Coffee strength is not the same as extraction. Strength is how much dissolved coffee is in the cup, measured as TDS. Extraction is how much of the bean you managed to dissolve. You can have a strong but under-extracted cup, and a weak but over-extracted one.
What the numbers mean
TDS stands for total dissolved solids and is expressed as a percentage of brew weight. Filter coffee usually lands between 1.15% and 1.45%. Espresso is 8% to 12%. Those are different drinks, so compare within a method.
| TDS | How it drinks | Usual method | What to do |
|---|---|---|---|
| 1.15 - 1.25% | Light, tea-like | Filter | More coffee or finer grind |
| 1.25 - 1.35% | Standard filter | Filter | In range |
| 1.35 - 1.45% | Strong filter | Filter | In range, good with milk |
| 1.45 - 1.55% | Very strong | Filter / immersion | Check extraction, may be muddy |
| 8 - 10% | Typical espresso | Espresso | In range |
| 10 - 12% | Ristretto-style | Espresso | In range for light roasts |
The extraction yield formula
Extraction yield tells you what percentage of the dry coffee ended up in the cup.
EY = (TDS x brew weight) / dry coffee weight
Espresso example: 18 g in, 36 g out, 10% TDS. EY = (0.10 x 36) / 18 = 20%.
Filter example: 15 g in, 240 g out, 1.30% TDS. EY = (0.0130 x 240) / 15 = 20.8%.
Where extraction should land
| Method | Target extraction yield |
|---|---|
| Filter, light roast | 19 - 22% |
| Filter, dark roast | 17 - 19% |
| Espresso, light roast | 19 - 22% |
| Espresso, dark roast | 17 - 20% |
| Immersion / French press | 18 - 21% |
Below the range the cup tastes sour, thin and salty. Above it, dry, woody and bitter. Inside, sweet with a clean finish.
Strength without a refractometer
A refractometer costs 150 to 400 dollars, more than most people want to spend. You can still control strength precisely with ratio alone, because ratio is the strongest predictor of TDS.
Filter ratio 1:15 gives roughly 1.35% TDS, 1:16 gives about 1.30%, 1:17 about 1.25%. With everything else fixed, ratio moves strength predictably.
Strength versus extraction, in practice
Weak and sour: both are low. Increase coffee or reduce water, and grind finer.
Strong and bitter: both are high. Reduce coffee or add water, and grind coarser.
Strong and sour: unusual, but happens with very high doses and short contact. Grind finer without changing the ratio.
Weak and bitter: usually channelling at a high brew ratio. Fix distribution before adding coffee.
Milk changes the target
Milk mutes both sweetness and acidity, so milk drinks want a stronger base. Push filter coffee to 1.40% TDS, or use a 1:2 espresso instead of 1:2.5, before adding 150 ml of milk.