The 2-tablespoon rule
Two level tablespoons of medium ground coffee per 8 oz cup of water is the rule most bag labels print, and the math checks out. An 8 oz cup is 237 milliliters of water. At a 1:16 ratio, 237 divided by 16 gives 14.8 grams of coffee. At 7.5 grams per tablespoon, 14.8 grams is 1.97 tablespoons, which rounds to 2. The folk rule and the scale agree.
That makes the rule easy to scale. Four cups need 8 tablespoons. Eight cups need 16 tablespoons, which is exactly 1 cup of grounds. Ten cups need 20 tablespoons. For a stronger cup, move to a 1:15 ratio: the same 8 oz cup then needs 15.8 grams, about 2.1 tablespoons, so the rule barely changes.
What counts as a cup
This is where most people go wrong. Coffee makers measure in 5 oz cups, not 8 oz. A 6 oz mug, the old diner standard, is 177 milliliters: at 1:16 it needs 11.1 grams of coffee, about 1.5 tablespoons. Two tablespoons in a 6 oz mug is actually a 1:12 ratio, noticeably stronger than intended.
So match the dose to the vessel. For an 8 oz mug, 2 tablespoons. For a 6 oz cup, 1.5 tablespoons. For a 5 oz coffee-maker cup, about 1.25 tablespoons (9.9 grams). If your coffee always tastes stronger than the bag suggests, you are probably dosing for 8 oz cups and brewing 5 oz ones.
Why tablespoons are approximate
A tablespoon of coffee is not a fixed weight. Dark roasts are less dense than light roasts, coarse grinds pack looser than fine grinds, and a heaping scoop can hold half again as much as a level one. Real-world measurements run from about 6 grams per tablespoon for a coarse light roast to about 10 grams for a fine dark roast. That is a 60 percent swing in dose from the same scoop.
At 7.5 grams per tablespoon, 2 tablespoons is 15 grams and the ratio lands at 1:16. At 10 grams per tablespoon, the same 2 scoops are 20 grams and the ratio is 1:12. Same scoops, very different cup. This is why every serious brewing guide eventually says the same thing: scoops get you in the neighborhood, a scale gets you to the address.
A quick reference table
For drip coffee at 1:16 with 8 oz cups, using 7.5 grams per tablespoon:
2 cups: 29.6 grams, 4 tablespoons. 4 cups: 59.1 grams, 8 tablespoons. 6 cups: 88.7 grams, 11.8 tablespoons. 8 cups: 118.3 grams, 15.8 tablespoons. 10 cups: 147.9 grams, 19.7 tablespoons. 12 cups: 177.4 grams, 23.7 tablespoons, about 1.5 cups of grounds.
Memorize the pattern rather than the table: 2 tablespoons per cup, 1 cup of grounds per 8 cups of water. The coffee ratio calculator converts any of these to grams or milliliters if you want to check a specific batch.
When to switch to a scale
Stay with tablespoons if you brew the same coffee the same way every day and it tastes good. The absolute accuracy does not matter once you have found a dose you like; repeatability does, and the same scoop used the same way is repeatable enough.
Switch to a scale when something changes: a new coffee, a new brew method, or a cup that suddenly tastes off. Weighing removes the biggest variable in the brew, and a 20 dollar kitchen scale pays for itself in saved beans within weeks. Weigh the coffee in grams, weigh the water in milliliters, and write down the ratio that tastes best.