Travel mugs and hotel carafes are smaller than your home jar, so it's easy to eyeball the ratio and end up with weak, sour coffee. Cold brew needs a higher dose than hot brewing—most people use 1:8 coffee to water by weight, meaning 1 gram of coffee for every 8 grams of water. That's about 1 cup of grounds per 4 cups of water.

If you don't have a scale, use a heaping tablespoon per 6 ounces of water, then adjust. If it's sour, add more coffee next time, not more water.