Why is my cold brew weak?

Weak cold brew usually comes down to one of three things: you did not use enough coffee, your grind was too coarse, or you did not steep long enough. Cold water is a slow, lazy solvent, so it needs more coffee and more time than hot brewing to pull out the same flavor. The fix is to push all three levers in the same direction.

The three things to change

Adjust one at a time so you know what worked:

Use more coffee. This is the biggest lever. For a concentrate you dilute later, aim for roughly 1:8 coffee to water by weight (for example, 100 g coffee to 800 g water). For ready-to-drink cold brew, more like 1:12 to 1:15. If you have been using a hot-coffee ratio like 1:16, that alone explains a thin cup. See coffee-to-water-ratio.
Grind medium-coarse, not extra coarse. Many guides say “coarse like sea salt,” but a grind that is too chunky under-extracts in cold water. Medium-coarse extracts more without turning muddy. Check the grind-size-guide.
Steep longer. Give it 12 to 18 hours. At room temperature it brews faster; in the fridge, lean toward the longer end. Under about 8 hours and you often get sour, hollow, under-extracted coffee.

Concentrate vs ready-to-drink

A lot of “weak” cold brew is really a dilution mistake. If you brewed a strong concentrate, you are meant to cut it with water, milk, or ice, usually around 1:1 up to 1:3 concentrate to liquid. Taste it neat first; if the undiluted base is already weak, the problem is the brew, not the dilution.

A few smaller things that sap strength:

Not enough contact: if the grounds float in a loose bag with little water movement, give a gentle stir at the start.
Ice melt: serving over lots of ice waters it down fast. Use cold-brew ice cubes or chill the brew instead.

Dial in the ratio first, then grind, then time. For the full method including filtering and storage, read cold-brew.