The short answer
Start at 1:16. One gram of coffee for every sixteen grams (millilitres) of water. For a standard 320 ml mug that works out to 20 g of coffee and 320 g of water.
From there, taste and adjust. A tighter ratio like 1:15 gives a stronger, heavier cup. A looser 1:17 stretches the same coffee into something lighter, closer to tea. Almost all good pour over lives between 1:14 and 1:18.
Why ratios beat scoops
Beans vary hugely in density. A level scoop of a dense light roast can weigh several grams more than the same scoop of a dark roast, so 'two scoops' means something different every time you change bags. Brewing by weight removes the variable, which is why a $15 kitchen scale improves more cups than most $100 gadgets.
Ratios also scale without thinking. Brewing for two? 40 g of coffee, 640 g of water. Still 1:16. The recipe never changes, only the quantities.
Common ratios at a glance
1:14 — strong and syrupy; can turn muddy with fine grinds.
1:15 — rich, full cup; suits darker roasts.
1:16 — the balanced default; works for almost everything.
1:17 — lighter body, more clarity; flatters bright, fruity light roasts.
1:18 — delicate and tea-like; unforgiving of stale beans.
When the ratio isn't the problem
Sour or thin? The grind is usually too coarse or the water too cool. Bitter and harsh? Too fine, or the brew ran too long. The ratio is rarely the culprit in either case.
Fix one variable at a time. Lock the ratio at 1:16, dial the grind until the cup balances, and only then play with the ratio to taste. A consistent burr grinder makes the dialling-in far easier than any blade grinder will.
Water temperature matters too. Aim for 90 to 96°C, which in practice means just off the boil. Cooler water under-extracts light roasts. Very hot water can push dark roasts into bitterness. No thermometer? Thirty seconds off the boil lands you in the zone.