Coffee Brew Ratio and Extraction, Explained
After reading this you can pick a brew ratio, solve for the dose or water in grams, and read a measured extraction yield off the SCA control chart to know whether your coffee is weak, strong, sour or bitter.
What the ratio and the extraction yield actually measure
Two numbers describe every cup of brewed coffee. The first is strength, how much dissolved coffee solids sit in the liquid you drink. The second is extraction yield, what fraction of the ground coffee you managed to dissolve out of the grounds. They are different, and confusing them is the most common mistake in brewing.
Strength is set mostly by the brew ratio: grams of water per gram of coffee. A drip brew at 1:16 tastes lighter than the same coffee at 1:14 because the same dissolved solids are spread through more water. Extraction yield is set by grind, time, temperature and agitation: how thoroughly the water pulls flavor out of each particle.
Here is the hook. Take 18 g of coffee and 300 g of water. That is a ratio of 300 \div 18 \approx 16.7, a sane pour-over. Whether that cup ends up sour or balanced does not depend on the ratio at all. It depends on how much of those 18 g you dissolve. This calculator handles both numbers so you can change one without guessing at the other.
When to use this, and when the numbers stop helping
Use the ratio side any time you want repeatable coffee. Weigh the dose, compute the water, and you get the same strength every morning. Use the extraction side when you own a refractometer and want to move a recipe toward the target instead of tasting blind.
The numbers stop helping at the edges. Espresso is not brewed coffee: its strength runs 8 to 12% TDS, roughly ten times a drip brew, and its extraction is measured on a different basis. The SCA drip box does not apply to it, so treat the espresso ratio as an out:in guide (yield mass divided by dose mass) and ignore the drip bands. Cold brew concentrate is meant to be diluted, so its brewed strength before dilution sits far to the right of the chart on purpose.
The SCA "Gold Cup" box is 18 to 22% extraction and 1.15 to 1.35% strength for brewed coffee. It is a preference target built from tasting panels, not a law of chemistry. Plenty of good coffee lives just outside it. Use it as a starting map, then trust your tongue.
The formulas, and why each one looks the way it does
Start with the ratio. If R is the water-to-coffee ratio, the water follows directly from the dose:
Here W is water in grams, D is the dose in grams, and R is dimensionless (grams of water per gram of coffee). At R = 16 and D = 18, W = 288 g. Solving for the dose just inverts it: D = W \div R.
Cup size is trickier, because spent grounds keep some water. Drip grounds hold roughly 2 g of water per gram of coffee. If you want a finished beverage of volume V mL (call it V grams, since brewed coffee is close to the density of water), you must brew with extra water to cover what the grounds absorb:
where a \approx 2 is the retention in grams of water per gram of coffee. Since D = W \div R, substitute and solve:
Notice the denominator is R - a, not R. That is the retention correction. Ignore it and a 250 mL cup at 1:16 comes out short, because 36 g of water never reaches the mug.
Now extraction yield. You dissolve some mass of coffee into the beverage. That dissolved mass is the beverage mass times its TDS fraction. Divide by the dose to get the fraction extracted:
where M_b is the beverage mass in grams (the coffee actually in your cup, not the water you poured), and TDS\% is the refractometer reading as a percentage. The \times 100 and the percent signs cancel so the answer is a clean percentage.
A worked example you can reproduce
Drip at 1:16 from an 18 g dose
This is the demo: method drip, solve for water, ratio 1:16, dose 18 g.
- Read the ratio.
1:16means R = 16 g water per g coffee. - Compute water: W = 16 \times 18 = 288 g. Pour 288 g of water over 18 g of grounds.
- Estimate the cup. The grounds retain about 2 \times 18 = 36 g. So the beverage is roughly 288 - 36 = 252 g, near a 250 mL mug.
- Now suppose you measure the finished cup: 252 g of coffee reading 1.35% TDS. Dissolved solids are 252 \times 0.0135 = 3.40 g.
- Extraction yield: 3.40 \div 18 = 0.189, or 18.9%.
That lands inside the Gold Cup box: 18.9% extraction and 1.35% strength. Balanced and on target. If the same cup had read 1.10% TDS, dissolved solids would be 252 \times 0.0110 = 2.77 g, an extraction of 2.77 \div 18 = 15.4\%: under-extracted and weak, likely sour.
Reading the control chart
The SCA chart plots strength (TDS, vertical) against extraction yield (horizontal). The ideal box is the rectangle where both fall in range. Four things can go wrong, and each has a different fix.
- Weak
- Strength below 1.15%. Too much water for the dose. Lower the ratio (more coffee per gram of water).
- Strong
- Strength above 1.35%. Raise the ratio, or dilute the cup.
- Under-extracted
- Yield below 18%. You did not dissolve enough. Grind finer, brew longer, or use hotter water. Tastes sour and sharp.
- Over-extracted
- Yield above 22%. You dissolved too much, pulling out bitter compounds. Grind coarser or shorten the brew.
Strength and extraction move somewhat independently. You can be strong and under-extracted at once (a tight ratio with a coarse grind), which tastes both intense and sour. The chart separates the two axes so you change the right variable.
How ratio maps to predicted strength
If you assume a fixed extraction yield, the ratio predicts strength. Hold extraction at 20% and vary the ratio for an 18 g dose. Dissolved solids are fixed at 0.20 \times 18 = 3.6 g. The beverage mass is R \times 18 - 2 \times 18 g, so strength is 3.6 \div M_b.
| Ratio | Water (g) | Beverage (g) | Strength TDS (%) | Verdict |
|---|---|---|---|---|
| 1:14 | 252 | 216 | 1.667 | Strong |
| 1:15 | 270 | 234 | 1.538 | Strong |
| 1:16 | 288 | 252 | 1.429 | Slightly strong |
| 1:17 | 306 | 270 | 1.333 | In box |
| 1:18 | 324 | 288 | 1.250 | In box |
The table shows why ratio and extraction must be read together. At a perfect 20% extraction, 1:16 is already a touch strong at 1.43% TDS. Loosen to 1:17 or 1:18 and you drop into the middle of the box.
Common mistakes that throw the numbers off
Do not use the water weight as the beverage mass in the extraction formula. The grounds keep about 2 g of water per gram of coffee, so your cup weighs less than the water you poured. Using 288 g instead of 252 g inflates the extraction reading by about 14%, enough to move a balanced brew into the over-extracted zone on paper.
Three more traps. First, TDS is a percentage, not a decimal: 1.35% means 0.0135 in the multiplication, so 252 \times 0.0135, never 252 \times 1.35. Second, a bigger ratio number means a weaker brew, which reads backwards to newcomers: 1:18 is lighter than 1:14. Third, refractometers need a clean, temperature-settled, filtered sample. An unfiltered French press sample full of fines reads high because suspended particles scatter light like dissolved solids do.
Related tools
If you brew as well as drink, three calculators here run on the same weigh-everything logic. The Brewing ABV and recipe calculator handles fermentation math: alcohol, attenuation and bittering. The Baker's percentage and dough calculator uses ratios exactly the way coffee does, scaling a recipe by mass. For low-temperature cooking, the Sous vide time and safety calculator covers come-up time and pasteurization holds.
Frequently asked questions
What is a good coffee-to-water ratio for pour-over?
Start at 1:16 to 1:17: 18 g coffee to 288 to 306 g water. That puts you near the middle of the SCA strength box when extraction lands around 20%. Adjust from there by taste.
Why is my coffee sour even though the ratio is right?
Sourness is under-extraction, not weakness. The ratio only sets strength. If a well-dosed cup tastes sour, you dissolved too little coffee: grind finer, extend the brew, or raise the water temperature. A refractometer reading below 18% extraction confirms it.
Do I need a refractometer to use the ratio side?
No. The dose, water and cup-size math work from weights alone. The refractometer only feeds the extraction-yield check, which you can leave blank. Weighing your coffee and water already fixes most consistency problems.
Why can't I use the SCA box for espresso?
Espresso strength runs 8 to 12% TDS, far above the 1.15 to 1.35% brewed band, and its extraction is measured on a different basis. Judge espresso by its out:in ratio (a 36 g shot from an 18 g dose is 1:2) and by taste, not by the drip chart.
How much water do the grounds keep?
About 2 g per gram of coffee for drip and pour-over. An 18 g dose holds back roughly 36 g of water. Immersion methods like French press retain a similar amount. This is why solving from a cup size uses R - 2 in the denominator rather than R.
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