Almost every complaint that starts with I cleaned it and it still looks bad ends at the tap. This is what comes out of it, state by state and city by city, and what each number means for the glass.
Hardness is dissolved calcium and magnesium, measured as milligrams per litre of calcium carbonate equivalent. It is invisible in the glass and unmistakable on it: when hard water dries on a pane, the water leaves and the minerals stay. That is the whole mechanism. Everything below follows from it.
The national spread is wider than most people expect. Idaho runs as low as 0 mg/L; Indiana reaches 633 mg/L, going by the figures their own utilities publish. That is a factor of 633, and it is why a technique that works perfectly in one state fails immediately in another without either person doing anything wrong.
Five bands, on the USGS classification. Find your reading in the table below, then read the band it lands in. The advice changes at every boundary, and it changes because the chemistry does.
Tap water is clean enough to be the last thing that touches the glass. Wash with it, rinse with it, squeegee it off, and nothing measurable is left behind when it dries. No distilled rinse, no deionized final pass, no spot-free rinse aid — the mineral load is too low to precipitate anything you would see. The failure mode at this end of the scale is not water chemistry, it is technique: streaks here come from a worn rubber, a dirty edge, or a pass that dried before it was pulled.
Tap water is still fine for most residential glass, with two exceptions worth knowing. Dark glass shows a faint mineral haze that lighter glass hides, and glass that is hot enough to flash the water off before you pull the squeegee will spot regardless of how soft the supply is. On both, a distilled final rinse costs about a dollar a window and removes the variable entirely. Everywhere else at this level, the water is not what is wrong.
This is the band where the water starts writing on the glass. Wash with tap — the surfactant holds the minerals in suspension while you work, so the wash pass is not the problem — then rinse with distilled and pull that. The rinse is the whole intervention: it replaces the mineral-bearing water sitting on the glass with water that has nothing in it to leave. This single change resolves most of the "I cleaned it and it still looks bad" complaints in this range, and it does not require buying a system.
A distilled rinse stops being an improvement and becomes the method. Tap water left to dry on glass at this concentration deposits a visible film within minutes, and the film is cumulative: each cleaning that ends in tap water adds a layer that the next cleaning has to get through first. Glass on a sprinkler line or under a runoff drip needs a maintenance interval, not just a better wash — the deposit is arriving faster than a cleaning schedule built around dust would predict.
The hardest water in North America — Phoenix, Las Vegas, West Texas, much of the limestone Midwest. At this concentration calcium carbonate precipitates out of any water that touches the glass and dries, so what you are looking at is essentially limestone, growing one molecular layer at a time. Tap water cannot be part of the final pass under any circumstance, including "just a quick rinse." Above roughly 300 mg/L, buying distilled by the gallon stops making economic sense against a DI filter or a pure-water pole system, and a residential owner cleaning their own glass twice a year is usually better served by a service that already owns one.
To put a number on your own address rather than your state, the Hard Water Scorer takes a ZIP code. If the damage is already done, the stain removal triage sorts what will come off from what has etched in — and the long version of the chemistry explains why that distinction is permanent.
The lowest and highest hardness the utilities we cover in each state publish in their own water quality reports. The range brackets those cities — it is not an average, and no single address sees the whole spread. A state marked PLANNING has no covered utility that publishes a figure, so its range is an estimate and sorts last. Open a state for its cities, utilities, and the reason its water is what it is. To compare cities directly, the city rankings order every city whose utility publishes a figure, hardest to softest, with dissolved solids and chloramine alongside.
50 STATES · 271 CITIES · 185 PUBLISH A FIGURE · READ IN 2026
Sources. Each covered city utility's own published figure, taken from its most recent annual water quality report (Consumer Confidence Report) or its water-quality page, read in 2026.
Method. A city figure is the one its utility publishes for the service area, as a single number or the range it reports; it is not a reading from any one tap. A state range runs from the lowest to the highest figure published by the utilities we cover in that state, so it brackets those cities rather than describing an average resident. A state where no covered utility publishes a figure shows a planning estimate, labelled as one.
The limit of this data. Private wells are outside all of it. A well is whatever the rock under it says, and the only number that describes one is a test of that tap.
Use it, quote it, link it. If you publish a figure from this table, we would rather you cite the page than copy the number silently — not for the traffic, but because a number without its provenance is how a wrong number outlives its correction. Our own working standards are on the editorial standards page, and the full source list is under sources.
This table is built from published municipal data, which means it is systematically blind to two things: private wells, and the gap between what a utility reports as a service-area average and what actually comes out of one tap. If you have a TDS meter, a softener installer's number, or a hardness figure off your own utility's report that disagrees with what we have — that is the interesting case, and we want it.
Readings that check out get folded into the next revision. Disagreements get looked at rather than filed — a reading that contradicts the table is worth more to us than one that confirms it.
SUBMIT A READING →OPENS YOUR MAIL CLIENT WITH THE FIELDS PREFILLED · water@windowwashingguide.com