How this instrument works
Cold brew is a different extraction process from hot brewing, not just a temperature change — steeping coarse grounds in room-temperature or cold water for 12 to 24 hours pulls out coffee's flavor far more slowly and selectively than a few minutes of hot water ever could, producing a naturally low-acid, smooth result. Because that slow extraction is so much less efficient than hot brewing, cold brew needs a much more concentrated ratio of coffee to water to end up with comparable strength.
That's why cold brew recipes split into two distinct ratios rather than one. A concentrate, meant to be cut with water, milk, or ice before drinking, is typically steeped around 1:4 coffee to water by weight — strong enough that it must be diluted, similar in spirit to how frozen juice concentrate needs water added back. A ready-to-drink batch, meant to be served straight from the fridge with no further dilution, uses a much weaker steep, commonly around 1:8, so the finished liquid is already at drinking strength.
Confusing the two ratios is the most common cold brew mistake — steeping at concentrate strength and drinking it undiluted produces an overwhelmingly strong, often unpleasantly bitter cup, while steeping at ready-to-drink strength and then diluting it further produces something thin and watery.
- Enter the coffee dose in Coffee (g) — use a coarse, even grind suited to a long steep.
- Choose Brew style — Concentrate (1:4) if you plan to dilute before drinking, or Ready-to-drink (1:8) if you want to serve it straight from the fridge.
- Read Water (mL) — the cold or room-temperature water to combine with your coffee for the chosen style.
- Steep for 12-24 hours at room temperature or in the fridge, then filter through a fine mesh or paper filter before serving or diluting.
Worked example — 100 g of coffee for a concentrate batch
A batch is steeped with 100 g of coarsely-ground coffee at the Concentrate (1:4) setting. Water needed = 100 × 4 = 400 mL of cold water for the steep.
After 12-24 hours steeping and filtering, that concentrate is strong enough that it should be cut roughly 1:1 with water, milk, or ice before drinking — steeped at this ratio, it is not meant to be consumed straight, unlike the same 100 g of coffee steeped at the weaker Ready-to-drink (1:8) setting, which would use 800 mL of water and need no further dilution.
Questions
What's the difference between concentrate and ready-to-drink cold brew?
Concentrate is steeped strong on purpose — around 1:4 coffee to water — specifically so it can be diluted with water, milk, or poured over ice before drinking, similar to how a frozen juice concentrate is meant to be cut with water. Ready-to-drink cold brew is steeped weaker, around 1:8, so the finished liquid is already at a normal drinking strength straight from the fridge, no dilution needed.
Why does cold brew need so much more coffee than hot brew for the same strength?
Cold water extracts coffee compounds far less efficiently than hot water does, so a much longer steep time (12-24 hours instead of a few minutes) and a much higher coffee-to-water ratio are both needed to reach comparable strength. A hot brew's roughly 1:17 ratio would produce an extremely weak, underextracted cold brew at the same steep time.
Can I use the concentrate ratio and just drink it black without diluting?
You can, but it will taste unusually strong and intensely caffeinated compared to a typical cup of coffee — a 1:4 concentrate is deliberately over-strength so that diluting it lands at normal drinking strength. If you want to drink cold brew straight from the steep with no dilution step, choose the Ready-to-drink (1:8) setting instead.
How long should I steep cold brew, and does the ratio change with steep time?
Most recipes steep for 12 to 24 hours at room temperature or in the refrigerator; steeping longer extracts more flavor and can shift a cup toward more bitterness, while steeping less than 12 hours often leaves it underextracted and thin regardless of the ratio used. This calculator's ratios assume a steep time in that typical 12-24 hour range.
Should the grind be different for cold brew than for hot brewing methods?
Yes — cold brew uses a notably coarser grind than drip or pour-over, closer to coarse breadcrumbs, because the long steep time would over-extract and turn muddy or bitter at a finer grind. A grind that's too fine also makes filtering the finished cold brew slower and harder, since fine particles clog a mesh or paper filter more easily.