The best substitute for distilled water depends on what you need it for. Reverse osmosis (RO) water is the closest match for most purposes, with mineral levels between 10 and 50 parts per million compared to distilled water’s near-zero. For some applications, like humidifiers or steam irons, even filtered tap water works fine. For others, like CPAP machines or car batteries, you need something with very low mineral content, or you can make distilled water yourself at home.
Why Distilled Water Is Used in the First Place
Distilled water is made by boiling water into steam and then cooling that steam back into liquid. The minerals, salts, and other dissolved solids get left behind in the original pot. The result is water with essentially 0 ppm (parts per million) of dissolved solids. That extreme purity is what makes it useful for things that can be damaged or contaminated by mineral buildup.
This is different from simply boiling water. Boiling kills bacteria, but it actually concentrates minerals rather than removing them. As some water leaves as steam, the remaining water holds a higher concentration of whatever was dissolved in it. So boiled water is not a substitute for distilled water when mineral content matters.
Reverse Osmosis Water
RO water is the most practical substitute. It’s made by forcing water through a very fine membrane that blocks most dissolved minerals, metals, and contaminants. The result typically has 10 to 50 ppm of dissolved solids, depending on the source water. That’s not quite as pure as distilled, but it’s close enough for the vast majority of household uses.
You can buy RO water in gallon jugs at most grocery stores. One thing to watch for: some bottled water brands that use reverse osmosis add minerals back in afterward for taste. That reintroduction of minerals can make the water unsuitable for applications where purity matters, like car batteries or lab work. Check the label for phrases like “minerals added for taste.”
Deionized Water
Deionized water has had its mineral ions (things like calcium, magnesium, and sodium) chemically removed. It’s comparable to distilled water in purity and works well for car batteries, steam irons, and other appliances where mineral buildup is the main concern. You can find it at hardware stores and auto parts stores, often sold specifically for battery use. It tends to be less common in regular grocery stores than distilled water, but it’s worth knowing about as an option.
The Best Substitute by Use Case
CPAP Machines
CPAP humidifier chambers are one of the most common reasons people buy distilled water. The CDC recommends using distilled water, sterile water, or water that has been boiled and then cooled for respiratory devices. Tap water can introduce waterborne pathogens into the mist you’re breathing all night. If you can’t find distilled water, boiling tap water for at least one minute and letting it cool completely before pouring it into the chamber is a reasonable backup. RO water also works here, since it removes the vast majority of dissolved solids and microorganisms.
Humidifiers
Ultrasonic humidifiers are particularly sensitive to mineral content. When they aerosolize tap water, the dissolved minerals get sprayed into the air as a fine “white dust” that settles on furniture and can be inhaled. Mouse studies have shown these mineral particles get taken up by immune cells in the lungs. While the research didn’t find severe inflammation from short-term exposure, using low-mineral water avoids the issue entirely. RO water, filtered water, or homemade distilled water all reduce white dust significantly. Evaporative humidifiers are more forgiving, though minerals still cause scale buildup inside the unit over time.
Steam Irons
Minerals like calcium and iron in tap water cause scale deposits that clog steam vents and eventually damage the iron. RO water or deionized water both work well here. Many iron manufacturers actually recommend a 50/50 mix of tap and distilled water, since completely mineral-free water can be slightly more corrosive to certain metal components. Check your iron’s manual for its specific recommendation.
Car Batteries
Traditional lead-acid batteries need water added periodically to replace what evaporates during charging. Minerals in regular water can cause buildup on the battery plates, leading to short circuits and reduced lifespan. Distilled or deionized water is strongly preferred here. RO water is an acceptable backup if nothing else is available, but this is one application where you want the lowest mineral content possible. Don’t use tap water.
Aquariums
RO water is actually the standard in the aquarium hobby, often preferred over distilled. Both strip out chlorine, heavy metals, and other contaminants that can harm fish. The slight mineral content in RO water can be beneficial since you’ll typically be remineralizing the water anyway to match your fish’s needs.
How to Make Distilled Water at Home
If you can’t find any of the alternatives above, you can distill water on your stovetop with equipment you already have. You need a large pot with a lid, a smaller heat-safe bowl, water, ice, and oven mitts.
Fill the large pot partway with tap water and place the small bowl inside so it floats or rests on the bottom. Flip the pot lid upside down so its handle points down toward the bowl. Bring the water to a simmer (not a rolling boil) and place ice on top of the inverted lid. As steam rises and hits the cold lid, it condenses into droplets that drip down the handle and into the bowl. That collected water is distilled.
The process is slow. Starting with 8 cups of water in the pot, expect to collect about 1¼ cups of distilled water per hour. It’s not practical for large volumes, but it works well in a pinch for filling a CPAP chamber or topping off a battery.
Quick Comparison
- Reverse osmosis water: 10–50 ppm dissolved solids. Best all-around substitute. Widely available in stores.
- Deionized water: Near 0 ppm dissolved solids. Excellent for batteries and appliances. Found at hardware and auto stores.
- Boiled and cooled water: Same mineral content as tap, but sterilized. Only useful when the concern is bacteria (like CPAP use), not minerals.
- Filtered tap water: Reduces chlorine and some contaminants but leaves most minerals. Fine for humidifiers and irons, not for batteries or labs.
- Regular tap water: Contains minerals, chlorine, and possible contaminants. Acceptable for evaporative humidifiers in a pinch, but not recommended for most applications where distilled water is specified.