Use this water hardness calculator to find total hardness from calcium and magnesium, solve for one missing value, or convert a known result between , , , , and . Enter alkalinity only if you need the optional carbonate and noncarbonate split.
How to calculate and interpret water hardness
Water hardness measures dissolved calcium and magnesium and is commonly reported as an equivalent concentration of calcium carbonate, or CaCO3. The calculator combines the mineral contributions, converts the total into common units, and assigns the exact category used by the engine. Hardness describes mineral scale and soap performance; it is not a health determination.
The calcium and magnesium fields require raw ion concentrations in , not values already reported as CaCO3.
TDS is not hardness. It includes ions beyond calcium and magnesium.
The engine uses Soft at 60 or below, Moderately hard at 120 or below, Hard at 180 or below, and Very hard above 180 .
Put a reported total into Water hardness and keep the stated unit: , , , , or .
How to find and enter your water hardness
A utility or lab report may list total hardness as CaCO3. Enter that result in Water hardness and select the unit printed on the report. If the report lists raw calcium and magnesium separately, enter those concentrations in the matching fields. Do not enter calcium hardness or magnesium hardness already expressed as CaCO3 because the calculator applies the conversion factors itself.
A home hardness strip or kit can provide an estimated total-hardness result. Follow the kit instructions, then enter the reported value in Water hardness with the printed unit. A total-hardness strip result does not belong in the separate calcium or magnesium fields. Treat a home-kit result as an approximation when accuracy matters, and do not substitute a TDS reading.
Worked example with 40 mg/L calcium and 12 mg/L magnesium
- Calcium contributes 40 × 2.497 = 99.88 .
- Magnesium contributes 12 × 4.118 = 49.416 .
- Their total is 149.296, displayed as 149.3 .
- The calculator also reports 8.72 , 8.36 , 14.93 , and the category Hard.
Units and USGS category boundaries
For dilute water, hardness in has the same numeric value as . are common in US softener manuals, while and appear in some appliance and international water specifications.
| Hardness, | Engine category | |||
|---|---|---|---|---|
| 60 | 3.51 | 3.36 | 6 | Soft |
| 120 | 7.01 | 6.72 | 12 | Moderately hard |
| 180 | 10.52 | 10.09 | 18 | Hard |
Values above 180 are Very hard. These categories describe mineral hardness and likely nuisance effects. They do not determine whether water is safe or healthy.
What the optional alkalinity split means
Enter alkalinity only when the report gives it in . The calculator sets carbonate hardness to the smaller of total hardness and alkalinity. Noncarbonate hardness is the remainder. These are also commonly called temporary hardness and permanent hardness, respectively. For calcium 72 , magnesium 18 , and alkalinity 140 , the engine reports 253.9 total, 140 carbonate, and 113.9 noncarbonate hardness.
What this calculator does not estimate
This calculator converts and classifies hardness; it does not size a water softener or predict salt use and regeneration timing. Those jobs require equipment-specific usable resin capacity at a stated salt dose, water demand, controller reserve, and possible compensation for iron or manganese.
It also does not convert TDS to hardness. A hardness value alone cannot identify a unique calcium-magnesium pair, but hardness plus one known mineral can solve the other through the same equation.
Frequently asked questions
Is 120 hard water?
The calculator classifies 120 as moderately hard because 120 is the inclusive upper boundary of that category. A result above 120 and through 180 is hard.
How do I calculate water total hardness?
Multiply raw calcium in by 2.497 and raw magnesium in by 4.118, then add the results. The total is .
Is 20 hard water?
Yes. A reading of 20 equals 342.36 , which is above the calculator's 180 threshold for very hard water. The category describes hardness and scale potential, not water safety.
What happens if I set my water softener hardness too high?
The softener may regenerate sooner than needed and use extra salt and water. Use a tested hardness value and follow the equipment manual because controller reserves and compensation settings vary.