Cold Brew at Home: Ratios, Steep Times, and the Concentrate Trick

By Groundwork Coffee Team · September 25, 2026 · 9 min read

Cold brew is the easiest brewing method to get right and the easiest to get bland. There's no pouring technique, no water temperature to nail, no bloom to time — just coffee, water, and patience. But "just wing it" produces the two classic failures: brown water (too weak) or ashtray (too bitter). The fix is exact ratios and steep-time windows, which is what this guide gives you — plus the concentrate trick that turns one Sunday batch into a week of iced coffee.

What You Need (a Jar, a Filter, and Coarse-Ground Coffee)

Cold brew needs less gear than any other method:

• A large jar or pitcher — a 1-quart (32 oz) mason jar is perfect for concentrate; a half-gallon jar works for ready-to-drink batches. Anything food-safe with a lid.
• Coarsely ground coffee — think rough breadcrumbs or cracked peppercorns. Coarse matters: fine grounds over-extract over 16 hours and turn the batch bitter and silty.
• A filter for straining — a fine-mesh sieve plus a paper coffee filter, a nut-milk bag, or a reusable metal mesh filter. Paper gives the cleanest cup; mesh is faster.
• Cold, filtered water — room-temperature or fridge-cold both work; filtered water tastes noticeably cleaner.

That's genuinely it. Dedicated cold brew makers (Toddy, OXO) add convenience — built-in filters, easier draining — but a mason jar you already own makes coffee that's just as good.

The Two Ratios: 1:8 Concentrate vs. 1:12 Ready-to-Drink

Pick your strategy before you measure anything:

• 1:8 concentrate — 1g coffee to 8g water. Example: 125g coffee to 1,000g (1 liter) water. Brews strong on purpose; you dilute it 1:1 with water or milk when serving. One liter of concentrate becomes two liters of drinkable coffee — the efficient choice.
• 1:12 ready-to-drink — 1g coffee to 12g water. Example: 80g coffee to 960g water (roughly a quart jar). Drink it straight over ice, no diluting. Simpler, but you get less coffee per batch and per bag of beans.

Our recommendation: brew concentrate at 1:8. It uses the same effort, stretches further, and diluting to taste at serving time gives you control — strong over ice for mornings, mellow with milk for afternoons. The concentrate trick below is built on this ratio.

Step-by-Step: The 12–18 Hour Immersion Method

1. Grind coarse. 125g of coffee at your grinder's coarsest or second-coarsest setting — rough breadcrumbs, not sand. If you're buying pre-ground (it happens), ask for "French press grind."
2. Combine. Add grounds to the jar, pour in 1,000g of cold filtered water, and stir gently for 10 seconds so every ground is wet. Dry clumps are the #1 cause of weak, uneven cold brew.
3. Steep. Lid on, room temperature on the counter (or in the fridge — fridge steeping is slightly cleaner-tasting but needs 2–4 extra hours). See steep-time windows below.
4. Strain twice. Pour through a fine-mesh sieve to catch the bulk, then through a paper filter (or nut-milk bag) for clarity. Don't squeeze the grounds — squeezing pushes bitter fines through.
5. Store. Sealed jar in the fridge. Concentrate keeps 7–10 days; flavor is best in the first 5.

Steep Time by Roast Level

Time is your extraction dial — and darker roasts extract faster, so they need less of it:

• Dark roast: 12–14 hours. Past 14 hours, dark roasts go bitter and ashy fast. When in doubt, pull it at 12.
• Medium roast: 14–16 hours. The sweet spot for most drinkers — chocolatey, smooth, forgiving.
• Light roast: 16–18 hours (up to 20 in the fridge). Dense light-roast beans need the full window; pulled early, they taste thin and grassy.

Room-temperature steeping runs about 2 hours faster than fridge steeping across the board. Pick one location and keep it consistent so your timing is repeatable.

The Concentrate Trick: Diluting 1:1

Here's the weekly system: brew 1 liter of 1:8 concentrate on Sunday evening. Each morning, pour 4 oz (120ml) of concentrate over ice and add 4 oz of water or milk — that's a 12 oz iced coffee at roughly regular strength. One batch = about eight servings = the whole work week from 125g of coffee (roughly a third of a 12 oz bag).

Dial your dilution to taste: 1:1 is balanced, 1:0.75 (less water) is stronger, 1:1.5 is lighter. Milk or oat milk instead of water makes it dessert-level creamy — cold brew's low acidity means dairy never curdles or tastes sour the way it can in iced hot coffee. For a quick cold-brew latte, do 4 oz concentrate + 6 oz cold milk over ice.

Troubleshooting: Weak, Bitter, or Cloudy Cold Brew

• Weak / watery: grind was too coarse, grounds weren't fully saturated (stir next time), or the ratio drifted — weigh both coffee and water. Also: steeping in the fridge for only 12 hours under-extracts; give it the full window.
• Bitter / harsh: steeped too long for the roast (dark roast past 14 hours is the classic), grind too fine, or you squeezed the grounds while straining. Next batch: coarser grind, shorter steep.
• Cloudy / silty: fines in the cup. Double-strain through paper, grind coarser, and pour gently — don't shake the jar before straining.
• Sour or "off": rare, but it means the batch sat too long or the jar wasn't clean. Cold brew is low-acid by nature; sourness signals spoilage, not under-extraction. When in doubt, throw it out.

Storage: How Long It Keeps and When It Turns

Concentrate (1:8): 7–10 days refrigerated in a sealed jar. Flavor peaks in days 1–5, then slowly dulls. Ready-to-drink (1:12): 3–4 days — the extra water means it oxidizes faster. Signs it's turned: a sour or winey smell, visible cloudiness that wasn't there before, or a sharp taste. Cold brew won't make you sick the way old milk will, but dead cold brew isn't worth drinking — the whole point is that it's cheap to make fresh.

Pro move: freeze leftover concentrate in ice-cube trays. Cold brew ice cubes keep iced coffee from diluting as they melt — the last sip tastes as strong as the first.