Cold brew is coffee made without heat. Ground coffee sits in cold or room-temperature water for many hours, then you strain it. There is no kettle, no timer to watch and no technique to get wrong. The only real decisions are how much coffee to use, how long to wait, and how much to water it down afterwards.
Hot water pulls flavour out of coffee fast, including the sharp, sour-bright acids and the bitter compounds that come later in the extraction. Cold water is slow and picks less of those up, so the cup tastes rounder and sweeter, with less bite. Time makes up for the missing heat: where a filter brew takes minutes, cold brew takes most of a day.
This is a taste difference, not a strength rule. Cold brew is not automatically weaker or stronger in caffeine. That depends on how much coffee you used and how much you dilute it, which is why the ratio matters more than anything else.
Ratios are by weight: grams of ground coffee to grams (or ml) of water. Sources differ a little, but they cluster around three styles.
Use a coarse grind, close to French press size. A fine grind over-extracts in a day and turns muddy and bitter, and it clogs the filter when you strain.
Most guides land between 12 and 24 hours. At room temperature the coffee extracts faster, so 12–16 hours is plenty, and it should go into the fridge as soon as you strain it. In the fridge extraction is slower and the result is often cleaner, so 16–24 hours works. Darker roasts tend to taste best at the short end, light roasts at the long end.
Go much longer and the flavour turns woody and flat, so there is no reward for forgetting it for two days. If a batch tastes weak, add more coffee next time before you steep longer.
Pour through a fine mesh sieve first, then through a paper filter or a clean cloth. The paper step removes the fine silt that otherwise makes the drink cloudy and keeps extracting in the jar. Store it in a sealed glass bottle with little air in it, label the brew date, and keep it in the fridge. Concentrate is best within about a week; sources say anything from 7 days to 2 weeks, so treat the longer end as the limit and throw it out if it smells off. Keep milk and sweetener out of the bottle and add them in the glass.
A glass of concentrate, even diluted, can hold a lot more coffee than you expect. The European Food Safety Authority treats up to 400 mg of caffeine a day, and up to 200 mg in a single sitting, as safe for healthy adults. Limits are lower in pregnancy and for some health conditions, so check local advice. Start with a small glass of a diluted strong batch and see how you feel, and keep it away from children.
Once the base ratio works, change one thing at a time. Add a strip of orange peel or a cinnamon stick to the jar for the last few hours. Pour it over ice with a splash of oat or dairy milk for a longer drink, or top it with sparkling water for something closer to a soda. For a warm cup, dilute with hot water; it will taste sweeter and smoother than a drip coffee.
You do not need to buy anything: a large clean jar, a sieve and a coffee filter are enough. If you plan to make it often, a few things help at any budget. A grinder that can produce a truly coarse, even grind makes the biggest difference, so grind fresh or ask for a coarse grind from the roaster. A jar with a wide mouth is easy to fill and clean. A kitchen scale lets you keep the ratio exactly the same from one batch to the next.