Glossary
Water hardness
How much dissolved mineral content, mainly calcium and magnesium, is in your water, affecting both taste and scale buildup.
Water hardness measures how much dissolved calcium and magnesium is present in your tap water, usually described in grains per gallon or parts per million. Coffee is around ninety eight percent water, so the minerals in it directly affect how flavors dissolve and taste, not just how the machine holds up over time.
Very soft or mineral-free water, like straight distilled water, actually brews flat and lifeless coffee because minerals help pull flavor compounds out of the grounds during extraction, while very hard water leaves a heavy, chalky, sometimes bitter taste and speeds up scale buildup inside a machine's boiler and pipes. A moderate middle ground works best for both taste and machine health.
Simple test strips can tell you roughly how hard your tap water is, and filtered or remineralized water is often worth using if your results are far from that middle range. A common mistake is switching to distilled water thinking it will improve taste, when it can actually make coffee taste worse while still protecting the machine from scale.