Understanding Grains Per Gallon (GPG), mg/L, and What They Mean for Your Home
Water hardness is the concentration of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions in your water. These minerals dissolve naturally as water moves through limestone and chalk deposits in the ground. While not a health hazard, hardness causes scale buildup, soap scum, dry skin, and appliance damage.
The U.S. Geological Survey (USGS) and Water Quality Association (WQA) classify water hardness based on the concentration of these minerals. The EPA does not set a Maximum Contaminant Level (MCL) for hardness because it is not considered a health risk.
Water hardness can be expressed in several different units. Understanding the conversions helps you interpret test results and compare water treatment products:
| Grains/Gallon (GPG) | mg/L or ppm | mmol/L | French Degrees | Clark Degrees |
|---|---|---|---|---|
| 1 | 17.1 | 0.17 | 1.71 | 1.20 |
| 3 | 51.3 | 0.51 | 5.13 | 3.60 |
| 5 | 85.5 | 0.86 | 8.55 | 6.00 |
| 7 | 119.7 | 1.20 | 11.97 | 8.40 |
| 10 | 171.0 | 1.71 | 17.10 | 12.00 |
| 15 | 256.5 | 2.56 | 25.65 | 18.00 |
| 20 | 342.0 | 3.42 | 34.20 | 24.00 |
| 25 | 427.5 | 4.28 | 42.75 | 30.00 |
Most Common: In the United States, water hardness is most commonly measured in grains per gallon (GPG) for water softener sizing, while water test kits often report in mg/L or ppm.
The USGS and WQA use the following standard classifications:
| Classification | GPG | mg/L (ppm) | Effects |
|---|---|---|---|
| Soft | 0 - 3 | 0 - 60 | No scaling issues; soap lathers easily; may be slightly corrosive |
| Slightly Hard | 3 - 7 | 60 - 120 | Minimal scale; slight soap reduction; generally acceptable |
| Moderately Hard | 7 - 10 | 120 - 180 | Noticeable scale buildup; soap performance reduced; consider treatment |
| Hard | 10 - 15 | 180 - 260 | Significant scale; appliance damage; dry skin; softener recommended |
| Very Hard | 15+ | 260+ | Severe scaling; major appliance damage; water softener essential |
| City | Hardness (GPG) | Hardness (mg/L) | Classification |
|---|---|---|---|
| Indianapolis, IN | 20+ | 340+ | Very Hard |
| Las Vegas, NV | 16-20 | 274-342 | Very Hard |
| Phoenix, AZ | 12-17 | 205-291 | Hard |
| San Antonio, TX | 15-20 | 257-342 | Very Hard |
| Minneapolis, MN | 12-18 | 205-308 | Hard |
| Chicago, IL | 7-11 | 120-188 | Moderately Hard |
| Denver, CO | 6-10 | 103-171 | Moderately Hard |
| Dallas, TX | 8-12 | 137-205 | Moderately Hard |
| Los Angeles, CA | 5-9 | 86-154 | Slightly Hard |
| New York, NY | 2-4 | 34-68 | Soft |
| Seattle, WA | 1-3 | 17-51 | Soft |
| Boston, MA | 2-5 | 34-86 | Soft |
| Portland, OR | 1-2 | 17-34 | Soft |
Soft Water (0-3 GPG): Soap lathers easily, no scale buildup, but water may be slightly corrosive to pipes over time due to low mineral content.
Slightly Hard (3-7 GPG): Minor spotting on dishes, minimal scale. Most people can live with this level without a softener.
Moderately Hard (7-10 GPG): Noticeable white spots on fixtures, soap doesn't lather as well, some scale in water heater. Consider a softener if you have skin sensitivity or appliance concerns.
Hard (10-15 GPG): Significant scale buildup in pipes and appliances, dry skin and hair, soap scum in shower, reduced water heater efficiency by up to 48%.
Very Hard (15+ GPG): Severe scaling, frequent appliance repairs, plumbing flow reduction, dramatically shortened water heater lifespan. A water softener is strongly recommended.
| Your Hardness | Recommendation |
|---|---|
| Below 7 GPG | Softener generally not needed unless skin sensitivity or specific appliance concerns |
| 7-10 GPG | Consider if you have scale issues, dry skin, or want to protect appliances |
| 10-15 GPG | Softener recommended for most households |
| Above 15 GPG | Water softener strongly recommended; consider whole-house system |
Temporary Hardness (Carbonate Hardness): Caused by calcium and magnesium bicarbonates. Can be removed by boiling (forms scale in kettles). This is why you see scale in your tea kettle but not necessarily in cold water pipes.
Permanent Hardness (Non-Carbonate Hardness): Caused by calcium and magnesium sulfates, chlorides, and nitrates. Cannot be removed by boiling. Requires ion exchange (water softener) or reverse osmosis to remove.
Most household water contains a mix of both types. Water softeners remove both temporary and permanent hardness.