Cold brew is the most forgiving coffee you can make, and the easiest to get wrong by volume. People pour water over grounds, wait a day, strain, and end up with far less drink than they expected, because the grounds drink a share of the water themselves. This calculator plans the whole batch: how much water to pour for your grounds, how much concentrate you will actually strain out, and how many servings you get after dilution. With those three numbers, a weekend batch stops being a surprise.
The maths starts with the steep ratio. At 1:5, one hundred grams of grounds need five hundred millilitres of water. But grounds absorb roughly twice their weight in water, so about two hundred millilitres never comes back, and the strained concentrate is closer to three hundred millilitres. That is not a mistake: it is the physics of the method, and this calculator subtracts the absorbed water for you. If you want a full litre of concentrate, you must start with more grounds and more water than the final number suggests.
Here is a worked example. You use 100 grams of grounds at 1:5, so you pour 500 millilitres of water. After absorption you strain about 300 millilitres of concentrate. Dilute it with one part water, and you get 600 millilitres of drink, which is two and a half servings of 240 millilitres. Want a full week of one serving a day, seven servings of 240 millilitres? That is 1,680 millilitres of diluted drink, or 840 millilitres of concentrate at one part dilution, which needs about 280 grams of grounds and 1,400 millilitres of steep water.
Ratio choice decides whether you make a concentrate or a ready drink. A 1:4 or 1:5 steep gives a strong concentrate that stores well in the fridge for up to two weeks and lets each person dilute to taste. A 1:8 steep gives a drink that is nearly ready as strained, which is simpler but takes more fridge space and goes stale faster. Concentrate is the better choice for most homes: one batch serves everyone, strong-drink lovers use less water, and light-drink lovers add more.
Grind and time matter as much as ratio. Use a coarse grind, like raw sugar, so the grounds strain cleanly and do not turn the brew muddy or bitter. Steep for 12 to 24 hours: at 12 hours the cup is sweet and light, at 24 it is deeper and heavier, and beyond 24 it often turns woody. Room temperature steeps faster than the fridge, so a fridge batch may need the longer end of the range. Taste at 16 hours if you are unsure, and strain when it tastes right rather than watching the clock.
Straining and storage decide the last part of the quality. Strain through a fine mesh first, then through a paper coffee filter or a clean cloth, so the final drink is clear and not gritty. Store concentrate in a sealed jar or bottle in the fridge, and dilute only the serving you are about to drink, because diluted coffee stales faster than concentrate. Label the jar with the date and the dilution you like, and your future self will thank you. Cold brew rewards planning once, then pays you back all week.
Water quality matters more in cold brew than in hot coffee, because there is no heat to hide off flavours. If your tap water tastes of chlorine or minerals, use filtered water, and you will taste the difference in the first batch. Sweetness also behaves differently cold: sugars taste less sweet when cold, so a cold brew that tastes balanced hot may need a touch more sweetener, or a splash of milk, when served over ice. Serve concentrate over plenty of ice rather than a little, because melting ice is part of the dilution, and a drink built for ice stays balanced as the ice melts instead of turning watery.