TL;DR
If your tap water is hard, distilled water is a genuine upgrade for glass and mirrors -- the spots you keep wiping away are dissolved minerals left behind when tap water evaporates, and distilled water contains essentially none. If your water is soft, the difference is barely visible and technique matters far more. Test your hardness first, because that single fact decides whether this is worth the effort or a waste of money.
Why tap water leaves spots
Tap water carries dissolved minerals, mainly calcium and magnesium, picked up from rock as the water moved through the ground. Water hardness is a measure of how much.
When water dries on glass, the water leaves and the minerals do not. What was invisible in solution becomes a visible white deposit. That is the entire mechanism. It is not soap residue, not dirty glass, and not a bad cloth.
Distilled water has been boiled and recondensed, which leaves the minerals behind in the boiler. Deionized water achieves a similar result chemically. Either one dries without depositing anything.
Source: Water quality and hardness classification guidance from public water utilities
Does distilled water make glass streak free?
It removes one of the two causes of spotting -- mineral deposits. It does not fix the other, which is product residue and technique. In hard water areas the improvement is obvious. In soft water areas it is marginal, and the streaks you are seeing come from something else.
Find out how hard your water is first
This determines everything, and it takes almost no effort.
Check your water utility's annual water quality report, which most publish online and which states hardness directly. If you are on a well, a home test strip kit is inexpensive.
The rough bands used across the industry: soft water sits below about 60 parts per million of dissolved mineral, moderately hard runs to about 120, hard to about 180, and very hard above that.
You can also read your fixtures. Persistent white crust on a faucet aerator or a showerhead, a film on shower doors that returns within days, and soap that never lathers well are all hard water signatures. If your fixtures are clean and soap lathers easily, your water is probably soft and distilled water will not change much.
Where distilled water actually helps
Windows and mirrors. The most visible payoff, especially on large panes where the drying edge is obvious.
Glass shower doors. These get the worst of it, because they see hot water constantly and dry slowly. Distilled water in the final wipe helps, though a daily squeegee helps more.
Homemade glass cleaner. If you mix your own, the water is most of the volume. Using distilled removes the mineral load from the recipe entirely -- worth doing if you already mix a solution.
Steam appliances and humidifiers. Not glass, but the same principle. Distilled water prevents scale in the heating element and prevents the fine white mineral dust some humidifiers emit.
Final rinse work. Professional window cleaners use purified water systems for exactly this reason, letting glass air dry without spotting.
Where distilled water is a waste of money
Mopping floors. The mineral load is irrelevant on a floor, and buying gallons of distilled water for mopping is pure expense.
General surface cleaning. Counters, appliances, and painted surfaces do not show mineral spotting the way glass does.
Soft water areas. If your hardness is genuinely low, you will not see a difference worth the cost or the plastic jugs.
Any situation where the streaks are from product. Too much cleaner, or a cleaner containing additives that leave a film, streaks regardless of the water in it.
The technique problems distilled water will not fix
Most streaking blamed on water is actually one of these.
Too much product. Excess cleaner is the single most common cause. A light mist is enough; more just leaves residue to buff off.
A dirty or softener-treated cloth. Fabric softener coats microfiber and makes it smear. Wash cleaning cloths without it.
Working in direct sun. The solution evaporates before you can wipe it, which locks residue and minerals onto the glass. Work in shade or early in the day.
Wiping in circles. Circles hide missed areas and push residue around. Work in one direction on one side of the glass and the perpendicular direction on the other, so you know which face a streak is on.
Old residue buildup. Glass cleaned for years with a product containing additives holds a film. One deep clean strips it, after which maintenance is easy.
The full sequence is in how to clean windows no streaks, the tool question is settled in squeegee vs cloth for streak free windows, and the mirror-specific method is in how to clean mirrors without streaks.
When the deposit is already baked on
Distilled water prevents new spotting. It does nothing to remove mineral deposits that have already bonded to glass, and those need an acid to dissolve them.
Etched glass is a separate and permanent problem. Long-standing hard water deposits, particularly on shower doors, can chemically etch the glass surface itself, and no cleaner reverses that. The removal options for deposits that have not yet etched are in how to remove hard water stains.
Never combine an acidic descaler with bleach or any chlorinated product. That combination produces chlorine gas. Use one product, ventilate, and rinse thoroughly before anything else touches the surface.
The permanent fix, if it is worth it to you
A whole-house water softener addresses the cause rather than the symptom, and it changes far more than glass -- fixtures, appliances, laundry, and how much soap the household uses. Whether it pays off depends on how hard the water is and what it costs locally, and that calculation is in does a water softener make cleaning easier.
Short of that, a squeegee after every shower prevents most glass door spotting for free, and it is more effective than any cleaner sold for the problem.
The verdict, with conditions
Use distilled water if your hardness reads above roughly 120 parts per million, or your fixtures show crust and your shower doors film within days. Use it for glass, mirrors, homemade glass cleaner, and any appliance that heats water.
Use tap water if your water is soft or moderately soft. Spend the effort on technique instead: less product, a clean untreated microfiber cloth, out of direct sun, and one direction per pass.
Do not use distilled water for floors, general surfaces, or any job where mineral spotting is not the visible problem. It is a targeted fix, not a general upgrade.
FAQ
Is distilled water better than tap water for cleaning windows?
In hard water areas, yes -- tap water leaves dissolved calcium and magnesium behind as visible spots when it dries, and distilled water contains essentially none. In soft water areas the difference is barely noticeable, and streaks are usually caused by excess product or technique.
How do you know if your water is hard?
Check your water utility's annual quality report, which states hardness directly, or use a test strip kit if you are on a well. Physical signs are reliable too: white crust on faucet aerators and showerheads, film returning on glass within days, and soap that will not lather.
Will distilled water remove existing hard water spots?
No. It prevents new mineral deposits from forming but does nothing to dissolve deposits already bonded to the glass, which require an acidic product. Long-standing deposits can also etch glass permanently, and no cleaner reverses etching once it has occurred.
Can you use distilled water in a homemade glass cleaner?
Yes, and it is one of the better uses for it since water makes up most of the volume in any homemade recipe. Using distilled removes the mineral load from the mixture entirely, which matters most on large panes where drying edges are visible.
Why do my windows still streak with distilled water?
Almost certainly technique or product. The most common causes are using too much cleaner, wiping with a cloth washed in fabric softener, working in direct sun so the solution dries before you wipe, or old residue buildup from years of a film-leaving product.
Is distilled water worth buying for general house cleaning?
No. For floors, counters, appliances, and painted surfaces the mineral content is irrelevant, and the cost and packaging add up quickly. Reserve it for glass, mirrors, and appliances that heat water, where the mineral deposit is the actual visible problem.
Glass shows every shortcut, which is why it is the surface people give up on first. If you want it handled properly on a schedule, get a quote or read more about house cleaning services.