Water hardness varies more than most people realize. The tap water in Seattle is nearly as soft as rainwater. The tap water in Phoenix is harder than most swimming pools. Understanding where your water falls on that scale tells you exactly what treatment, if any, you need.
How Hardness Is Measured
Two units are common. Grains per gallon (gpg) is what most water softener manufacturers use. Milligrams per liter (mg/L) is what the EPA and USGS use. The conversion is straightforward: 1 gpg equals 17.1 mg/L.
The standard hardness scale:
- Soft: 0-3.5 gpg (0-60 mg/L)
- Moderately hard: 3.5-7 gpg (61-120 mg/L)
- Hard: 7-10.5 gpg (121-180 mg/L)
- Very hard: above 10.5 gpg (above 180 mg/L)
The Softest Regions
The Pacific Northwest produces some of the softest municipal water in the country. Seattle typically measures 1-3 gpg. The combination of granitic geology, high rainfall, and reliance on surface water (snowmelt from the Cascades) means very little mineral pickup.
New England is similarly soft. Boston measures around 1-3 gpg. The region draws heavily from reservoirs and has low-mineral bedrock in many areas.
Parts of the Southeast fall in the moderately soft range. Atlanta typically measures 4-5 gpg. Much of the Southeast draws from rivers and reservoirs rather than groundwater, which tends to be softer.
The Hardest Regions
The Desert Southwest is where hardness gets extreme.
Phoenix averages 15-25 gpg, and the range is real, not just statistical noise. The city blends Colorado River water (very hard, from dissolving through Colorado Plateau limestone) with Salt River water (moderately hard). The blend ratio shifts seasonally, which is why hardness varies within the year.
Las Vegas draws from Lake Mead, which is Colorado River water. Hardness runs 14-16 gpg consistently.
Tucson measures 15-20 gpg. San Antonio runs 14-17 gpg. These are year-round figures, not seasonal peaks.
The Mountain West is hard but not quite Desert Southwest levels. Denver typically measures 10-14 gpg. Salt Lake City runs 13-16 gpg, drawn partly from hard groundwater sources.
The Midwest varies by city and source. Chicago suburban water varies widely, from 8-10 gpg depending on which suburb and which source (Lake Michigan water vs. local groundwater). Indianapolis runs 10-12 gpg. Indianapolis gets its water from surface sources mixed with some groundwater, both of which run through Indiana’s limestone-heavy geology.
Texas is hard almost everywhere. Dallas runs 10-12 gpg. San Antonio, as noted, is in the 14-17 gpg range.
Why Location Varies So Much
Hardness comes from geology. As water moves through rock and soil, it dissolves calcium and magnesium from limestone, dolomite, and chalk formations. The longer water stays in contact with soluble rock, and the more limestone present, the harder the water becomes.
The Southwest has extensive limestone and dolomite geology, low rainfall that concentrates minerals in surface water, and high evaporation rates. All three factors push hardness up.
The Pacific Northwest has granitic bedrock (granite doesn’t dissolve easily), very high rainfall, and relies heavily on snowmelt and surface reservoirs. All three factors keep hardness down.
City Water vs. Well Water
For city water, your utility is required to publish water quality data annually. That Consumer Confidence Report (CCR) lists hardness. If you can’t find it on the utility’s website, call and ask. They’ll have the number.
Well water is different. Your hardness depends entirely on your local geology and your specific well depth. Two wells a mile apart can have meaningfully different hardness levels. There’s no shortcut for well water, you need to test your own.
How to Test Your Water
For city water, check your CCR first. It’s the fastest and most accurate method, and it’s free.
Home test strips work well for hardness specifically. Unlike many home water tests that provide unreliable results for trace contaminants, hardness test strips are reasonably accurate. A multi-test strip pack runs $5-10 at hardware stores. Dip, wait 30 seconds, compare the color to the chart.
A certified lab test gives the most accurate result and is worth doing if you’re sizing a water softener precisely or have well water with unknown chemistry.
Calling a water softener company is the lazy option that actually works. Most companies do free hardness tests as part of the sales process. The result is accurate. The tradeoff is the sales call.
What Your Number Means for Treatment
Under 3.5 gpg: you don’t need a softener. You might not notice hard water effects at all.
At 3.5-7 gpg: minor scale, reduced soap efficiency. A softener is optional.
At 7-10.5 gpg: visible scale on fixtures, dishwasher spots, early signs of appliance impact. A softener is worth evaluating.
Above 10.5 gpg: significant appliance effects, water heater efficiency loss, laundry and skin effects. A softener pays for itself through appliance longevity and energy savings. See how hard water affects appliances for the cost breakdown.
Once you have your number, comparing softener options becomes a much simpler decision. And if you want to understand exactly what a softener does to the water, how ion exchange works explains the process clearly.