Water Hardness Calculator & Unit Converter
Convert between ppm, gpg, German degrees, French degrees, Clark degrees, and mmol/L. See your hardness classification instantly.
Water Hardness Classification
Hard
Hard water can cause scale buildup in pipes and appliances. A water softener may be beneficial.
Converted Values
Grains per Gallon
7.01
gpg
°dH (German)
6.72
°dH
°fH (French)
12
°fH
°e (Clark)
8.4
°e
mmol/L
1.2
mmol/L
Household Impact
Shower
May dry out skin & hair
Kitchen
Spots on dishes
Laundry
Clothes may feel stiff
Appliances
Scale buildup
How to Use This Calculator
- Enter your hardness value — you can find this number on your annual Consumer Confidence Report (CCR), a home water test strip, or a lab water test kit.
- Select the unit — choose the unit your value is measured in. US water reports typically use ppm (mg/L) or gpg. European reports may use °dH, °fH, or °e.
- Read the results — the calculator instantly shows your value in all other units, plus your hardness classification and what it means for your home.
Not sure about your water hardness? Search your city on TapWater.org to find your local water hardness data.
Understanding Water Hardness Units
mg/L (ppm)
Milligrams per liter, equivalent to parts per million. The standard unit used by the EPA, USGS, and most US water utilities.
Grains per Gallon (gpg)
The unit preferred by the water softener industry in North America. 1 gpg = 17.1 ppm. Most softener sizing charts use gpg.
German Degrees (°dH)
Deutsche Härtegrade — the standard in Germany, Austria, and Switzerland. 1 °dH = 17.85 ppm. Commonly seen on European appliance manuals.
French Degrees (°fH)
Used in France and some parts of Europe. 1 °fH = 10 ppm. Often listed on French household appliance specifications.
Clark Degrees (°e)
Also called English degrees. Used primarily in the United Kingdom. 1 °e = 14.3 ppm. Named after Thomas Clark who developed the test.
mmol/L
Millimoles per liter — the SI scientific unit. Used in laboratory and academic settings. 1 mmol/L = 100.1 ppm.
For a deeper dive into hardness units and their history, see our Water Hardness Scale guide.
Water Hardness Classification Scale
The USGS and Water Quality Association classify water hardness into four levels based on dissolved calcium and magnesium content:
| Classification | ppm (mg/L) | gpg | Household Impact |
|---|---|---|---|
| Soft | 0 – 60 | 0 – 3.5 | No scale buildup. Gentle on skin and appliances. |
| Moderately Hard | 61 – 120 | 3.6 – 7.0 | Slight mineral buildup over time. May need more soap. |
| Hard | 121 – 180 | 7.1 – 10.5 | Scale buildup on fixtures and pipes. Water softener recommended. |
| Very Hard | 180+ | 10.5+ | Significant scale. Reduces appliance efficiency. Softener strongly recommended. |
Frequently Asked Questions
What is considered hard water?
According to the USGS, water is classified as hard when it exceeds 120 mg/L (ppm) or about 7 grains per gallon (gpg). Water above 180 mg/L (10.5 gpg) is considered very hard. The full scale is: Soft (0-60 ppm), Moderately Hard (61-120 ppm), Hard (121-180 ppm), and Very Hard (above 180 ppm).
How do I convert ppm to grains per gallon?
To convert ppm (mg/L) to grains per gallon (gpg), divide the ppm value by 17.1. For example, 120 ppm / 17.1 = 7.0 gpg. To convert gpg back to ppm, multiply by 17.1.
What water hardness level requires a water softener?
Most water softener manufacturers and the Water Quality Association recommend considering a water softener when water hardness exceeds 7 gpg (about 120 ppm). At this level, you may notice scale buildup on fixtures, water spots on dishes, and reduced soap efficiency. Above 10.5 gpg (180 ppm), a water softener is strongly recommended to protect appliances.
What is the difference between gpg and ppm?
Both are units for measuring water hardness, but they are used in different contexts. PPM (parts per million), also written as mg/L, is the standard scientific unit used by the EPA, USGS, and most water utilities. GPG (grains per gallon) is commonly used by the water softener industry in North America. 1 gpg equals approximately 17.1 ppm.
How do German degrees (°dH) compare to ppm?
1 German degree of hardness (°dH) equals approximately 17.85 ppm (mg/L). German degrees are the standard hardness unit in Germany, Austria, and Switzerland. To convert °dH to ppm, multiply by 17.85. To convert ppm to °dH, divide by 17.85.