Water Hardness Calculator

Determine your water hardness level from calcium and magnesium concentrations. Get results in mg/L, GPG, German degrees (°dH), French degrees (°fH), and English degrees (°eH) with detailed classification.

Concentration of calcium ions in mg/L or ppm

Concentration of magnesium ions in mg/L or ppm

Water Hardness Formula: Total Hardness = 2.497 × [Ca²⁺] + 4.118 × [Mg²⁺] Where concentrations are in mg/L and result is in mg/L as CaCO₃. Why these factors: 2.497 = 50.04 / 20.04 (Molar mass CaCO₃ / Equivalent weight Ca²⁺) 4.118 = 50.04 / 12.15 (Molar mass CaCO₃ / Equivalent weight Mg²⁺) Unit Conversions: 1 gpg = 17.12 mg/L CaCO₃ 1 °dH = 17.848 mg/L CaCO₃ 1 °fH = 10.0 mg/L CaCO₃ 1 °eH = 14.286 mg/L CaCO₃ Classification: • Soft: 0-60 mg/L (0-3.5 gpg) • Moderate: 61-120 mg/L (3.5-7.0 gpg) • Hard: 121-180 mg/L (7.0-10.5 gpg) • Very Hard: >180 mg/L (>10.5 gpg)
Example 1: Typical Municipal Water Ca²⁺ = 40 mg/L, Mg²⁺ = 12 mg/L Hardness = 2.497(40) + 4.118(12) = 99.9 + 49.4 = 149.3 mg/L as CaCO₃ = 8.72 gpg Classification: Hard Calcium contributes 66.9% Magnesium contributes 33.1% Example 2: Softened Water Ca²⁺ = 5 mg/L, Mg²⁺ = 2 mg/L Hardness = 2.497(5) + 4.118(2) = 12.5 + 8.2 = 20.7 mg/L as CaCO₃ Classification: Soft Excellent for household use. Example 3: Hard Well Water Ca²⁺ = 85 mg/L, Mg²⁺ = 30 mg/L Hardness = 2.497(85) + 4.118(30) = 212.2 + 123.5 = 335.8 mg/L as CaCO₃ = 19.6 gpg Classification: Very Hard Water softener strongly recommended. Scale will form rapidly on fixtures. Example 4: International Standards US: < 60 mg/L soft; > 120 mg/L hard UK: < 50 mg/L soft; > 200 mg/L hard WHO: < 100 mg/L acceptable EU: Desirable range 60-240 mg/L CaCO₃ Common Global Water Hardness: • New York City: ~24 mg/L (Soft) • London: ~300 mg/L (Very Hard) • Tokyo: ~60 mg/L (Moderate) • Most bottled water: 50-150 mg/L

What is water hardness and how is it measured?

Water hardness measures the concentration of dissolved calcium and magnesium ions, expressed in equivalent calcium carbonate (CaCO₃). Hard water contains high levels of these minerals, which can cause scale buildup in pipes, reduce soap lathering, and leave deposits on fixtures. Hardness is most commonly reported in mg/L or grains per gallon (gpg), where 1 gpg = 17.12 mg/L as CaCO₃.

What are the classification levels for water hardness?

Water hardness is classified as: Soft (0-60 mg/L or 0-3.5 gpg), Moderately Hard (61-120 mg/L or 3.5-7.0 gpg), Hard (121-180 mg/L or 7.0-10.5 gpg), and Very Hard (>180 mg/L or >10.5 gpg). Most municipal water supplies aim for soft to moderately hard water. Water above 180 mg/L typically requires a water softener for household use.

Why is hardness expressed as calcium carbonate equivalent?

Hardness is expressed as CaCO₃ equivalent because it provides a common basis for comparing different water samples regardless of the specific ions present. The conversion uses equivalent weights: Ca²⁺ has an equivalent weight of 20.04 (atomic mass 40.08 ÷ valence 2), while CaCO₃ has an equivalent weight of 50.04. Thus, 1 mg/L Ca²⁺ = 2.497 mg/L as CaCO₃.

Can water hardness be reduced at home?

Yes, the most common method is ion-exchange water softening, which replaces Ca²⁺ and Mg²⁺ ions with Na⁺ or K⁺ ions. Reverse osmosis systems also effectively reduce hardness. For temporary hardness (bicarbonate-based), boiling can precipitate calcium carbonate. For permanent hardness (sulfate/chloride-based), chemical treatment or ion exchange is needed. Magnetic descalers are not scientifically proven to be effective.

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