Brew Ratio

Brew ratio is the single most important variable in coffee brewing consistency. It defines the relationship between the mass of dry coffee used and the mass of water used to brew it. Expressed as 1:X, where the first number is coffee and the second is water: a 1:15 ratio means 1 gram of coffee to 15 grams of water. Get this wrong, and no amount of precise grind size, perfect temperature, or careful technique will produce a consistently good cup.

Why Ratio, Not Volume

Most brewing instructions historically described coffee in tablespoons and water in cups or millilitres. This is imprecise for two reasons: tablespoons vary by how tightly the coffee is scooped, and density varies enormously between roast levels and grind sizes. A tablespoon of coarsely ground dark-roast coffee weighs significantly less than a tablespoon of finely ground light-roast coffee.

Weight is the only reliable measurement. A scale that measures to 0.1g is the most impactful piece of equipment any coffee brewer can own.

Standard Ratios by Method

Different brew methods produce optimal results at different ratios, primarily because their extraction efficiency and serving size differ:

• Filter (pour-over, batch brew): 1:15 to 1:17. The SCA Golden Cup standard targets 1:15–1:17.5 for filter coffee.
• AeroPress: 1:10 to 1:16. At 1:10–1:12, brew as a concentrate and dilute. At 1:14–1:16, drink directly.
• French press: 1:14 to 1:16. The immersion method extracts efficiently, so lower ratios are possible.
• Espresso: 1:1.5 to 1:3. The high concentration is espresso's defining characteristic.
• Cold brew concentrate: 1:5 to 1:8. Dilute 1:1 before drinking.
• Moka pot: Determined by the basket volume; cannot be meaningfully varied as with other methods.

Strength vs. Extraction

Strength and extraction are related but independent variables, and confusing them is the most common source of brew problems.

Strength refers to the concentration of dissolved coffee solids in the cup, expressed as TDS (Total Dissolved Solids, usually as a percentage). A 1.3% TDS means 1.3% of the liquid in the cup is dissolved coffee. This is determined primarily by ratio: more coffee per unit of water produces a stronger (higher TDS) cup.

Extraction yield refers to the percentage of the coffee's mass that dissolved into the water. A 20% extraction yield means 20% of the coffee's dry weight ended up in the cup. This is determined primarily by grind size, temperature, and brew time. Under-extraction (below 18%) tastes sour, salty, and hollow. Over-extraction (above 22%) tastes bitter and astringent.

You can have a strong but under-extracted cup (too much coffee, too coarse grind), or a weak but well-extracted cup (correct grind, too little coffee). The ideal is within the SCA target: 1.15–1.35% TDS and 18–22% extraction yield simultaneously.

The Brewing Control Chart

The SCA Brewing Control Chart plots TDS against extraction yield, creating four quadrants: ideal (well-extracted, appropriate strength), weak (under-extracted or under-dosed), bitter (over-extracted), and strong (high TDS but possibly under-extracted if ratio is too low). A refractometer measures TDS; extraction yield is calculated from TDS, ratio, and brew mass. This chart was developed by MIT researcher Ernest Earl Lockhart in the 1950s and remains the foundation of coffee quality measurement.

Using a Refractometer

A refractometer measures TDS optically: a drop of brewed coffee on the prism; the instrument reads how much light bends, which correlates to dissolved solids concentration. Coffee refractometers (such as those from Atago or VST) are calibrated for coffee's specific refractive index.

The extraction yield calculation is: EY% = (TDS% × Brew Weight) / Dry Coffee Weight

For example: 1.3% TDS, 280g brew weight, 18g dry coffee = (0.013 × 280) / 18 = 20.2% extraction. Well within the ideal range.

Practical Application

Start with a reference ratio (1:16 for filter), brew, taste, and adjust:

• Too weak, flat: Increase coffee dose (move ratio toward 1:14) or improve extraction (finer grind).
• Too strong, overpowering: Decrease dose (move ratio toward 1:18) or extend the brew with more water.
• Sour, thin: Under-extraction. Finer grind, higher temperature, or longer contact time — do not simply add more coffee.
• Bitter, astringent: Over-extraction. Coarser grind or shorter contact time — do not simply add more water.

Ratio adjustment controls strength; grind adjustment controls extraction. They are different knobs, and using the wrong one produces the wrong result.