The formula
grams of coffee = millilitres of water ÷ ratio
Where it comes from
The ratio says how many grams of water go per gram of coffee. 1:16 means 16 g of water per gram, and it is the starting point the Specialty Coffee Association recommends for filter methods. A lower ratio gives a more concentrated cup; a higher one, a milder cup.
How to work it out by hand
- Work out the total water by multiplying the cups by their size
- Divide the water by the ratio you chose
- The result is the grams of coffee
What is worth knowing
The ratio sets the concentration, but the extraction is set by the grind and the time. If the coffee tastes bitter, it is usually over-extracted: grind coarser or shorten the contact time. If it tastes sour and thin, it is under-extracted: grind finer. Changing the amount of coffee to fix bitterness is the usual mistake, because it treats the symptom rather than the cause.