Water Hardness Scale: What the Numbers on Your Water Report Actually Mean
You got a number from your water report or a test strip — now you need to know what it means. Learn how to interpret water hardness in PPM, grains per gallon, and what the numbers mean for your home.
You got a number from your water report or a test strip — now you need to know what it means. The water hardness scale tells you whether your water is soft, moderately hard, hard, or very hard based on how much dissolved calcium and magnesium it contains.
Here's the quick answer, followed by everything you need to interpret your results.
The Water Hardness Scale
The U.S. Geological Survey (USGS) classifies water hardness into four levels. This is the standard scale used across the United States:
| Classification | PPM (mg/L) | Grains per Gallon (GPG) | What You'll Notice |
|---|---|---|---|
| Soft | 0–60 | 0–3.5 | Soap lathers easily, no residue on fixtures, spot-free dishes |
| Moderately Hard | 61–120 | 3.6–7.0 | Slight film on glass shower doors, minor spotting on dishes |
| Hard | 121–180 | 7.1–10.5 | Visible white buildup on faucets, soap scum in shower, stiff laundry |
| Very Hard | 180+ | 10.5+ | Heavy limescale on everything, reduced water heater efficiency, dry skin and hair |
If your water report shows a number in mg/L or PPM — those are the same thing. One milligram per liter equals one part per million.
Understanding the Units
Water hardness gets measured in different units depending on who's doing the measuring. In the US, you'll mostly encounter two: PPM and grains per gallon. Here's how they relate.
Parts per million (PPM) is the most common unit on city water quality reports. It measures milligrams of calcium carbonate (CaCO₃) per liter of water. Your annual Consumer Confidence Report from your water utility will almost always use this unit.
Grains per gallon (GPG) is what water softener companies and plumbers use. If you're shopping for a water softener or getting a quote from a plumber, they'll ask for your hardness in grains. One grain per gallon equals 17.1 PPM.
To convert between them:
- PPM to GPG: Divide by 17.1
- GPG to PPM: Multiply by 17.1
For example: Your water report says 150 PPM. Divide by 17.1 = 8.8 GPG. That puts you in the "Hard" category.
Quick Conversion Reference
| PPM (mg/L) | GPG | Classification |
|---|---|---|
| 25 | 1.5 | Soft |
| 50 | 2.9 | Soft |
| 75 | 4.4 | Moderately Hard |
| 100 | 5.8 | Moderately Hard |
| 125 | 7.3 | Hard |
| 150 | 8.8 | Hard |
| 175 | 10.2 | Hard |
| 200 | 11.7 | Very Hard |
| 250 | 14.6 | Very Hard |
| 300 | 17.5 | Very Hard |
| 400 | 23.4 | Very Hard |
| 500 | 29.2 | Very Hard |
If your water report uses a different unit — like German degrees (°dH), French degrees (°fH), or millimoles per liter (mmol/L) — here's a conversion table:
| Unit | Equivalent to 1 PPM |
|---|---|
| 1 mg/L CaCO₃ | = 1 PPM |
| 1 GPG | = 17.1 PPM |
| 1 °dH (German degree) | = 17.8 PPM |
| 1 °fH (French degree) | = 10 PPM |
| 1 °Clark (English degree) | = 14.3 PPM |
| 1 mmol/L | = 100.1 PPM |
How Hard Is Your Tap Water?
Water hardness varies dramatically across the United States. The Southwest and Great Plains tend to have the hardest water, while the Pacific Northwest and Northeast have some of the softest.
Here's what real water hardness looks like in major US cities:
| City | Hardness (PPM) | GPG | Classification |
|---|---|---|---|
| Seattle, WA | 22 | 1.3 | Soft |
| Portland, OR | 27 | 1.6 | Soft |
| New York, NY | 35 | 2.0 | Soft |
| San Francisco, CA | 47 | 2.7 | Soft |
| Miami, FL | 98 | 5.7 | Moderately Hard |
| Denver, CO | 90 | 5.3 | Moderately Hard |
| Chicago, IL | 130 | 7.6 | Hard |
| Houston, TX | 155 | 9.1 | Hard |
| Dallas, TX | 165 | 9.6 | Hard |
| Phoenix, AZ | 285 | 16.7 | Very Hard |
| Las Vegas, NV | 280 | 16.4 | Very Hard |
| San Antonio, TX | 357 | 20.9 | Very Hard |
| Indianapolis, IN | 382 | 22.3 | Very Hard |
→ Check your city's exact water hardness
The reason for these differences comes down to geology. Water picks up calcium and magnesium as it flows through limestone, chalk, and dolomite rock. Cities that draw water from mountain snowmelt or surface reservoirs (like Seattle and New York) tend to have soft water. Cities sitting on limestone bedrock or drawing from deep wells (like Phoenix and San Antonio) end up with very hard water.
How to Find Your Water Hardness Number
There are three ways to find out how hard your water is:
Check your city's annual water report. Every US public water system is required to publish a Consumer Confidence Report (CCR) by July 1 each year. Search "[your city] water quality report" or look up your city on our site. Hardness isn't an EPA-regulated contaminant, but most utilities report it voluntarily.
Use a test strip. A 17-in-1 test strip kit costs about $25 on Amazon and gives you a rough reading within minutes. Good for a quick check, but not precise enough for sizing a water softener. → Our recommended test kits
Send a sample to a lab. A certified lab test like Tap Score ($59–$199) gives you an exact hardness number along with dozens of other parameters. Worth it if you're making purchasing decisions about filtration or treatment.
If you're on a private well, you won't have a CCR — testing is your only option, and the EPA recommends testing at least annually.
What Your Number Means for Your Home
Not all hardness levels need action. Here's what each range actually looks and feels like in daily life.
Under 60 PPM — Soft Water
This is the easy zone. Soap lathers quickly, dishes come out of the dishwasher spot-free, and your shower glass stays clear. Your skin feels smooth after a shower — no tight, dry sensation. Coffee and tea taste clean without any mineral aftertaste. Appliances like water heaters and washing machines run at full efficiency with no scale buildup to worry about.
One caveat: very soft water (under 25 PPM) can be slightly corrosive to older copper pipes, potentially leaching small amounts of metal. This is rare in modern plumbing but worth noting if you live in a pre-1980s home with original pipes. Cities like Seattle (22 PPM) and Portland (27 PPM) fall in this range.
60–120 PPM — Moderately Hard
Most people don't notice moderately hard water in daily life, and most don't need a softener. You might spot a light white film on glass shower doors after a few weeks, or see faint spots on wine glasses fresh from the dishwasher. A rinse aid usually takes care of it. Soap still lathers well, skin and hair feel normal, and your appliances aren't under any real stress.
Your water heater may develop a thin mineral layer over years, but nothing that significantly impacts performance. If you're in Denver (90 PPM) or Miami (98 PPM), this is your world — functional and low-maintenance.
120–180 PPM — Hard Water
This is where hardness starts making itself known. White crusty deposits appear around faucet aerators and on showerheads within weeks. You'll use noticeably more soap and shampoo to get a good lather. Glasses come out of the dishwasher with a cloudy haze, and your shower doors develop stubborn water spots that need vinegar or CLR to remove.
Behind the scenes, mineral scale is building up inside your water heater, reducing its efficiency and lifespan. Tankless water heater manufacturers like Rinnai and Navien often recommend treatment above 120 PPM — and some void their warranty if you don't. Washing machine heating elements and dishwasher spray arms start accumulating deposits too. Cities like Chicago (130 PPM) and Houston (155 PPM) sit in this range.
A water softener becomes a smart investment here, especially if you have expensive appliances to protect.
Over 180 PPM — Very Hard Water
At this level, hard water is a daily presence. Thick white limescale forms on every surface that touches water — faucets, showerheads, toilet bowls, kettle interiors. You'll notice it within days, not weeks. Soap barely lathers, and your skin and hair feel dry and filmy after every shower. Laundry comes out stiff and colors fade faster because minerals trap detergent in the fibers.
The real cost is hidden in your appliances. The Department of Energy estimates hard water above 180 PPM can reduce water heater efficiency by up to 48%. Scale narrows pipe diameter over time, lowering water pressure throughout your home. Dishwashers and washing machines need more frequent repairs. If you're in Phoenix (285 PPM), Las Vegas (280 PPM), or San Antonio (357 PPM), a water softener isn't optional — it's protecting thousands of dollars in plumbing and appliances.
Hard Water Isn't a Health Risk
One important thing the scale doesn't tell you: hardness has nothing to do with safety. The EPA does not regulate water hardness because calcium and magnesium are not harmful to drink. In fact, the World Health Organization has noted that hard water may contribute beneficial minerals to your diet — an 8-ounce glass of moderately hard water contains about 50–75 mg of calcium.
The problems with hard water are entirely practical: scale buildup, reduced appliance efficiency, dry skin, and higher soap usage. If your water report shows high hardness but clean results for actual contaminants like lead, bacteria, and nitrates, your water is safe to drink regardless of how hard it is.
Sources: U.S. Geological Survey (USGS) water hardness classification, EPA Consumer Confidence Reports, Water Quality Association standards. City hardness data sourced from municipal water quality reports and EPA databases. Last updated: February 2026.