How brew ratios work
A brew ratio is simply coffee to water by weight. A 1:16 ratio means one gram of coffee for every sixteen grams of water — so 20g of coffee needs 320g of water. Because a millilitre of water weighs a gram, you can read grams of water as millilitres.
Lower numbers mean a stronger, more concentrated cup. Higher numbers mean a lighter one. Most methods have a range that works well, listed on each method's guide, and the difference between the ends of that range is real but not dramatic — it is worth trying both.
Why the yield is less than the water
Coffee grounds hold onto water. Roughly two grams stay behind in the bed for every gram of dry coffee, which is why brewing 320g of water over 20g of coffee leaves you with about 280g in the cup rather than 320g. The calculator accounts for this when it estimates yield.
Immersion methods that you press or plunge, like the AeroPress, recover a little more than that estimate. Treat the yield figure as a close guide rather than an exact measurement.
If the ratio is right but the coffee isn't
Ratio sets strength. It does not fix a cup that tastes sour or bitter — that is usually grind size. Sour, sharp, and thin means under-extraction, so grind finer. Bitter, hollow, and drying means over-extraction, so grind coarser. Calibrating the grind walks through the whole diagnosis.