Compared
Hardness vs. alkalinity
Hardness and alkalinity are different water measures that people mix up, partly because both are reported in the same units. Hardness is the amount of dissolved calcium and magnesium, the minerals that cause scale. Alkalinity is the water's ability to resist a change in pH, its buffering capacity.
Last reviewed 2026-08-04
| Hardness | Alkalinity | |
|---|---|---|
| What it measures | Dissolved calcium and magnesium | Ability to resist a pH change (buffering) |
| What it causes | Scale, and soap that will not lather | Steadier pH that is harder to shift |
| Units | mg/L as calcium carbonate | mg/L as calcium carbonate |
| Why they get confused | Reported in the same units as alkalinity | Reported in the same units as hardness |
The bottom line
Hardness is how much scale-forming mineral is in the water; alkalinity is how strongly the water resists a pH change. Same units, different properties.
Common questions
- Are hardness and alkalinity the same?
- No. Hardness is dissolved calcium and magnesium; alkalinity is the water's resistance to a pH change.
- Why are they confused?
- Both are commonly reported in mg/L as calcium carbonate, so the numbers look similar.
- Which one causes scale?
- Hardness. The calcium and magnesium are what form scale.