Cold brew is not iced coffee
Iced coffee is brewed hot and then chilled, usually poured over ice. Cold brew never meets hot water at all: coarse grounds sit in cold or room-temperature water for the best part of a day, and the water pulls the coffee apart slowly.
That difference is the whole reason the drinks taste different. Heat extracts fast and pulls out a wider range of compounds, including many of the acids that give hot coffee its brightness. Cold water working over sixteen hours extracts a narrower set, which is why cold brew tastes rounder, sweeter and heavier, and why it tastes flat to people who like a bright Kenyan filter.
Neither is better. If you want the fruit and acidity of a good single origin, brew it hot and chill it. If you want something smooth and strong that keeps in the fridge for a week, cold brew is the better tool.
The method
1. GRIND COARSE. Coarser than filter, about what you would use in a French press, with visible chunky particles like raw sugar. Fine grounds over a long steep give you a muddy, bitter brew and they are miserable to filter out. A grinder that holds a consistent coarse setting matters more here than it does for espresso, and a Baratza Encore is the usual answer.
2. COMBINE AND STIR. Grounds and cold filtered water into a jar. Stir properly so there are no dry pockets, because unlike a pour-over nothing is going to agitate this for you afterwards.
3. WAIT. Somewhere between 12 and 24 hours. Stumptown puts the ideal at 16 hours and says 14 to 18 works well, which matches the general consensus. Past about 24 hours you get very little extra extraction and a growing risk of harshness.
4. FRIDGE OR COUNTER. Both work. The fridge is slower and safer for a long steep; the counter extracts a little faster. If you steep at room temperature, move it to the fridge as soon as it is done.
5. FILTER TWICE. Strain the grounds out through a fine mesh, then run it through a paper filter or a cloth if you want it clean rather than silty. Skipping the second pass is why homemade cold brew often has a gritty finish that the bought stuff does not.
Concentrate or ready to drink, and why the ratios disagree
Search for a cold brew ratio and you will find everything from 1:4 to 1:17, which looks like nobody knows. They are answering two different questions.
A CONCENTRATE is brewed strong and diluted before you drink it. Ratios here run roughly 1:4 to 1:8 by weight. Stumptown's own recipe is 12 ounces of coffee to 64 ounces of water, cut 1:1 with water or milk in the glass. Concentrate is the practical choice: it takes less fridge space and it lets you decide the strength cup by cup.
READY TO DRINK is brewed at the strength you will actually drink it, somewhere around 1:12 to 1:17 by weight, and goes straight in the glass. Less faff, more fridge space, and no way to adjust it later.
Neither is more correct. What matters is knowing which one a recipe is giving you, because brewing a 1:5 concentrate and drinking it neat is unpleasant, and brewing 1:15 and then diluting it gives you brown water. If a recipe does not say whether to dilute, look at the ratio and you will know.
Start at 1:5 for concentrate or 1:15 for ready to drink, then move in one direction and keep notes. There is no in-brew adjustment with this method, so the only way to dial it in is across batches.
How long it keeps
Stumptown puts homemade cold brew at 7 to 10 days in the fridge, and that matches what most roasters say. It does not spoil dramatically at the end of that, it just flattens out and loses whatever aromatic character it had.
Concentrate keeps at the longer end because there is less water in it. Once you dilute a glass, drink it that day. Cold brew cut with milk should be treated like any other milk drink and finished the same day.
Keep it sealed. An open jug in a fridge picks up every other smell in there, and cold brew is bland enough to show it.
One habit worth having: date the jar. A week is long enough that you will otherwise be guessing, and guessing usually means throwing out coffee that was fine.
Caffeine and acidity, with the actual numbers
Cold brew is sold on being smoother, less acidic and easier on you. Two studies by Fuller and Rao, published in Scientific Reports in 2017 and 2018, measured this properly, and the results are more interesting than the marketing.
ON ACIDITY, the pH is essentially the same. The 2018 study measured cold brew at pH 4.96 to 5.13 and hot brew at 4.85 to 5.10, which is a rounding error apart. What does differ is titratable acidity, the total quantity of acid present rather than the strength of it: hot brew came out substantially higher across every sample. So cold brew does contain fewer acids, but not in the way most people mean when they say less acidic, because the pH your stomach meets is basically unchanged.
The same study measured antioxidant activity and hot brew won there too, at about 20.72 against 13.36 mmol Trolox equivalents per litre on the Ethiopian sample. That is the honest trade nobody puts on the bottle. We are reporting chemistry rather than giving health advice, and if acidity affects you personally that is a question for a doctor rather than a coffee site.
ON CAFFEINE, cold brew came out ahead at the same coarse grind. The 2017 study measured a dark roast at 990 mg per litre cold against 840 mg per litre hot. The reason is time rather than temperature: a six minute hot brew does not give caffeine long enough to diffuse out of large particles, while a cold steep running past 400 minutes extracts them fully. At a medium grind the two methods came out level, which tells you the effect is about particle size and contact time.
As for whether cold brew is stronger than espresso, per ounce it is not close: espresso is far more concentrated. Per drink it often is, because a cold brew is served in a glass many times the size of a shot. A 12 ounce cold brew and a single espresso are not the same drink in any sense, including caffeine.