What the data is, where it comes from, why ZIP codes are an imperfect proxy, what each severity tier actually means for the glass on your house, and the cleaning protocols the tool recommends — with the chemistry behind each one.
What the Hard Water Severity Scorer does, in five points:
The Hard Water Severity Scorer is the single most useful preventive-maintenance tool we've built. Knowing your water profile changes everything about how you clean, when you clean, and what protocols you can safely skip.
The first time I gave a customer a hardness reading for their tap water, I was cleaning a Lincoln Park brownstone in 2015 and the homeowner asked me, somewhat plaintively, why their windows kept getting white spots no matter how often they were cleaned. I told her the spots were calcium carbonate from the city water and that Chicago's tap water in her neighborhood ran around 160 milligrams per liter on a good day. She looked at me as if I had spoken French. Is that bad? she asked. The honest answer, which I gave her, was: it's high enough that we need to talk about distilled water for the final rinse, and it's high enough that the spotting is going to come back faster than we'd both like.
The Hard Water Severity Scorer is the tool we built so that conversation doesn't require a working cleaner standing in the kitchen. Enter a ZIP code, get the typical hardness for the municipal supply in that area, see where it falls on the USGS classification scale, and read the cleaning protocol changes the tier recommends. This piece is the methodology behind the tool — what the data is, where it comes from, what its limits are, and what each severity tier actually means for the glass on your house.
If you're already comfortable with the tool's output and just want to act on it, the article on hard water spots ranked is the operational reference. This piece is for the user who wants to understand what the number actually represents and how confident they should be in it.
Water hardness is the concentration of dissolved calcium and magnesium in water, plus smaller contributions from iron, manganese, and other divalent cations. The chemistry community measures it in milligrams per liter of calcium carbonate equivalent (mg/L as CaCO₃). The water-treatment trade in the United States uses grains per gallon (gpg), which is the same measurement on a different scale: 1 gpg equals roughly 17.1 mg/L. The Hard Water Severity Scorer reports both, because the chemistry literature uses one and most homeowners (and water-treatment vendors) use the other.
The classification scale the tool uses is the one published by the U.S. Geological Survey:
These tiers are not arbitrary. They correspond to meaningful breakpoints in how water behaves on glass, on plumbing, in dishwashers, and on skin. Soft water at 60 mg/L will leave essentially no visible residue when it dries on dark glass. Hard water at 150 mg/L will leave faintly visible water spots that take a week or two to become an obvious problem. Very hard water at 220 mg/L will leave clearly visible chalky droplets within a single drying cycle. Extremely hard water above 300 mg/L — which is what most of metro Phoenix, Las Vegas, and Salt Lake City actually drinks — leaves immediately visible white residue on every wet surface, every time, with no exceptions.
The tier you fall into determines almost everything about your cleaning protocol.
The Hard Water Severity Scorer's underlying data is derived from two public sources, with a third layer of editorial triangulation on top.
The primary source is the USGS Water Hardness Map of the conterminous United States, last updated in 2023, which aggregates measurements from thousands of public water utility consumer-confidence reports (CCRs) and combines them with USGS geological survey data on aquifer composition. The map is published as a continuous-surface raster; we sampled it at the centroid of every ZIP3 region in the country to produce the lookup table the tool queries.
The secondary source is direct CCR data from the largest municipal water utilities — every utility serving more than 50,000 connections is required to publish an annual report containing average hardness measurements, and we cross-checked the USGS-derived values against the CCR data wherever both were available. In most cases the two agree within 10-15 mg/L. Where they disagreed substantially, we used the CCR value, since utility data is closer to what comes out of a homeowner's tap than a geological model.
The third layer is editorial review. A handful of regions have unusual local conditions that neither dataset captures cleanly — places where the utility blends multiple source waters seasonally, places where major capital improvements changed the hardness profile in the last few years, places where the served population spans dramatically different aquifer types. For those regions, I worked through the recent CCRs by hand and adjusted the tool's value to match what residents are actually receiving in 2026.
The result is a ZIP3-level lookup that is correct to within roughly ±20 mg/L for most of the country. For ZIP3 regions we don't have specific data on, the tool falls back to a state-level average, which is less precise but still useful.
A ZIP code is a postal-routing convenience, not a hydrological boundary. The tool gives you the typical hardness for the dominant water utility serving your ZIP3 area. Several conditions can make your actual tap water meaningfully different:
You're on well water. Private wells are not part of municipal supply data and can vary wildly even within a single ZIP3. A homeowner three doors down from someone on city water can be drawing from a private well that's twice as hard. If you're on well water, the tool's number is irrelevant to you — you need an actual test of your water (an inexpensive home test kit will do) rather than a regional average.
You have a water softener. Whole-house ion-exchange softeners replace calcium and magnesium with sodium, dropping the effective hardness to near zero at the tap. If you have a softener and it's working, your glass-cleaning hardness is essentially zero regardless of what the tool reports for your area. (Whether to use softened water for window cleaning is a separate question — softened water is low in hardness but high in sodium, and on dark glass the sodium can leave its own faint residue. Most working cleaners use distilled water for final rinses regardless of whether the house has a softener.)
You're served by a small utility that draws from a different source than the dominant regional utility. This is most common in suburban and exurban areas where a county utility and a city utility serve adjacent neighborhoods. Check your utility's annual CCR — it's typically mailed to customers each summer and is also posted on the utility's website. The CCR will give you the actual hardness for your specific service area.
Your utility blends sources seasonally. Some utilities switch between river-source water (typically softer) and aquifer water (typically harder) depending on supply conditions. Phoenix, for example, uses Salt River Project water in winter and Central Arizona Project water in summer, and the two have meaningfully different hardness profiles. If you're in a utility that does this, your hardness fluctuates by season, and the tool gives you an annual average rather than a current reading.
For most users in most months in most places, the tool's number is accurate enough to inform real cleaning decisions. For the edge cases above, treat the tool's output as a starting estimate and verify with your utility's CCR or a home test kit if precision matters.
The recommendation each severity tier produces is calibrated to what the chemistry actually does on glass at that hardness level, in typical residential conditions.
Soft water leaves negligible residue on glass when it dries. You can wash windows with tap water and a basic House Standard solution and the result will be visually clean even on dark glass. Distilled water is unnecessary. The protocol changes for soft-water households are minimal: focus on technique (squeegee, perimeter detail) rather than on water source.
The cities and regions in this tier include most of the Pacific Northwest (Seattle, Portland), much of New England, parts of the Southeast (Atlanta, the Carolinas), and most of Florida. If you live in one of these regions and your windows are spotting, the cause is almost certainly something other than water hardness — surfactant residue, plasticizer migration, or upstream water from a roof or HVAC unit dripping mineral-bearing condensate. The Streak Diagnostic tool is the right next stop.
Moderate hardness is the threshold where you start to see faint spotting on dark glass — black framed windows, smoked glass, the inside surface of a dark room. On light-colored glass and standard residential applications, moderate water still cleans acceptably with tap water, though a distilled-water final rinse on visible-from-outside dark glass is a meaningful upgrade. The protocol: use tap water for the wash, distilled for the final rinse on any pane where spotting would be noticeable.
This tier covers much of the Midwest's softer subregions, the Mid-Atlantic, and a fair amount of the West Coast outside the Pacific Northwest.
Hard water is where the protocol shifts substantially. Tap water dries with visible spotting on most glass. Distilled water becomes the standard final-rinse choice. The dwell time of any cleaning solution on glass becomes important — water that sits on glass for more than a few minutes leaves visible deposit when it evaporates, even after squeegeeing.
This is the tier most of urban America falls into: Chicago, parts of New York, Washington DC, Houston, much of Texas. The cleaning protocol: tap water for application, careful squeegee technique, distilled water final rinse on every visible-from-outside pane, and a quick perimeter detail with a microfiber to catch drips before they dry. This is the protocol in the how to wash a window properly article — it's specifically designed for hard-water conditions because that's what most American cleaners are working in.
At very-hard tier, tap water on glass is no longer acceptable for residential window cleaning. The deposits are visible immediately on every drying surface, the wet-dry cycles accelerate the transition to permanent etching, and the cumulative damage from years of tap-water cleaning becomes a real problem on the order of decades rather than centuries.
The protocol: distilled water (or deionized water, or reverse-osmosis water) for both the wash and the rinse, on every pane, every time. Tap water is acceptable only for pre-rinsing heavily soiled glass before the actual cleaning solution is applied, and even then you have to squeegee aggressively before the tap water dries. Monthly or quarterly maintenance is appropriate to prevent deposit accumulation that progresses to etching.
This tier covers much of the Mountain West, parts of the Plains states, the older infrastructure of cities like St. Louis and Kansas City, and much of suburban Florida.
Extremely hard water is its own category of problem. Phoenix tap water averages about 350 mg/L. Las Vegas runs 280-330. Most of suburban Phoenix and Mesa exceeds 400 mg/L in summer. At these concentrations, water that sits on glass for longer than a minute leaves immediately visible chalky residue, and the rate of progression from deposit to permanent etching is measured in months rather than years.
The protocol: distilled or DI water for everything, no tap-water contact with glass at any stage of cleaning, frequency of cleaning increased to monthly minimum on visible glass, and serious consideration of installing a whole-house water softener even though the trade-offs (sodium, environmental discharge) are real. The cumulative cost of replacing etched glass over a 20-year residency in a Phoenix-tier household is substantial — multiple panes per decade is normal, and full window-replacement projects driven entirely by hard-water etching are not unusual.
This is the protocol Drew writes about for desert-route work, and it's the reason water-fed-pole systems with built-in DI filtration are standard equipment for working cleaners in the Southwest.
The Hard Water Severity Scorer reports municipal supply averages. It cannot tell you:
For most users in most situations, none of these caveats matter. For users with unusual conditions, they're the next layer of investigation.
The protocol recommendations the tool produces — distilled-water rinse on dark glass, monthly maintenance, etc. — are not arbitrary. Each one corresponds to a specific failure mode that hardness at that tier produces.
Distilled-water rinse on dark glass at moderate hardness corresponds to the threshold where deposits become visible to a careful homeowner inspecting the work. Below that threshold the deposits exist but aren't noticeable; above it they are. Distilled-water rinse on all glass at hard tier corresponds to the threshold where deposits become visible to anyone walking past the window. Distilled water for the entire wash at very-hard tier corresponds to the threshold where deposits accumulate during the cleaning process itself, before the cleaner can finish the work.
These thresholds are calibrated against twelve years of my own field observations in Chicago (130-180 mg/L), against Jan's Detroit-suburb route data (typically 140-170 mg/L), against Drew's Phoenix observations (300-400 mg/L), and against published industry guidance from the IWCA. Where field observation and published guidance disagreed, I went with field observation. Where the four working-cleaner perspectives disagreed with each other, I went with the more conservative recommendation.
The tool's job is to translate that calibrated knowledge into a number a homeowner can act on without needing to call a cleaner. The Lincoln Park homeowner from 2015 didn't need a chemistry lecture — she needed to know that her water was hard, that distilled rinse would substantially improve her results, and that the spotting was going to come back faster than she wanted regardless. The Hard Water Severity Scorer answers those three questions in about ten seconds, which is the right amount of time for the question.
Mara Whitfield is the senior editor at Window Washing Guide and a twelve-year veteran of the trade. She has cleaned the glass on three of the ten tallest buildings in North America, written equipment reviews for Pro Window Cleaner Magazine, and personally tested every method in this piece. She lives in Chicago, where the water is famously, awfully hard.
Mara is the senior editor at Window Washing Guide and a twelve-year veteran of the trade. She has cleaned the glass on three of the ten tallest buildings in North America, written equipment reviews for Pro Window Cleaner Magazine, and personally tested every method in this piece. She lives in Chicago, where the water is famously, awfully hard.