around 10%, according to Wikipedia. The ranges differ too: SCA cupping water should sit at 125–175 ppm, while a 1.3% coffee is 13,000 ppm. A conductivity reading suits brewing water; coffee strength is what SCA 310 measures with a coffee refractometer.

How researchers check the number

The reference method for TDS is gravimetric: evaporate the liquid and weigh what is left. UC Davis teams checked their VST refractometers that way — the drip study against oven drying, the immersion study against weighed solutions of instant coffee in water — and both reported accurate readings. Drying the spent grounds instead is a different measurement: the immersion paper found it “invariably” underestimates extraction, because wet grounds hold back strong brew and baking drives off some coffee mass.

TDS reading calculator

TDS reading (%)

Beverage weight (g)

Dose (g dry coffee)

Instrument accuracy (± % TDS)

Target TDS for dilution (%)

Dissolved coffee4.16 g

Extraction yield20.8%

Water to add13 g

solids = 1.30% × 320 g = 4.16 g · 13,000 ppm · EY = 1.30% × 320 g ÷ 20 g = 20.8%

1.30% is inside the classic 1.15–1.35% box and the SCA 310 target of 1.15–1.55%.

With ±0.03, the true TDS is 1.27–1.33%, so EY is 20.3–21.3%.

Adding 13 g of water brings 320 g at 1.30% to 1.25% (333 g). Extraction yield stays 20.8%: the dissolved coffee does not change.

dissolved coffee (g) = TDS (%) ÷ 100 × beverage (g)
ppm = TDS (%) × 10,000
EY (%) = TDS (%) × beverage (g) ÷ dose (g) (percolation; immersion: see the extraction calculator)
EY range = (TDS ± accuracy) × beverage ÷ dose
water to add (g) = beverage × (TDS ÷ target − 1)

The extraction formula is the one UC Davis drip research uses (PE = TDS × M_brew ÷ M_grounds). The dilution line is mass conservation: water adds weight but no coffee solids, so strength drops and extraction yield stays put. The accuracy presets are the two manufacturers’ published figures above.

Common questions

Can I use a Brix refractometer for coffee?

Degrees Brix are defined for sucrose solutions — one gram of sugar per 100 g of solution is 1 °Bx — and when other solids are dissolved, Wikipedia notes, Brix “only approximates” the dissolved solids content. Coffee refractometers apply a coffee-specific TDS formula instead, and ATAGO lists Brix and TDS as separate scales with different accuracy figures (±0.10% Brix, ±0.15% TDS).

Why does espresso need filtering?

Espresso carries more particles than drip coffee, which scatter the reading; ATAGO calls a 0.5% fluctuation common, and the VST manual requires a syringe filter. Espresso is also far stronger — ristretto runs over 5% TDS — so check that the instrument’s range and accuracy cover it (the VST manual gives ±0.05 above 5%).

Do I need a refractometer at all?

No. Strength is the product of brew ratio and extraction, so a scale and a fixed coffee-to-water ratio already pin down much of it. A refractometer adds the missing measurement, which matters mainly when you want to compare extraction between brews.