Water Hardness Conversion: PPM to GPG Calculator
Convert water hardness between PPM, GPG, mg/L, and mmol/L with explanation of each unit.
The Core Conversion
The two most common units for measuring water hardness are parts per million (PPM) and grains per gallon (GPG). The conversion factor is straightforward:
1 GPG = 17.1 PPM
To convert PPM to GPG: divide by 17.1. To convert GPG to PPM: multiply by 17.1.
Quick examples
- 120 PPM ÷ 17.1 = 7.0 GPG
- 220 PPM ÷ 17.1 = 12.9 GPG
- 350 PPM ÷ 17.1 = 20.5 GPG
- 5 GPG × 17.1 = 85.5 PPM
- 10 GPG × 17.1 = 171 PPM
- 15 GPG × 17.1 = 256.5 PPM
If you do not know your water\'s hardness level, check your city\'s water and you will see both PPM and GPG values.
All Four Units Explained
PPM (Parts Per Million)
PPM measures the concentration of calcium carbonate equivalent in water. One PPM means one milligram of calcium carbonate per liter of water. This is the most widely used unit in the US. Municipal water reports, EPA standards, and most consumer-facing water quality data use PPM.
mg/L (Milligrams Per Liter)
For water hardness purposes, mg/L and PPM are identical. The terms are interchangeable because one liter of water weighs approximately one kilogram, making "parts per million" and "milligrams per liter" equivalent. If your water report lists hardness in mg/L, treat the number exactly as you would PPM.
GPG (Grains Per Gallon)
GPG is the unit used by the water softener industry. One grain equals 1/7000th of a pound, and the measurement refers to grains of calcium carbonate per US gallon. Every water softener is rated in grains of capacity (for example, a 32,000-grain softener). Knowing your hardness in GPG is essential for sizing a softener correctly.
mmol/L (Millimoles Per Liter)
This is the international scientific unit. It measures the molar concentration of calcium and magnesium ions. The conversion is: 1 mmol/L = 100.1 PPM (commonly rounded to 100 PPM). This unit appears primarily in scientific literature and international water quality reports. Most US homeowners will never encounter it.
Complete Conversion Table
| PPM (mg/L) | GPG | mmol/L | Classification |
|---|---|---|---|
| 0 | 0 | 0 | No hardness |
| 25 | 1.5 | 0.25 | Soft |
| 50 | 2.9 | 0.50 | Soft |
| 60 | 3.5 | 0.60 | Soft (upper limit) |
| 75 | 4.4 | 0.75 | Moderately hard |
| 100 | 5.8 | 1.00 | Moderately hard |
| 120 | 7.0 | 1.20 | Moderately hard (upper limit) |
| 150 | 8.8 | 1.50 | Hard |
| 180 | 10.5 | 1.80 | Hard (upper limit) |
| 200 | 11.7 | 2.00 | Very hard |
| 250 | 14.6 | 2.50 | Very hard |
| 300 | 17.5 | 3.00 | Very hard |
| 400 | 23.4 | 4.00 | Extremely hard |
| 500 | 29.2 | 5.00 | Extremely hard |
USGS Hardness Classification Scale
The US Geological Survey defines four hardness categories. These are the standard classifications used across the water industry:
| Classification | PPM Range | GPG Range | What It Means |
|---|---|---|---|
| Soft | 0-60 | 0-3.5 | No treatment needed. Soap lathers easily. Minimal scale |
| Moderately Hard | 61-120 | 3.6-7.0 | Slight scale on fixtures. Most people do not notice issues |
| Hard | 121-180 | 7.1-10.5 | Noticeable scale. Soap does not lather well. Spots on dishes and glass |
| Very Hard | 181+ | 10.6+ | Heavy scale. Appliance damage. Stiff laundry. Water treatment recommended |
To see where common US cities fall on this scale, visit our hard water map of the United States.
Why You Need to Know Both Units
Reading your water report
Your city\'s annual Consumer Confidence Report (also called a water quality report) lists hardness in PPM or mg/L. This is the number you compare against EPA guidelines and health standards. When you look up your city on our site, you will see PPM values alongside GPG.
Sizing a water softener
Softener capacity is rated in grains. To calculate the softener size you need, use this formula:
Daily softening demand = Hardness (GPG) x Daily water use (gallons) x Number of people
For example: 12 GPG hardness, 75 gallons per person per day, 4 people = 12 x 75 x 4 = 3,600 grains per day. A 32,000-grain softener regenerating every 7-9 days would handle this load. Visit our softener guide for more on sizing.
Comparing test results
Home test kits, lab results, and water reports may use different units. Knowing the conversion ensures you are comparing equivalent values. A home test strip that reads "15 GPG" is the same as a lab result that reads "257 PPM."
Common Sources of Confusion
Hardness vs. TDS
Total Dissolved Solids (TDS) is not the same as hardness. TDS includes all dissolved minerals (sodium, potassium, chloride, sulfate, and others) while hardness measures only calcium and magnesium. A city with 300 PPM TDS might have only 150 PPM hardness. These numbers are not interchangeable.
Calcium hardness vs. total hardness
Some water reports list "calcium hardness" and "total hardness" separately. Calcium hardness measures only calcium. Total hardness includes both calcium and magnesium. The USGS classification scale and softener sizing both use total hardness. When in doubt, use total hardness.
Temporary vs. permanent hardness
Temporary hardness (carbonate hardness) can be removed by boiling. Permanent hardness (non-carbonate hardness) cannot. The PPM and GPG values on your water report represent total hardness, which is the sum of both types. Water softeners remove both.
Quick Reference
Bookmark this page for fast conversions. The key number to remember is 17.1. Divide PPM by 17.1 to get GPG; multiply GPG by 17.1 to get PPM. For your specific city\'s hardness in both units, check your city\'s water.
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