Distilled water and purified water are not two grades of the same thing. Distilled describes a process: water boiled and its vapor condensed. Purified describes an outcome: water treated by any method until very little dissolved material remains. All distilled water is purified, but plenty of purified water was never distilled.
How distillation works
A distiller boils water and condenses the steam in a separate chamber. Anything that will not travel as vapor at that temperature stays behind in the boiling tank: dissolved minerals, metals, salts, and most bacteria and their remains. The result is water with almost nothing dissolved in it and a characteristically flat taste, because taste in water comes largely from dissolved minerals. Distillation is slow and consumes a lot of electricity for the volume it makes, and the boiling tank scales up and needs regular descaling. Its strength is that it is indifferent to what is in the water; the mechanism does not care.
What purified actually covers
Purified is a description of the finished water rather than of how it got there. Reverse osmosis, deionization, distillation and multi-stage filtration can all produce purified water, and the label alone does not tell you which was used. In practice most bottled purified water on a shelf started as a municipal supply and went through reverse osmosis, sometimes followed by deionization and a polishing carbon stage. That matters because different processes leave different things behind: distillation carries over some volatile organic compounds that a membrane would reject, while a membrane leaves a little dissolved material a distiller would not.
Which one your appliance needs
For most household equipment the question is really about minerals, not purity in the abstract. Steam irons, humidifiers, CPAP machines and car batteries want water with no scale-forming minerals, and distilled water is the traditional and reliable answer there. Reverse osmosis water is close and usually fine for the same purposes. Neither is what a drinking water decision should turn on. If your complaint is scale in the kettle and spotting on glassware, buying jugs of distilled water is treating the symptom at one point; the actual fix is a softener, because the USGS defines hardness as dissolved calcium and magnesium, with soft water at 0 to 60 mg/L as calcium carbonate and very hard above 180 mg/L.
Making it at home versus buying it
A countertop distiller produces small batches slowly and uses a lot of power, which suits low-volume needs such as a humidifier. An under-sink reverse osmosis unit produces far more water per day for far less energy and mounts out of sight, which is why it is the usual household choice for drinking and cooking water, as our under-sink reverse osmosis guide covers. Buying bottled purified water works for occasional needs but is an expensive and inconvenient habit for daily use. Whichever route you take, base the decision on a water test rather than on the wording on a bottle label.
Questions people ask about distilled water vs purified water
Is distilled water the same as purified water?
Distilled water is one kind of purified water. Purified describes the result and can be achieved by reverse osmosis or deionization instead of distillation.
Which should I put in my humidifier?
Distilled water is the conventional choice because it contains virtually no scale-forming minerals. Reverse osmosis water is generally acceptable for the same purpose.
Does distilled water taste different?
Yes, it tastes flat. Much of the flavor of drinking water comes from dissolved minerals, and distillation removes essentially all of them.
Can I make distilled water at home?
Yes, with a countertop distiller. It produces slowly and uses substantial electricity, and the boiling tank needs regular descaling.