Water hardness by zip code: how hard is the well water where you live?

Hardness is dissolved calcium and magnesium, reported as the equivalent amount of calcium carbonate. Labs give it in mg/L (the same as ppm); softeners are sized in grains per gallon. One grain per gallon is 17.1 mg/L. The bands below are the USGS scale that our hardness converter also uses, and they describe how water behaves, not whether it is safe. The EPA sets no health limit for hardness.

County counts are for the 2,568 counties with at least 5 samples. Hard water is the norm for wells, not the exception: 67% of those counties have a median above 7 gpg, and the typical US well in this data measures 11.1 gpg (190 mg/L).396 counties are above 20 gpg, which the map shows in its darkest shade. Below about 3.5 gpg a softener is hard to justify; above 7 gpg it starts to pay back through less scale and less soap.

The ten counties with the highest and lowest median, among counties with at least 20 samples from 10 or more different wells. Each well counts once, so one heavily sampled well cannot set a county’s figure.

Hardest 10 counties

Most of the hardest counties are in the southern Great Plains and the arid Southwest, where aquifers in gypsum-bearing rock add calcium sulfate to the usual carbonate. These are extreme figures from relatively few wells, and some rest on a single study: check the wells and years columns before quoting one.

Softest 10 counties

The softest well water is in the Southeast — Mississippi, Georgia, the Carolinas, Virginia and parts of Florida — where many wells draw from sandy coastal plain aquifers with little limestone for the water to dissolve.

Every state

Median of all wells sampled in the state, the share of its counties (with 5+ samples) whose median is hard, and the hardest county with at least 10 samples. Sort by any column. South Dakota has the hardest well water by state median (29.2 gpg), followed by Iowa (21.6 gpg); Mississippi has the softest (1.1 gpg). District of Columbia has too few samples for a state figure.

Across the country, deeper wells tend to be softer: wells under 100 feet have a median of 14.0 gpg, wells deeper than 600 feet 6.7 gpg. The pattern is not just a regional mix — in 408 of the 603 counties with at least 10 shallow (under 100 ft) and 10 deep (300 ft+) wells, the shallow wells are harder, and the typical deep-to-shallow ratio is 0.72.

That is a tendency, not a rule: in 190 of those counties the deep wells are harder. Whether a deeper well would be softer for you depends on which aquifer it reaches, and a well driller who knows the local formations is the person to ask.

Everything on this page is groundwater from wells. If you are on a public supply, your water may come from a river or reservoir, may be blended from several sources, and may be softened at the treatment plant — so the county figure here can be far off.

Utilities publish hardness in their annual Consumer Confidence Report. Search for yours on EPA’s Consumer Confidence Report page, or call the number on your water bill. Private wells have no such report, which is the gap this dataset is meant to fill.

A county median tells you what to expect, not what you have. Neighbouring wells can differ by several times, because depth, aquifer and local rock all change over short distances. Use the lookup to decide whether hardness is likely to be a problem, then measure your own well.

• Home test strips or a titration kit are accurate enough for hardness, cost little, and let you retest after any treatment is installed.
• A lab test is worth it if you are buying equipment anyway: iron and manganese change softener sizing, and a well should be tested for bacteria and nitrate regardless. See what a well water test costs.
• Got a result? The water test interpreter reads the whole report, and the softener sizing calculator turns hardness and iron into a grain capacity. Budget with what a softener costs.

Source. Groundwater samples collected by the U.S. Geological Survey and published through the Water Quality Portal (USGS, EPA and the National Water Quality Monitoring Council): characteristic “Hardness, Ca, Mg” (USGS parameter 00900) at sites typed as wells, for all 50 states and DC, downloaded September 25, 2026. The county comes from each well’s station record. Zip codes are matched to counties with the Census Bureau’s 2020 ZCTA-to-county relationship file: each zip goes to the county covering most of its land, and any other county covering at least 20% is shown too (4,236 of 33,791 zips).

Filters. Of 453,907 records we kept 434,761. We dropped 936 with no usable number (non-detects, blanks), 9,928 above 2,000 mg/L (brines and mine water rather than drinking-water wells), 2,525 exact duplicates, and 5,757 from wells whose names identify them as landfill or contamination monitoring wells or piezometers. Every kept record was reported in mg/L as CaCO₃, so no unit conversion was needed. Hardness can also be calculated from separate calcium and magnesium results; we did not do that, so a county where only those were measured shows less data here than exists.

Why medians, and why per well. Each well’s samples are reduced to that well’s median first; the county median, quartiles and “hard” share are then taken across wells. Medians are not dragged about by the occasional extreme sample, and one-well-one-vote stops a single monitoring well sampled dozens of times from outvoting the rest of a county. The “harder than X% of counties” rank compares against the 2,568 counties with at least 5 samples.

Age. Samples run from 1889 to 2023; half were taken before 1977, and only 26% since 1990. Aquifer chemistry changes slowly, so old samples are still informative, but the “Since 1990” map view is there if you want to check. Confidence is labelled High with 30+ samples including some since 1990, Low under 5 samples, and Moderate between.

Coverage. 2,959 of 3,142 counties have at least one sample, 2,568 have 5+, and 1,720 have 30+. USGS sampled where it was studying something, not on a grid, so some counties rest on a single project.

A county median is not your well. It describes the wells USGS happened to sample, often decades ago and sometimes from a different aquifer than yours. Hardness is an aesthetic and scale issue rather than a health hazard, but test your own well before buying equipment. More on how the site’s tools work is on the methodology page.

The full county table — 2,959 counties with samples, wells, median, quartiles, years and percentile — is free to download and reuse under CC BY 4.0: credit Well Water Fixes with a link to this page. The underlying USGS data is in the public domain.

Download the CSV (262 KB)

Writing about a particular county? Look it up above and open “Embed this county’s figure” under the result for a small copy-and-paste badge that links back to the data.