Two tablespoons of ground coffee per six-ounce cup. If you stop reading here you’ll be fine, and your coffee will be better than most people’s.
The awkward thing about cups
A “cup” in coffee-maker terms is six ounces, not the eight ounces of an actual measuring cup, and not the twelve or fourteen your mug probably holds. This is why a machine that claims twelve cups fills about six mugs, and why following the marks on the carafe often produces something weak.
For a normal mug — call it twelve ounces — you want roughly three to four tablespoons.
Why grams are better, and when to bother
A tablespoon of coffee can weigh anywhere from about four to seven grams depending on the grind and how you scooped it. That’s a swing of nearly half, which is enough to turn a good cup into a disappointing one without you changing anything you were aware of.
The reliable version is 60 grams of coffee per litre of water. For one large mug that’s about 15 grams of coffee to 250 grams of water. A $12 kitchen scale is the single cheapest upgrade available to a home coffee drinker, and it is a considerably better first purchase than almost any piece of equipment.
That said: if weighing your coffee would make you stop enjoying your mornings, use the tablespoon and don’t think about it again. Consistency matters more than precision.
Adjusting it
More coffee makes it stronger, not more bitter. Bitterness comes from brewing too long or grinding too fine, which is a different problem with a different fix — the two are worth telling apart.
So if your coffee tastes weak, add coffee. If it tastes bitter, don’t remove coffee — grind coarser or brew for less time. Reaching for the scoop is the reflex, and about half the time it’s the wrong lever.
Cold brew is the exception
Cold brew uses far more coffee — roughly one part coffee to eight parts water — because it’s made as a concentrate and diluted afterward. Using the hot-coffee ratio for cold brew produces something disappointing, and it’s the most common reason a first attempt fails. Our numbers are here.