Water Hardness Calculator
Enter your water hardness value and see instant conversions, classification, and what it means for your home.
Classification
Hard
At this level, you may notice scale buildup on fixtures and appliances. A water softener could be beneficial. Learn about water softener options →
US average: ~100 ppm. Check your city →
Converted Values
gpg
7.01
gpg
°dH
6.72
°dH
°fH
12
°fH
°e
8.4
°e
mmol/L
1.2
mmol/L
Household Impact
Shower
May dry out skin & hair
Kitchen
Noticeable spots on dishes & fixtures
Laundry
Clothes may feel stiff, more soap needed
Appliances
Scale buildup reduces efficiency
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. |
PPM to GPG Conversion Table
PPM (parts per million) and GPG (grains per gallon) are the two most common water hardness units in the United States. Use this table as a quick reference to convert between them, or enter any value into the calculator above for instant results.
| ppm (mg/L) | gpg | Classification |
|---|---|---|
| 25 | 1.46 | Soft |
| 50 | 2.92 | Soft |
| 75 | 4.38 | Moderately Hard |
| 100 | 5.84 | Moderately Hard |
| 120 | 7.01 | Moderately Hard |
| 150 | 8.76 | Hard |
| 200 | 11.68 | Hard |
| 250 | 14.60 | Very Hard |
| 300 | 17.53 | Very Hard |
| 400 | 23.37 | Very Hard |
| 500 | 29.21 | Very Hard |
Looking for a different value? Enter any number into the calculator above to convert instantly.
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.
What is the average water hardness in the US?
The US average water hardness is approximately 100 mg/L (5.8 gpg), which falls in the moderately hard range. However, hardness varies dramatically by region — cities in the Southwest like Phoenix (285 ppm) and Las Vegas (290 ppm) have very hard water, while cities like Portland (10 ppm) and Seattle (25 ppm) have naturally soft water.
Is hard water bad for you?
Hard water is not a health risk — the World Health Organization and the EPA consider calcium and magnesium in drinking water to be safe and even potentially beneficial. However, hard water above 120 ppm can cause practical issues: scale buildup in pipes and appliances, reduced soap effectiveness, and dry skin and hair. The concern is primarily about household effects, not health.
How do I test my water hardness at home?
There are three ways to find your water hardness: (1) Check your water utility's annual Consumer Confidence Report (CCR), which lists hardness in ppm or mg/L. (2) Use a home test strip — dip it in water and match the color to the chart. Results are approximate but quick. (3) Order a lab water test kit for precise results. Once you have your number, enter it into the calculator above.