Water Hardness Calculator

Convert water hardness between units and classify as soft, hard, or very hard.

Calcium Ca2+ (mg/L) (mg/L)

Magnesium Mg2+ (mg/L) (mg/L)

Bicarbonate HCO3- (mg/L) (mg/L)

Please check your inputs and try again.

What Water Hardness Measures

Hardness is the total concentration of divalent metal cations in water — overwhelmingly calcium and magnesium, which dissolve out of limestone, chalk and dolomite as groundwater passes through them. Monovalent ions such as sodium do not contribute, which is why softened water can be high in dissolved solids yet still test as soft.

The convention of reporting everything “as CaCO₃” exists because calcium and magnesium have different molar masses but equivalent chemical effect. Converting both to a single reference compound lets one number describe the combined burden. It does not mean calcium carbonate is present in the water.

Total hardness (mg/L as CaCO₃) = (Ca²⁺ / 40.08 + Mg²⁺ / 24.31) × 100.09

Term	Meaning	Note
Ca²⁺	Calcium concentration, mg/L	Divided by 40.08, its molar mass
Mg²⁺	Magnesium concentration, mg/L	Divided by 24.31 — lighter, so contributes more per mg
100.09	Molar mass of CaCO₃	Converts moles back to the reference scale
HCO₃⁻	Bicarbonate	Determines how much hardness is temporary

Because magnesium is the lighter ion, a milligram of magnesium contributes about 1.65 times as much hardness as a milligram of calcium. Water with modest calcium but high magnesium can therefore test harder than expected.

Reading the Result

mg/L as CaCO₃	Grains per gallon	German degrees (°dH)	Classification
0–60	0–3.5	0–3.4	Soft
61–120	3.6–7.0	3.5–6.7	Moderately hard
121–180	7.1–10.5	6.8–10.1	Hard
>180	>10.5	>10.1	Very hard

Hardness also divides by type. Temporary hardness is the portion paired with bicarbonate; boiling drives off CO₂, precipitates CaCO₃, and removes it — this is limescale in a kettle. Permanent hardness is paired with sulfate or chloride and survives boiling, requiring ion exchange or reverse osmosis to remove.

Worked Examples

Example 1: Very hard water: Ca=80, Mg=24

TH=(80/40.08+24/24.31)*100

Result: TH=298 mg/L CaCO3 - very hard

Scale risk in appliances

Example 2: Soft water: Ca=20, Mg=5

TH=70 mg/L

Result: TH=70 - slightly hard

Minimal scaling risk

Example 3: Temporary vs permanent hardness

Ca=100, Mg=12, HCO₃⁻=180 mg/L

Result: TH ≈ 298 mg/L, of which ~148 is temporary

Bicarbonate can only pair with part of the hardness. The remainder is permanent and will not respond to boiling — relevant when sizing a softener.

Example 4: Magnesium-dominant water

Ca=20, Mg=45 → (20/40.08 + 45/24.31) × 100.09

Result: TH ≈ 235 mg/L — very hard

Calcium alone would suggest soft water. Magnesium contributes far more per milligram, which is why both ions must be measured.

Example 5: Unit conversion

TH = 171 mg/L as CaCO₃

Result: 10.0 gpg, or 9.6 °dH

Divide mg/L by 17.1 for grains per gallon, or by 17.85 for German degrees. Appliance manuals frequently specify one of these rather than mg/L.

Common Mistakes

⚠️

Assuming hardness means the water is unsafe

Hardness is an aesthetic and operational issue, not a health one. Hard water causes scale and poor lather but is not harmful to drink, and may contribute usefully to dietary calcium and magnesium.

⚠️

Treating all hardness as removable by boiling

Only temporary (bicarbonate) hardness precipitates on boiling. Hardness paired with sulfate or chloride is permanent and requires ion exchange or membrane treatment.

⚠️

Weighting calcium and magnesium equally

Magnesium’s lower molar mass means each milligram contributes roughly 1.65 times more hardness than a milligram of calcium. Adding the raw mg/L figures underestimates hardness in magnesium-rich water.

⚠️

Confusing hardness with total dissolved solids

Softened water exchanges calcium and magnesium for sodium. TDS is unchanged or higher, but hardness falls to near zero because sodium is monovalent.

Frequently Asked Questions

Why express as CaCO3?

Universal standard allows comparison of different hardness ions. 1mM Ca2+ = 100 mg/L CaCO3 hardness.

Treating hard water?

Ion exchange softeners replace Ca2+/Mg2+ with Na+. Reverse osmosis removes all ions. Sequestrants (citrate, EDTA) chelate ions without removing them.

Why is hardness reported as CaCO₃?

Calcium and magnesium have different molar masses but equivalent effect on hardness. Expressing both against a single reference compound gives one comparable number. It does not imply calcium carbonate is dissolved in the water.

Is hard water bad for health?

No. Hardness is an aesthetic and operational concern — scale, poor lather, spotting on glassware. Some studies suggest hard water may modestly contribute to dietary calcium and magnesium intake.

What is the difference between temporary and permanent hardness?

Temporary hardness is paired with bicarbonate and precipitates when boiled, forming limescale. Permanent hardness is paired with sulfate or chloride and survives boiling.

How do I convert mg/L to grains per gallon?

Divide by 17.1. For German degrees (°dH) divide by 17.85, and for French degrees divide by 10.

Why does magnesium count more than calcium?

Because hardness is calculated per mole, and magnesium’s molar mass of 24.31 is lower than calcium’s 40.08. The same mass of magnesium supplies about 1.65 times more ions.

Formula Explorer connections

Interpretation: This analytical relationship converts an instrument signal, separation measure or optical response into concentration, identity or performance. Assumption: Calibration, blank correction, linear range, path length, matrix effects and instrument settings must match the sample and method.

Refractometer & Brix Calculator →Beer-Lambert Law Calculator →COD and Oxygen Demand Calculator →Chemistry Formula Explorer →