Water hardness data
The wrong-number problem
The wrong-number problem is what we call a pattern in published water hardness data: the numbers published for the same city rarely agree, and a city's own utility sometimes disagrees with itself. A renter looking up their water can find three different figures in three tabs, none of them sourced.
This page lays out the pattern with examples we found ourselves. Each one is dated, and each one can be checked.
Where wrong numbers come from
Most hardness numbers online are copies. Somebody measured the original, but that was several websites ago. A ranking site copies from another ranking site. A blog rounds a range into a single figure. A conversion from grains per gallon gets done wrong, or not at all. Each copy moves the number a little further from anything a utility published.
Utilities contribute to it too, usually by accident. Water hardness genuinely changes with season and source, and utility websites get updated unevenly. A page can carry two figures from two different years at the same time.
Documented examples
These come from our verification passes in August 2026.
A utility disagreeing with itself
The Las Vegas Valley Water District's own site answered "how hard is our water" with 291 parts per million while a second block on the same page still said 280. Two different numbers on the same utility page. We recorded 291, the figure in the current main answer, and the Las Vegas page shows the source.
A conversion that doesn't add up
San Diego's utility page stated its hardness range as 272 to 284 parts per million, and also as 16 to 18 grains per gallon. Those grains convert to 274 to 308. The two ranges can't both be right. We recorded the figures and noted the discrepancy.
Data that can't be checked at all
The Chicago water quality documents we checked are image scans with no readable text. A machine can't read them and neither can a copy-paste. That gap is part of why so many sites guess at Chicago's number, and why our Chicago record carries no verification date.
What a right number looks like
A right number carries its source and the date someone last checked it. The figure alone is half an answer. Our methodology page documents how we hold hardness data to that standard, and every city page on this site names its utility and links to it. When a number here gets re-checked, the page shows the month.
If you want the number for your own area, our lookup tool shows it by zip code, with the source next to it.
What is the wrong-number problem?
It's the pattern where published water hardness figures for the same city disagree with each other, and sometimes with the city's own utility. The name is ours, and this page documents the pattern with dated examples.
How do I find the correct water hardness for my city?
Go to the utility itself. Its annual water quality report or its hardness page carries the measured figure. Our lookup tool collects those utility figures by zip code and links each one to its source.