Convert between water hardness units (mg/L CaCO₃, gpg, dGH, mmol/L, Clark degrees, French degrees). Calculate from Ca²⁺ and Mg²⁺ concentrations and classify water hardness.
Water hardness is a measure of the dissolved calcium and magnesium ions in water, traditionally expressed as milligrams per liter of calcium carbonate (CaCO₃) equivalent. Hard water causes scale buildup in pipes and appliances, reduces soap effectiveness, and affects brewing and aquarium chemistry.
Hardness can be reported in many different units across countries and industries: mg/L as CaCO₃ (US), grains per gallon (US water treatment), German degrees (°dH or dGH), French degrees (°fH), Clark degrees (UK), and millimoles per liter (SI). Converting between these systems is a frequent source of confusion.
This calculator converts between all major hardness units, calculates total hardness from individual Ca²⁺ and Mg²⁺ concentrations, classifies water according to USGS and WHO scales, estimates the amount of water softener salt or ion-exchange resin needed, and provides guidance for aquarium and brewing applications.
For best results, combine calculator output with direct observation and periodic check-ins with a veterinarian or qualified advisor. Small adjustments made early usually improve comfort, safety, and long-term outcomes more than large corrective changes made later.
Knowing your water hardness in the right units is essential for water treatment sizing, aquarium management, beer and coffee brewing, industrial boiler operation, and environmental monitoring. This calculator eliminates unit-conversion errors. It also supports faster maintenance decisions for scaling control and equipment longevity in both home and industrial systems over long service cycles.
Total Hardness (mg/L as CaCO₃) = Ca²⁺ (mg/L) × 2.497 + Mg²⁺ (mg/L) × 4.118. Conversions: 1 gpg = 17.12 mg/L, 1 dGH = 17.85 mg/L, 1 °fH = 10 mg/L, 1 Clark = 14.29 mg/L, 1 mmol/L = 100.09 mg/L.
Result: 211.5 mg/L as CaCO₃ (12.4 gpg) — Hard
Ca²⁺ at 60 mg/L contributes 60 × 2.497 = 149.8, Mg²⁺ at 15 mg/L contributes 15 × 4.118 = 61.8. Total = 211.5 mg/L as CaCO₃, classified as "Hard" by USGS standards.
| mg/L as CaCO₃ | Classification | |----------------|----------------| | 0 – 60 | Soft | | 61 – 120 | Moderately Hard | | 121 – 180 | Hard | | > 180 | Very Hard |
Most US groundwater ranges from 100 to 300 mg/L. Surface water is typically softer (20-120 mg/L). The national average is about 150 mg/L.
| From | Factor to mg/L CaCO₃ | |------|----------------------| | 1 gpg (grains/gallon) | × 17.12 | | 1 °dH (German degree) | × 17.85 | | 1 °fH (French degree) | × 10.00 | | 1 °Clark (UK degree) | × 14.29 | | 1 mmol/L CaCO₃ | × 100.09 | | 1 mg/L Ca²⁺ | × 2.497 | | 1 mg/L Mg²⁺ | × 4.118 |
The WHO considers water hardness to have no adverse health effects at typical levels and suggests hard water may provide a beneficial dietary supplement of calcium and magnesium. However, very hard water can cause aesthetic issues (scale, soap scum) and increase energy costs by reducing heat exchanger efficiency.
USGS classifies 0-60 mg/L CaCO₃ as soft, 61-120 as moderately hard, 121-180 as hard, and >180 as very hard. The WHO considers >120 mg/L to be hard.
Hardness comes from dissolved calcium and magnesium minerals — primarily CaCO₃ (calcite), CaMg(CO₃)₂ (dolomite), and CaSO₄ (gypsum) — picked up as water percolates through rock. This keeps planning practical and lowers the chance of preventable errors.
Temporary (carbonate) hardness can be removed by boiling — it precipitates as CaCO₃ scale. Permanent (non-carbonate) hardness from sulfates and chlorides cannot be removed by boiling.
Ion-exchange softeners replace Ca²⁺ and Mg²⁺ with Na⁺. The resin is regenerated with NaCl brine. One cubic foot of resin typically has 30,000-32,000 grain capacity.
Most tropical freshwater fish prefer soft to moderately hard water (4-12 dGH). African cichlids prefer hard water (12-25 dGH). Marine tanks need specific Ca/Mg for coral growth.
Brewing water profiles vary by style. Pilsners need very soft water (50 mg/L), while IPAs benefit from harder water with higher sulfate. Burton-on-Trent water is ~350 mg/L.