Reverse Osmosis vs Distilled Water: Which Purity Do You Actually Need?

Updated July 2026 · Editorial team · Topic: reverse osmosis / drinking water

People treat these two as interchangeable — both make "pure" water, both strip out the junk, pick whichever. But they get there by completely different physics, and that difference decides which one belongs under your sink and which one belongs next to your CPAP machine. Choosing wrong means either overpaying for purity you'll never use or buying a jug of water for a job a faucet should handle.

Short answer: Both remove nearly all dissolved solids, but distillation goes further and slower. Distilled water reads about 1 ppm TDS and its boiling step also kills bacteria; RO reads 10–30 ppm and relies on filtration, not heat. RO costs pennies per gallon at the tap; a countertop distiller uses roughly 3 kWh per gallon and takes 4–6 hours to make one. For everyday drinking, RO wins; for medical devices and labs, distilled wins.

Reviewed by the ClearTap editorial team. We publish plain specs, model compatibility and NSF/EPA-based standards so you can judge for yourself — no lab-test theatre and no upsell. We do not run a water lab; our guidance is built from published specifications and NSF/EPA standards, not invented tests. General information about water quality only, not medical or drinking-water advice: for legal or health decisions about your water, test it with a certified laboratory.

Two machines, two methods

Distillation copies the water cycle in a box. It boils water into steam, leaves the dissolved solids and most metals behind in the boiling chamber, then condenses the vapor back into a jug. Because it uses a phase change, it leaves salts and minerals stranded and — crucially — the heat inactivates living organisms along the way.

Reverse osmosis never heats anything. It pushes water under pressure through a semipermeable membrane whose openings are too small for dissolved ions to pass. The contaminants get rinsed to a drain while the clean water collects on the other side. No boiling means no energy-hungry heat, but also no thermal kill step — RO's protection comes from the pore size and its carbon stages, not temperature.

Head to head, by the numbers

	Reverse osmosis	Distillation
How it separates	Membrane, pressure-driven	Boiling then condensing
Typical output TDS	10–30 ppm	~1 ppm
Inactivates bacteria?	No thermal kill; relies on filtration	Yes, via boiling
Speed	Continuous at the tap	~4–6 hours per gallon
Energy / cost	Wastes some water, ~pennies/gal	~3 kWh/gal (~$0.30–0.45 in power)
Where it fits	Whole-home drinking, cooking	CPAP, batteries, aquariums, lab work

The purity gap is real but rarely matters

Distilled water is technically cleaner — about 1 ppm versus RO's 10–30. That last stretch toward zero sounds impressive, and for a handful of uses it genuinely counts. A CPAP humidifier fed hard water scales up and grows a crust; a lead-acid battery topped with mineral water shortens its own life; a reef aquarium reacts to trace metals a drinking glass would never notice. In every one of those cases the demand is measured in gallons per week, which is exactly what a slow distiller can supply.

For drinking, cooking, and coffee, the gap between 1 ppm and 20 ppm is not something a human mouth or body registers. What you'd feel instead is the difference in convenience: RO runs the moment you lift the lever, while distillation asks you to wait hours and pay for electricity every gallon. That trade is why homes pick RO and hospitals pick distilled.

A note on volatile compounds: Neither method is a magic wand for everything. Some low-boiling-point organic compounds can ride the steam over in a basic distiller, and some small volatiles can slip an RO membrane. That's why quality distillers add a carbon post-filter and quality RO systems sandwich the membrane between carbon stages. The takeaway: purity depends on the full system, not the headline method.

What we're not claiming

We haven't tested a shelf of distillers and RO units side by side in a lab — we don't have one. The figures here come from the physics of each process and published equipment specs, not from bench results we ran. Your distiller's exact output and your RO's exact rejection depend on your feed water and maintenance, so read these as the reliable shape of the comparison rather than a promise about your specific gallon.

Common mistakes

Buying a distiller for daily drinking water. The speed and energy cost make it a poor fit for a household's full volume. RO handles that job cheaper and instantly.
Using RO water in a CPAP or steam iron long-term. RO's 10–30 ppm still leaves enough mineral to scale delicate devices over months; those uses want distilled.
Assuming distilled means sterile forever. Once it sits open in a jug, distilled water can pick up airborne organisms like any other. The kill step happens during boiling, not in storage.
Ignoring the flat taste of both. Near-zero-mineral water tastes empty to many people. A little remineralization fixes RO for drinking; distilled is usually left as-is for its non-drinking jobs.

FAQ

Is distilled water purer than reverse osmosis water?

Slightly. Distilled water reads around 1 ppm TDS versus 10–30 ppm for RO. For drinking, that gap is not something you can taste or that affects your body.

Can I drink distilled water every day?

Yes, it is safe to drink, though many people find it flat. Its bigger drawback for daily use is the slow, energy-hungry process of making enough of it.

Does reverse osmosis kill bacteria like distillation does?

Not by heat. RO blocks bacteria physically through its membrane rather than killing them. Distillation inactivates organisms during boiling, which is why it suits sterile-leaning uses.

Which is cheaper to run at home?

Reverse osmosis, by a wide margin for volume. It costs pennies per gallon at the tap, while a distiller burns roughly 3 kWh and several hours per gallon.

Related:

General information, not medical advice. Test your water first. Prices and specifications vary by model and region.