HardWaterLab
Softeners

Open data: water hardness by ZIP and county

Everything on this site is built from public monitoring data, so the derived files are published too. No sign-up, no API key, no rate limit. Use them commercially if you want.

Files

33,534 US ZIP codes joined to the hardness median for the county each sits in, plus 3,096 county records. A packed fixed-width string with a county lookup table.
Published data files
FileContentsSize
/data/zip-hardness.json~401 KB

Source and licence

The underlying measurements come from Water Quality Portal (USGS, EPA and state agencies), at https://www.waterqualitydata.us/, retrieved 2026-09-19. The Water Quality Portal is a joint service of the USGS, the EPA and the National Water Quality Monitoring Council. As a work of the US federal government the source data is public domain.

The derived files here are free to reuse, including commercially. Attribution is not legally required and is genuinely appreciated: a credit line pointing at this site is how other people find the source.

How it is built, and what it is not

County figures are medians of usable samples, never means, because hardness distributions are skewed. A county with fewer than three usable samples is omitted rather than published thinly. The ZIP join uses the US Census ZCTA to county relationship file, and where a ZIP spans a county line the county holding the most land area wins.

This is ambient water, not tap water. The samples come from wells, streams, springs and lakes. A utility treats, blends and sometimes softens before the water reaches anyone, so a municipal supply can be considerably softer than the ground beneath it. Read the full method and its limits before you rely on a figure.

Update cadence

The aggregate is rebuilt when the Water Quality Portal export is refreshed, roughly annually. The retrieved field in the file states the date the source was pulled, and every page on this site carries a machine-readable modified date. Do not assume a figure is current without checking that field.

Embed the ZIP lookup

If you would rather show the tool than rebuild it, the ZIP lookup embeds in one line. Paste both lines: the frame, and the credit below it.

Citing this

HardWaterLab, water hardness by ZIP code and county, derived from the Water Quality Portal (USGS, EPA and the National Water Quality Monitoring Council), retrieved 2026-09-19. https://hardwaterlab.com/data/

Frequently asked questions

Can I use this data commercially?
Yes. The source is a work of the US federal government and is public domain, and the derived files published here are free to reuse commercially. A credit line is appreciated but not required.
Is there an API?
No, and deliberately. The whole dataset is a single static JSON file small enough to fetch once and query in the browser, which is faster than an API round trip and cannot rate limit you. Fetch it and hold it.
How often is the data updated?
The aggregate is rebuilt when the Water Quality Portal export is refreshed, roughly annually. The retrieved field inside the file is the authoritative date, so check it rather than assuming.
Does this tell me the hardness of my tap water?
No. These are ambient samples from wells, streams, springs and lakes, not treated tap water. A utility that treats, blends or centrally softens can deliver water considerably softer than the ground it sits on. For a tap figure, use your utility annual report or a test strip.

Related tools

Hardness by ZIP code

The tool this data drives.

Method and limits

How a county figure is produced, and what it cannot tell you.

Hardness by state

Every state, county by county.