Concentrate first, dilution second
Cold brew is really two recipes in one. You brew a strong concentrate at 1:8, then you dilute it when you serve it, usually with an equal part of water or milk. That 1:1 dilution halves the strength, so the cup you drink lands at an effective ratio of 1:16, right in the normal brewed-coffee range.
This two-step structure is the point. The concentrate keeps for a week or more in the fridge, so one batch covers many servings, and each person dilutes to their own taste. Someone who likes it strong pours more concentrate; someone who likes it mild adds more water. The ratio work happens once, at brew time.
Worked example 1: 1 liter of concentrate
One liter of water is 1,000 grams. Divide by 8 to get 125 grams of coarse ground coffee, which is about 16.7 tablespoons, or just over 1 cup of grounds. Combine the coffee and cold or room-temperature filtered water in a jar, stir to wet all the grounds, cover, and refrigerate for 12 to 24 hours.
After steeping, strain through a fine mesh sieve, then again through a paper coffee filter or cheesecloth for a clean cup. You will end up with a little under a liter of concentrate, since the grounds absorb some water. That concentrate makes roughly 8 servings when diluted 1:1.
Worked example 2: 4 cups of concentrate
Four cups of water is about 946 milliliters. Divide 946 by 8 to get 118.3 grams of coffee, about 15.8 tablespoons, which is just under 1 cup of grounds. This is a convenient batch for a half-gallon mason jar with headroom for the grounds.
The ratio scales linearly, so any batch size works the same way. Two liters of water need 250 grams of coffee. A quart jar with 3 cups (710 ml) of water needs 88.8 grams, about 11.8 tablespoons. The coffee ratio calculator has a cold brew setting that does this division for any water amount.
The dilution math
Diluting 1:1 with water or milk doubles the total liquid without adding coffee, so the effective ratio doubles from 1:8 to 1:16. A 12 oz serving made from 6 oz of concentrate and 6 oz of water contains 22.2 grams worth of coffee extraction in 355 ml of liquid, an effective 1:16, the same strength as a standard drip cup.
Tune the dilution to taste. A 2:1 concentrate-to-water pour gives an effective 1:12, bold and heavy. A 1:2 pour gives an effective 1:24, light and refreshing over ice. Start at 1:1 and move in small steps; the concentrate is forgiving.
Steep time and grind
Twelve hours is the minimum for a full-flavored steep; 16 to 18 hours is the sweet spot for most beans; past 24 hours the cup starts picking up woody, over-extracted notes. Steep in the fridge rather than on the counter for a cleaner flavor and safer food handling, especially for batches you will keep all week.
Grind coarse, like raw sugar or coarser. Fine grounds over-extract during the long steep and make straining miserable. If your cold brew tastes harsh or ashy, the grind was too fine or the steep ran too long, not the ratio.
Storage and serving
Strained concentrate keeps 7 to 14 days refrigerated in a sealed jar. Keep it cold from the moment it finishes steeping. If it smells sour or tastes off, discard it; the long steep gives microbes plenty of time if the jar was not clean.
Serve over ice, diluted 1:1 with water or milk. Because the concentrate is strong, ice melt is already accounted for: the melting ice just becomes part of the dilution. For a creamier drink, dilute with milk or oat milk instead of water; the 1:1 starting point still applies.