What is the most common unit for water hardness?

In the United States, grains per gallon (gpg) is widely used in residential water treatment, while mg/L as CaCO₃ (ppm) is standard in laboratory and municipal reports. European countries typically use French degrees (°f) or German degrees (°dH).

What hardness level is considered safe for drinking water?

The WHO does not set a health-based guideline for hardness. Water below 75 mg/L CaCO₃ is considered soft; above 300 mg/L is hard. Hard water can cause scale in pipes and appliances but is not a health risk and often supplies beneficial calcium and magnesium.

What is the difference between mg/L as CaCO₃ and mg/L as Ca²⁺?

Both express calcium concentration, but mg/L as CaCO₃ normalizes the value to the molecular weight of calcium carbonate (100.087 g/mol), while mg/L as Ca²⁺ uses the atomic weight of calcium (40.078 g/mol). Multiply mg/L Ca²⁺ by 2.497 to convert to mg/L CaCO₃.

How hard should water be for brewing beer?

Optimal brewing hardness depends on beer style. Pale ales typically use soft water (50–100 mg/L CaCO₃), while stouts and porters suit harder water (150–250 mg/L CaCO₃). Brewers usually target specific calcium, magnesium, sulfate, and chloride levels rather than total hardness alone.

What level of hardness causes limescale buildup?

Noticeable limescale in kettles and pipes typically starts around 150 mg/L CaCO₃ (moderately hard). At levels above 200 mg/L, scale buildup accelerates, increasing maintenance costs and reducing appliance efficiency.

Water Hardness Calculator

Convert water hardness between mg/L CaCO₃, grains per gallon, mmol/L, French degrees, German degrees, and mg/L Ca²⁺.

Enter Hardness Value

Water classification:

Conversion Results

Unit	Value	Symbol

Hardness Classification Reference

Category	mg/L CaCO₃	gpg	°f	°dH
Soft	< 75	< 4.38	< 7.5	< 4.20
Moderately soft	75 – 150	4.38 – 8.76	7.5 – 15	4.20 – 8.40
Moderately hard	150 – 300	8.76 – 17.52	15 – 30	8.40 – 16.80
Hard	> 300	> 17.52	> 30	> 16.80

Source: WHO Guidelines for Drinking-water Quality; US EPA secondary standards.

Summary

Enter a water hardness value in any unit and instantly convert it to all six common hardness units. The tool also classifies water as soft, moderately soft, moderately hard, or hard based on standard thresholds.

How it works

Enter a water hardness value and select the source unit (mg/L CaCO₃, gpg, mmol/L, mg/L Ca²⁺, French degrees, or German degrees).
The tool converts the input to mg/L as CaCO₃ as the common baseline, then derives all other units.
Results for all six units appear simultaneously in the output table.
A classification badge indicates whether the water is soft, moderately soft, moderately hard, or hard per standard thresholds.

Use cases

Water treatment engineers sizing softening or filtration systems.
Aquarium hobbyists matching tank water to species requirements.
Brewers calculating mineral additions for recipe water profiles.
Boiler operators monitoring feed-water hardness to prevent scaling.
Lab technicians translating supplier certificates between unit systems.
Homeowners interpreting water quality reports from different countries.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu