A bottle labeled “purified water” and a bottle labeled “distilled water” sit on the same store shelf, and most shoppers assume the two names describe competing processes. They do not.
Table Of Contents
−Under FDA rules, distilled water is one specific way to produce purified water, not a rival category sitting next to it.
This article settles the confusion using the actual federal definition, walks through the three methods that qualify as purified water, and covers the real tradeoff those methods create once the minerals are gone.
The Short Version
- Distilled water is a type of purified water. Distillation, reverse osmosis, and deionization can all produce water that meets the FDA’s purified water definition.
- The tradeoff: purified water supplies essentially none of the calcium and magnesium the WHO links to lower cardiovascular mortality.
- Where it earns its place: CPAP machines, steam irons, aquariums, and lab equipment. Infant formula does not require it.
- When buying equipment, look for NSF/ANSI 58 or 62 and check the output with a TDS meter.
What Purified Water Actually Means Under Federal Rules
The FDA defines these terms in 21 CFR 165.110, the same regulation that governs bottled water labeling nationwide.
Purified water there is water produced by distillation, deionization, reverse osmosis, or another suitable process. It must meet the United States Pharmacopeia’s purified water specification regardless of which method made it.
That specification is a conductivity test, USP chapter 645, requiring a reading below 1.3 microsiemens per centimeter at 25 degrees Celsius. In practice, that means almost no dissolved solids are left behind, whether a distiller boiled them out or a membrane rejected them.
Purified Water Versus Mineral Water
Mineral water sits at the opposite end of the same regulation:
| Label | Dissolved solids | Source or method |
|---|---|---|
| Purified water | Near zero | Distillation, deionization, reverse osmosis, or another suitable process |
| Mineral water | At least 250 parts per million of total dissolved solids | A protected underground spring or bore hole |
The FDA prohibits adding minerals to mineral water to reach that number after the fact.
So the two labels are not just marketing language competing for shelf space. They mark opposite ends of the same TDS scale, one required to read near zero and the other required to read measurably mineral rich.
Distillation Is One Way to Meet That Standard
Distillation earns its purified water status the oldest way on record. The unit boils tap water, captures the steam, and condenses it back into liquid in a separate chamber, leaving dissolved minerals, most bacteria, and non-volatile contaminants behind in the boiling tank.
Home units fall under NSF/ANSI 62, the standard covering structural integrity, material safety, and any distillation performance claim a manufacturer wants to print on the box.
Not Every Distiller Is Certified
Not every countertop distiller on the market actually carries that certification. The Waterwise 4000, one of the more established countertop models, lists its wetted parts as FDA rated stainless steel rather than NSF certified.
It produces about one gallon every four to four and a half hours, up to six gallons in a day. That is a real limitation for a family of four, and it is worth checking before buying rather than after delivery.
If you are comparing specific models, start with our water distillers roundup, which lists certification status alongside output rate for each pick.
Reverse Osmosis and Deionization Get You There Too
Two other methods reach the same purified water bar without a boiler.
Reverse Osmosis
Reverse osmosis forces tap water through a semipermeable membrane under pressure, rejecting dissolved solids along with anything else too large to fit through the pores.
NSF/ANSI 58 governs point of use RO systems, and TDS reduction is the only claim the standard requires every certified unit to prove. Lead, arsenic, nitrate, and cyst reduction are optional add on certifications, so two RO systems can both carry the NSF 58 mark while covering very different contaminant lists.
If you are weighing a unit for your own kitchen, read our full breakdown of how reverse osmosis works before assuming a certification badge alone tells the whole story.
Deionization
Deionization takes a different route. Resin beads inside the cartridge swap dissolved mineral ions for hydrogen and hydroxide ions, which is fast and cheap at small volumes but does nothing to bacteria, viruses, or organic contaminants that carry no charge.
That is the detail missing from most “DI water is basically distilled water” claims. It is worth reading in full in our deionized water versus distilled water comparison, since the two read almost identically on a TDS meter and behave very differently once safety is the question.
What You Lose When the Minerals Are Gone
Purified water’s near zero TDS reading is the whole point for lab and industrial use. It is also the tradeoff nobody prints on the bottle.
The World Health Organization’s 2005 review of nutrients in drinking water found that measurable calcium and magnesium in tap water correlates with lower cardiovascular mortality across more than 80 epidemiological studies conducted since 1957. It recommends these minimums for that protective association:
- Calcium: roughly 20 to 30 milligrams per liter.
- Magnesium: 10 milligrams per liter.
Purified water supplies essentially none of either.
Who This Actually Affects
If you drink filtered tap water most days and purified water occasionally, that gap is not a health event.
It becomes relevant for anyone who drinks purified water exclusively for years, or who cooks in it routinely. The same WHO review found that vegetables, cereal, and meat lose measurably more of their own mineral content when boiled in demineralized water compared to mineralized water.
Corrosion in Whole House Systems
There is a second, more immediate issue for a whole home distiller or RO system rather than a countertop unit. The same WHO review flags that demineralized water, if not remineralized or otherwise stabilized, is corrosive to plumbing. It can pull lead or copper out of old solder joints and fixtures downstream.
That risk applies to point of entry systems feeding an entire house with older copper piping. It does not apply to a countertop unit that only fills a pitcher.
When Purified Water Actually Solves a Real Problem
Several kinds of equipment specify distilled or purified water because mineral deposits foul the equipment over time, not because tap water is unsafe to touch:
- CPAP humidifiers.
- Steam irons.
- Car cooling systems.
- Reptile and aquarium setups.
- Lab or medical equipment.
That is a maintenance requirement, not a health one, and it accounts for most of what these gallons on the store shelf actually get used for.
Infant Formula Does Not Require Distilled Water
Infant formula is the one place this gets misread most often. The CDC does not list distilled water as a special requirement for formula preparation. For most babies, filtered or unfiltered tap water mixed according to the formula instructions is fine.
The CDC does recommend an added precaution for babies younger than two months, born premature, or otherwise immunocompromised. Boil the water and let it cool about five minutes before mixing, specifically to kill Cronobacter bacteria that can survive inside powdered formula itself.
That boiling step, not a switch to distilled water, is the guidance worth following unless a pediatrician has said otherwise.
Buying Purified Water Equipment Without Guessing
Two checks matter more than the marketing copy on the box:
- Look for the NSF/ANSI 58 or 62 mark specific to the device type. FDA rated materials and NSF certification are not the same claim, and only one of them comes with independent testing behind it.
- Buy a cheap meter and test the output directly instead of trusting the spec sheet. A properly functioning distiller or RO unit should read close to zero on total dissolved solids.
A reading that climbs over time on an RO system usually means the membrane needs replacing. A distiller that never gets there likely has scale building up in the boiling chamber.
Which Method to Choose
For everyday drinking water, filtered tap water covers what almost every household actually needs. Purified water earns its place for the specific equipment and rare medical situations above, not as a default upgrade.
If one of those situations applies, match the method to the actual constraint:
- Distillation: when equipment cost per gallon matters more than speed.
- Reverse osmosis: when daily volume matters more than upfront price.
Then confirm the unit carries the certification that matches its type before it ships.
Jay
Jay is a health and wellness enthusiast with expertise in water quality and nutrition. As a knowledgeable advocate for holistic well-being, Jay successfully manages Type 2 Diabetes through informed lifestyle choices. Committed to sharing reliable and authoritative insights, Jay combines firsthand experience with a passion for enhancing health."