Municipal water hardness vs well water
If you are on a private well, every number on this site is irrelevant to you, and that is worth saying first because almost nothing else on the internet will say it. A utility figure describes a treated, monitored, blended system. A well describes one hole in one piece of ground. This guide sets out why the two cannot be substituted, and what you can actually use instead.
The difference in one table
| Municipal supply | Private well | |
|---|---|---|
| Who tests it | The utility, on a schedule, under federal and state rules | The owner, when they choose to |
| Who is responsible for quality | The utility, up to the point of delivery | The owner, entirely |
| Is there an annual report | Yes, sent to customers | No. There is no report to read |
| Is hardness published | Sometimes. It is not a regulated contaminant, so it is voluntary | Only if the owner pays for a test |
| How much it varies | Across the system and the year. Denver Water publishes 55 to 124 mg/L; Indianapolis 148 to 460 mg/L | Between neighbouring properties, and with depth, season and drawdown |
| Is it treated | Yes, to a specification the utility publishes | Only by equipment the owner installs |
| Can a neighbour's figure stand in for yours | Roughly, if you are on the same system and understand the range | No. Two wells on one street can differ substantially |
Why a utility figure cannot be borrowed for a well
A municipal figure is the end point of a process. Source water is collected, often blended from several sources, treated, and then sampled at plants and at points in the distribution network. The published number is a summary statistic over that whole arrangement. Phoenix's 2025 report gives total hardness as a range of 172 to 302 ppm, 10 to 17.6 grains per gallon, with "NA" where an average would be, and the water behind it is drawn from the Salt, Verde and Colorado rivers through multiple treatment plants.
A well has none of that. Its hardness comes from the specific geology the water sits in and moves through. Limestone and dolomite dissolve readily and produce hard water; granite and basalt produce soft water. That geology can change over a few hundred metres and with depth, which is why a well-water figure is a property-level fact, not a town-level one.
There is a second asymmetry that matters practically. A municipal figure is stable enough to plan around because the utility is actively managing it. A well is not managed, so its hardness can move with the season, the water table, how hard the well is being pumped, and the age of the casing.
The one thing that is the same
The chemistry and the units. Hardness is calcium and magnesium expressed as calcium carbonate, whether it comes from a reservoir or from a borehole. The USGS classification applies to both:
| Class | USGS wording (mg/L as CaCO₃) | In grains per gallon |
|---|---|---|
| Soft | Soft: 0-60 milligrams per liter (mg/L) as calcium carbonate | 0 to 3.5 |
| Moderately hard | Moderately hard: 61-120 mg/L as calcium carbonate | 3.6 to 7 |
| Hard | Hard: 121-180 mg/L as calcium carbonate | 7.1 to 10.5 |
| Very hard | Very hard: more than 180 mg/L as calcium carbonate | above 10.5 |
So the converter and the grain capacity calculator work identically for well water. They need a hardness figure; they do not care where it came from. What differs is where you get that figure.
Getting a number for a well
- A laboratory test is the only figure worth sizing from. A certified lab gives total hardness in mg/L, usually alongside iron, manganese, pH and total dissolved solids, which all affect whether a softener is the right equipment.
- Your state or county health department is the usual starting point for a list of certified labs and for which parameters they recommend testing in your area.
- Iron and manganese change the equipment decision, not just the sizing. A softener sized correctly for hardness can still be the wrong unit if iron is present, which is a case for reading the specific unit's documentation rather than a general calculator.
- Test strips give you an order of magnitude. They are enough to tell soft from very hard. They are not a basis for buying equipment, and the accuracy of a given strip against a lab result is a claim only its own manufacturer's data can support — nothing on this site has tested any strip.
- Retest after any major change to the well, the pump, or the depth, and periodically regardless.
The mixed case: a utility customer who also has a well
Plenty of properties are on municipal water indoors and a well for irrigation, or have been switched from well to municipal supply without the well being decommissioned. Two points follow. The utility report covers only the municipal line. And if the two ever cross-connect, that is a plumbing and regulatory problem well outside the scope of a hardness figure.
A related case appears on the municipal side: a system can have two supplies of very different hardness. Portland's Bull Run supply is published at 7 to 11 ppm, about half a grain per gallon, while the city's groundwater supply is published at about 80 ppm, about 5 grains. Which one reaches an address changes. That is not the same as being on a well, but it has the same consequence — one published number does not describe your water all year.
What this site does and does not cover
This site publishes utility figures only, each read from the utility's own report, because that is the only hardness data that can be verified from a public document. There is no well data here and there will not be, since a well figure cannot be verified from anything but a test on that property. The 10 systems covered are listed here, the sources and assumptions are on the methodology page, and how to read a water quality report covers the municipal case in detail.