Almost always the ratio, and people reach for time instead.
The ratio is the lever
Cold water extracts far less efficiently than hot, so cold brew needs considerably more coffee per litre:
- Concentrate: 1:8 by weight — 125 g of coffee to 1 litre of water. Dilute one to one to serve.
- Ready to drink: 1:15 — about 65 g per litre.
If you used a filter-coffee ratio of 1:16 and did not dilute, that is your answer.
Time helps less than you expect
Beyond about 18 hours the extraction curve flattens, and past 24 hours you start pulling out woody, papery compounds rather than more sweetness.
Extending from 12 to 18 hours is worth doing. Extending from 18 to 36 makes it worse, not stronger.
Other causes
Grind too coarse. Coarse is right; very coarse is not. Aim for French press texture rather than gravel.
Cold fridge and a short steep. Refrigerated brewing is slower than room temperature. If you steep in the fridge, use the longer end — 16 to 24 hours.
Under-agitated. Stir once at the start so every ground is wet. Dry pockets never extract.
And check what you diluted with
Concentrate diluted 1:2 rather than 1:1 will taste weak because it is.
Last reviewed 8 August 2026