Pour a cup of filter coffee that tastes thin, and the usual advice is "use more coffee". Pour one that tastes harsh, and it is "use less". Both are true, but only if you know what "more" and "less" are measured against. The answer is a ratio: grams of coffee for every litre of water. Get that one number right and most of the other filter-coffee advice gets easier.
The Specialty Coffee Association's long-standing guideline for brewing at home is 55 grams of ground coffee per litre of water, with about 10% leeway either side, so roughly 50 to 60 g/L. As a ratio that is about 1 part coffee to 18 parts water. It is a starting point built around a cup that tastes balanced to most people, not a law.
It is easier to think in a mug than in a litre. A typical 250 ml mug at 55 g/L needs about 14 g of coffee, and a 1.25 litre pot of water needs about 69 g. (The calculator below adds a little water on top, to make up for what the grounds soak up.) Those are weights: a "scoop" or a "tablespoon" of ground coffee can vary a lot with the grind, the roast and how heaped it is, so a cheap kitchen scale is the single most useful tool here.
Dry coffee grounds soak up water, roughly twice their own weight, and that water stays in the filter. So if you fill the reservoir to the "4 cups" line and brew, you get a little less than four cups out. The calculator allows for this: it shows the water to pour in and, separately, what to expect in the pot. If you want exactly two full mugs, pour in a bit more than two mugs' worth. The exact loss varies with grind, roast and filter type, so treat the pot figure as an estimate and adjust once after your first brew.
Two different things hide behind "strength". The first is how much coffee dissolved into the water in total: that is the concentration, and it is what the ratio mostly controls. The SCA target for the finished brew is about 1.15% to 1.35% dissolved coffee, which tastes like a normal cup of filter. The second is how much of the grounds you managed to dissolve: that is the extraction, with a target of roughly 18% to 22% of the dry coffee. Extraction is controlled by grind size, water temperature and brew time, not by the ratio.
That is why a ratio alone cannot rescue a bad cup. If you use plenty of coffee but grind it so coarse that the water races through, you will get a weak, sour, hollow cup that is under-extracted. If you grind very fine and brew long, you pull out the harsh, dry, bitter compounds as well, even with a modest dose. Change the ratio to change how strong the cup is; change the grind to change how it tastes.
Ratio sets the volume of the flavour. Grind, heat and time set which flavours you get.
Start at 1:18 (about 55 g/L) and brew a full pot. Then change one thing at a time.
Small steps work best. A full step on the slider is only a gram or two for a mug, and it is easy to overshoot.
The 55 g/L guideline is for a standard batch brewer. Pour-over, French press and an AeroPress often settle at slightly stronger ratios, around 1:15 to 1:17, because they brew in different ways. Very light, dense roasts sometimes want a finer grind or a bit more coffee. Many people simply prefer a stronger cup: 1:16 is perfectly respectable. Whatever you choose, write it down. A ratio you can repeat is worth more than a perfect one you cannot.
One note on caffeine: a stronger brew means more caffeine per mug. Up to about 400 mg a day is the commonly cited limit for most healthy adults (the European Food Safety Authority's figure), and a mug of filter coffee varies widely, so if you are pregnant or sensitive to caffeine, go lighter and check with a health professional.