How much does water quality affect coffee taste?

Enormously — coffee is 98% water. Hard water mutes acidity and produces flat flavors. Too-soft water over-extracts and tastes sharp. Ideal brewing water has 50-175 ppm total dissolved solids, a pH of 6.5-7.5, and no chlorine. If your tap water tastes bad, your coffee will too. Third Wave Water mineral packets or a simple Brita filter make a noticeable difference.