Category: Basics

Published October 12, 2026. Every claim is checked against the sources listed at the end of this page. How we research.

What TDS in water means

Total dissolved solids is a simple idea: if you evaporated a liter of water, the residue left behind is roughly its TDS. In natural and tap water, most of that weight is ordinary minerals picked up as water moves through soil and rock. The World Health Organization lists the main ones as calcium, magnesium, sodium and potassium (the positive ions) and bicarbonate, chloride, sulfate and nitrate (the negative ions).

TDS is reported in milligrams per liter (mg/L). For drinking water, 1 mg/L is close enough to 1 part per million (ppm) that meters and reports use the two interchangeably. So a reading of 180 means about 180 milligrams of dissolved material in every liter.

Two things are not part of TDS. Particles you can see, such as sand, rust flakes or cloudiness, are “suspended” solids, not dissolved ones, and a filter or a settling glass deals with them. Dissolved gases such as chlorine or radon are not counted either. That matters when people use TDS as a shortcut for “how clean is my water”: it is a measure of dissolved minerals and salts, not of everything that can be wrong with water.

Natural water varies enormously. The WHO notes TDS in natural waters ranging from under 30 mg/L to as much as 6,000 mg/L, depending on how soluble the local rock is. For scale, the US Geological Survey classes water under 1,000 ppm as fresh, and seawater holds about 35,000 ppm.

What is a normal TDS level for drinking water?

There is no single “normal” number, but there are two official reference points: the US EPA’s secondary guideline of 500 mg/L, and the WHO’s taste ratings, which come from panels of people tasting water at different TDS levels. Together they give a practical scale:

TDS (mg/L or ppm)	WHO taste rating	What it usually means at home
Under 50	Excellent (may taste flat)	Reverse osmosis, distilled or deionized water, or very soft surface water
50–300	Excellent	Common for US city water; little scale
300–500	Good	Normal; some scale on kettles; within the EPA guideline
500–900	Fair (300–600 good, 600–900 fair)	Above the EPA guideline; often hard water, sodium, chloride or sulfate
900–1,200	Poor	Noticeable mineral or salty taste, heavy scale
Over 1,200	Unacceptable	Common only in some wells and arid regions; lab test advised
Over 1,000	—	USGS: “slightly saline” water starts here

The EPA guideline is one of its “secondary” standards. The agency describes these as non-enforceable guidelines for cosmetic and aesthetic effects, things like taste, color and staining, and lists the effects of high TDS as hardness, deposits, colored water, staining and a salty taste. Federal rules do not require utilities to meet it, though some states set their own requirements.

If you are on city water, your utility’s annual water quality report (the Consumer Confidence Report) often lists TDS or conductivity in its table of secondary or “other” results. The ZIP code tap-water check on our homepage helps you find yours.

Is high TDS bad for you? Is low TDS bad?

High TDS on its own is mostly a taste and nuisance problem. The WHO states that it does not propose a health-based guideline value for TDS, because the common minerals that make up TDS are not toxic at the levels found in drinking water. What high TDS does cause is practical: scale in kettles, water heaters and on glass, spots on dishes, and a salty or bitter taste.

The catch is that a high number can hide a specific problem. A TDS of 800 made mostly of calcium and magnesium (hard water) is very different from a TDS of 800 with high nitrate from a farm field, or high sodium for someone on a low-salt diet. TDS cannot tell those apart. If your reading is high, the next step is a test that names the components: hardness, sodium, chloride, sulfate and, for wells, nitrate. Hard water, meaning high calcium and magnesium, is the most common cause.

Low TDS is not harmful either. Reverse osmosis and ZeroWater-type filters produce water with a TDS close to zero, and people drink it every day. The WHO notes that very low-TDS water can taste flat and insipid, and that it tends to be corrosive to pipes, which is a concern for water utilities rather than for a glass of filtered water. Most of the minerals people need come from food, not water. We cover the evidence in more depth in is reverse osmosis water good for you?

What a TDS meter measures, and what it misses

A handheld TDS meter does not weigh anything. It measures electrical conductivity: dissolved minerals are charged ions, and the more ions in the water, the better it conducts electricity. The meter then multiplies conductivity by a conversion factor to estimate TDS. North Dakota State University Extension notes that the right factor changes with the water, from about 0.5 for low-mineral water to about 0.8 for very salty water, which is why two meters can disagree and why a meter reading is always an estimate.

This method has two big blind spots:

• Things that are not charged ions do not register. Many organic chemicals, including many pesticides, solvents and PFAS at the levels found in tap water, add essentially nothing to conductivity. Bacteria and viruses do not register at all.
• Things that are dangerous at tiny amounts are too small to see. The EPA’s maximum contaminant level for PFOA and PFOS is 4 parts per trillion, and lead is a concern at a few parts per billion. A tap reading of 200 ppm is about 50 million times larger than 4 parts per trillion. Even a lead level that would alarm a utility changes a TDS reading by a fraction of one digit.

So a TDS meter is useful for three jobs: seeing how mineral-rich your water is, checking whether a reverse osmosis system or ZeroWater-type filter is still doing its job, and tracking changes in a well over time. It is not a safety test, and a low reading is not proof that water is free of lead, PFAS or germs. Be wary of anyone selling a filter by showing a TDS meter drop on a carbon filter, or claiming that a high reading proves your water is unsafe. For lead, see how to test for lead in tap water; for PFAS, how to find out if your water has PFAS.

TDS and water filters: which ones change the number

Different filter types change TDS very differently, which is useful to know when you test before and after:

Treatment	Effect on TDS	Why
Activated carbon (most pitchers, fridge filters, faucet filters)	Little change	Carbon targets chlorine and organic chemicals, not dissolved minerals
Reverse osmosis (NSF/ANSI 58)	Large drop	The membrane rejects most dissolved ions; the certification test requires at least a 75% cut
Deionizing pitcher (for example ZeroWater)	Near zero	Ion exchange resin removes almost all ions until it is used up
Distillation	Near zero	Minerals stay behind when water is boiled off
Ion exchange water softener	Little change	Swaps calcium and magnesium for sodium or potassium; the total stays similar
Salt-free conditioner	No meaningful change	Changes how minerals form scale, does not remove them
Remineralizing cartridge after RO	Small rise	Adds calcium and magnesium back on purpose

Two consequences follow. First, if your carbon pitcher reads the same before and after, the filter is not broken; TDS is simply the wrong test for it. Our comparison of ZeroWater vs Brita shows why the two pitchers read so differently. Second, a water softener will not fix a high TDS reading, even though it fixes hard-water scale. If your TDS is high because of hardness, see salt vs salt-free softeners and the softener size calculator; if it is high because of sodium, chloride or sulfate, reverse osmosis is the household fix.

What is a good TDS for RO water?

Under NSF/ANSI 58, the certification standard for home reverse osmosis systems, the test water has a TDS of 750 mg/L and the system must bring it down to 187 mg/L or less, a cut of at least 75 percent. Certified systems often do better: one current under-sink data sheet, for example, reports a 91 percent reduction. In practice, RO output is usually a small fraction of your tap reading.

An easy way to judge your own system is to compare the two numbers, not to aim for a magic value. As an example only: tap water at 300 ppm and RO water at 20 ppm is a 93 percent reduction, which is healthy. If, months later, the RO reads 60 with the same tap water, rejection has dropped to 80 percent, and it is time to check the prefilters and membrane. A remineralizing stage will raise the reading on purpose, so measure before it if you can. For what RO removes besides minerals, see what reverse osmosis removes.

How to test TDS at home

1. Use a TDS or conductivity meter with automatic temperature compensation; readings change with water temperature.
2. Run the cold tap for a few seconds, then fill a clean glass. Do not test hot water.
3. Dip the probe to the marked depth, stir gently to clear air bubbles, and wait for the number to settle.
4. Rinse the probe with the water you are about to test, and dry it after use.
5. For a filter check, test tap water and filtered water within a few minutes of each other and compare.
6. Write the number down with the date. For a well or an RO system, the trend over months matters more than one reading.

If your reading looks odd, check the meter in a known sample. Bottled water labels sometimes list TDS or “dissolved solids at 180°C”, and distilled water should read close to zero.

When to get a lab test instead of a TDS reading

A TDS meter is a starting point. Use a certified laboratory when:

• You are on a private well. Test at least for bacteria, nitrate and arsenic, then add what local conditions call for. Our well water results chart explains the numbers.
• Your TDS is over 500 and you want to know why, or over 1,000 at any time.
• The reading jumps suddenly, for example after flooding, nearby drilling or a change in taste or color.
• Someone in the home is an infant, pregnant, or on a low-sodium diet, and you need sodium or nitrate numbers, not a total.
• You are worried about lead, PFAS or bacteria. No home TDS meter can detect them.

If you are trying to decide on a filter, the filter finder matches your concern to the right filter type in about 15 seconds, and how to choose a water filter walks through it step by step.

Sources and how we checked this page

Every claim on this page comes from the US EPA, the World Health Organization, the US Geological Survey, North Dakota State University Extension and manufacturer data sheets, checked on October 12, 2026. We did not test water or meters ourselves. This page is general information, not medical advice.

• US EPA: secondary drinking water standards (TDS 500 mg/L)
• World Health Organization: total dissolved solids in drinking-water, WHO/SDE/WSH/03.04/16 (PDF)
• US Geological Survey: saline water and salinity
• NDSU Extension: using EC and TDS meters to field-test water quality (WQ1923)
• US EPA: PFAS national primary drinking water regulation
• A. O. Smith: example NSF/ANSI 58 performance data sheet with the 750 mg/L TDS test (PDF)
• ZeroWater: How long will my ZeroWater filter last? (TDS chart)
• US EPA: Consumer Confidence Reports (find your water quality report)

Published October 6, 2026. Every claim is checked against the sources listed at the end of this page. How we research.

Does boiling water remove lead?

No. The EPA answers this question with one word: no. The CDC is just as direct and says boiling water will not reduce the amount of lead in it. Lead is a metal. It does not turn into steam at the temperature of a kettle, so it stays in the pot while some of the water leaves.

Some state health agencies go a step further. The Massachusetts Department of Public Health warns that if there is lead in your water, boiling it will increase lead levels. That happens because the same amount of lead ends up in less water. The effect is small for a quick boil, but it never works in your favor.

This matters most for anyone who boils water for a baby. Infant formula, cereal and baby food made with tap water can carry lead from old pipes, solder or brass fixtures. Boiling may make the water free of germs, but it does nothing about the metal. The EPA advises using only cold water for drinking, cooking and making baby formula, because hot water tends to pick up more lead from plumbing.

What actually lowers lead at the tap

• Use cold water only for drinking, cooking and formula. Heat what you need on the stove after you draw it cold.
• Flush the pipes when water has been sitting for several hours. The EPA suggests running the tap, taking a shower or doing a load of laundry first.
• Use a certified filter. The CDC points to filters certified to NSF/ANSI 53 for lead and NSF/ANSI 42 for particulates. Our guide to NSF 53 vs 58 explains which standard covers what.
• Verify the claim yourself. A box that says “reduces lead” is not proof. Look the model up in the certifier’s database, as shown in how to check if a filter is really NSF certified.
• Test if in doubt. Your utility can tell you whether your home has a lead service line, and many offer or can recommend lead testing.

The federal lead action level is 15 parts per billion (ppb) today. Under the EPA’s Lead and Copper Rule Improvements, finalized in October 2024, it drops to 10 ppb starting November 1, 2027, and most water systems must replace lead service lines within 10 years. As of fall 2026 the rule remains in effect while water industry groups challenge it in federal court. None of this changes the advice at home: a utility’s action level is a trigger for that utility to act, not a safe level for your glass.

Why boiling concentrates some contaminants

Boiling separates water from anything that does not evaporate at the same temperature. Pure water leaves as steam. Metals, salts and most man-made chemicals stay behind. The Kansas Department of Health and Environment puts it plainly for nitrate: excessive boiling can make nitrates more concentrated, because they remain when the water evaporates.

The math is simple. Imagine one liter of water with 10 ppb of lead. Boil it long enough that 20% of the water turns to steam. You now have 0.8 liters holding the same amount of lead, which works out to 12.5 ppb. Nothing was removed. The lead just has less water to hide in.

A short boil to make tea loses little water, so the change is small. A pot left simmering for 20 minutes, or water boiled again and again in the same kettle, loses much more. Either way the direction is the same. For lead, PFAS, fluoride, nitrate and arsenic, boiling can only keep the level the same or raise it.

Distillation works on the same physics, but in reverse. A distiller boils the water and then collects the steam and condenses it into a clean container. NSF describes distillers as heating water to the boiling point and collecting the water vapor as it condenses, leaving contaminants such as heavy metals behind. A pot on a stove keeps the wrong part.

Does boiling water remove it? Contaminant by contaminant

Here is the short version for everything people ask about. Each row is based on federal or state health agency guidance or a published study, listed in the sources at the bottom of this page.

Contaminant	Does boiling remove it?	What happens	What works instead
Lead	No	Stays in the water; can concentrate	Filter certified to NSF/ANSI 53 for lead; cold water; flushing
Bacteria, viruses, parasites	Yes	Killed by a 1-minute rolling boil	Boiling, or UV certified to NSF/ANSI 55 for ongoing well problems
PFAS (forever chemicals)	No	Do not boil off	Reverse osmosis or carbon with a certified PFOA/PFOS claim
Fluoride	No	Stays in the water	Reverse osmosis certified for fluoride; distiller
Nitrate	No	Becomes more concentrated	Reverse osmosis or another certified nitrate treatment
Arsenic	No	Stays in the water	Treatment with a certified arsenic claim; test first
Chlorine	Partly, slowly	Evaporates over 15 to 20 minutes	Carbon filter certified to NSF/ANSI 42
Chloramine	Not effectively	Too stable to boil off quickly	Filter with a listed chloramine reduction claim
Hardness (calcium carbonate)	Partly	Some drops out as scale in the kettle	Water softener for whole-home hardness
Microplastics	Partly, in hard water	Trapped in scale, then must be filtered out	Filter certified to NSF/ANSI 401 for microplastics

Notice the pattern. Boiling is excellent at one job, killing living things, and a weak or harmful choice for nearly everything else. If you are not sure which group your concern falls into, the quick check at the top of this page gives a one-line answer for each.

When boiling is the right answer: germs and boil water notices

Boiling is the best tool you have when the problem is biological. The CDC calls boiling the surest way to kill disease-causing organisms, including viruses, bacteria and parasites. That is why utilities issue boil water advisories after a main break, a pressure drop or a positive test for bacteria such as E. coli.

The CDC’s instructions are short:

1. If the water is cloudy, let it settle and filter it through a clean cloth, paper towel or coffee filter first.
2. Bring the clear water to a rolling boil for 1 minute. At elevations above 6,500 feet, boil for 3 minutes.
3. Let it cool, then store it in clean, covered containers.
4. Use boiled or bottled water for drinking, cooking, brushing teeth, making ice and mixing formula until the advisory is lifted.

The same CDC page adds an important line: boiling does not remove chemicals. If your utility issues a “do not drink” or “do not use” order instead of a boil notice, the problem is usually chemical, and boiling will not help. Follow the utility’s instructions exactly.

Private well owners face a different situation. If a well tests positive for coliform bacteria again and again, boiling every glass is not a long-term plan. Find and fix the source, then consider ongoing disinfection such as an ultraviolet system certified to NSF/ANSI 55. Our guide to choosing a water filter walks through matching treatment to a test result.

PFAS, fluoride, nitrate and arsenic: boiling will not help

Does boiling water remove PFAS?

No. The New Mexico Department of Health says it directly: boiling water does not remove PFAS. These “forever chemicals” are built to resist heat, which is why they were used in nonstick coatings and firefighting foam. A kettle does not touch them.

In 2024 the EPA set limits of 4.0 parts per trillion each for PFOA and PFOS, plus limits for several other PFAS. In May 2026 the EPA proposed keeping the PFOA and PFOS limits, dropping the others, and moving the compliance deadline from 2029 to 2031. As of this update that proposal is not final. To see whether your own water has PFAS, check your utility’s PFAS results. To lower PFAS at home, look for reverse osmosis or a carbon filter whose listing shows a PFOA/PFOS or total PFAS reduction claim. What reverse osmosis removes covers the details.

Does boiling water remove fluoride?

No. A CDC Agency for Toxic Substances and Disease Registry (ATSDR) report states that heating or boiling water does not remove fluoride. The EPA’s enforceable limit for fluoride is 4.0 mg/L. Reverse osmosis systems certified for fluoride reduction and countertop distillers are the reliable options; see water filters that remove fluoride.

Does boiling water remove nitrates?

No, and this is the case where boiling does the most harm. Kansas health officials warn that boiling, freezing or letting water stand does not reduce nitrate, and that excessive boiling makes it more concentrated. The EPA limit is 10 mg/L (as nitrogen). The EPA warns that infants under six months who drink water above that limit could become seriously ill, a condition known as blue-baby syndrome. Nitrate is mostly a private well issue, often near farmland or septic systems.

Does boiling water remove arsenic?

No. The Virginia Department of Health says heating or boiling your water will not remove arsenic. The EPA limit is 10 ppb. Arsenic occurs naturally in groundwater in parts of the country, so well owners should test for it rather than assume.

Chlorine vs chloramine: one boils off, one mostly does not

Chlorine is the one chemical that boiling does remove, given time. Local health departments say boiling water for about 15 to 20 minutes lets the chlorine evaporate. Leaving a pitcher uncovered in the fridge also lets some chlorine escape. Both are slow, and neither is needed: a basic carbon pitcher or faucet filter certified to NSF/ANSI 42 for chlorine taste and odor does the job in seconds.

Many US cities now use chloramine, a mix of chlorine and ammonia that lasts longer in the pipes. Utilities that use it say boiling is not an effective way to remove it. If your water report lists chloramine, choose a filter whose certification listing names chloramine reduction specifically. A chlorine claim alone does not cover it. Your utility’s annual water quality report says which disinfectant it uses.

Not sure what is in your water? The ZIP code tap water check on our homepage points you to your utility’s report.

Microplastics: the hard-water exception

Here boiling does more than you might expect, but only under the right conditions. A 2024 study in Environmental Science & Technology Letters by researchers at Guangzhou Medical University and Jinan University boiled tap water for 5 minutes, let it cool, and poured it through a simple filter such as a coffee filter.

In hard water, the calcium carbonate that forms limescale crystallized around the tiny plastic particles and trapped them. Results depended on hardness:

• About 90% removed in hard water at 300 mg/L of calcium carbonate.
• About 84% removed at 180 mg/L.
• About 25% removed in soft water under 60 mg/L.

Three limits are worth knowing. First, the filtering step is essential; boiling alone just leaves the trapped particles in the scale. Second, the researchers tested only three types of plastic (polystyrene, polyethylene and polypropylene). Third, most people in soft-water areas will see little benefit. If microplastics are your main concern, a filter certified to NSF/ANSI 401 for microplastics reduction is the more dependable route. That claim is tested on particles from 0.5 to 1 micron, and the model’s listing will say so.

Is boiled water the same as distilled or filtered water?

No, and the difference explains everything above. Boiled water is the same water, minus most living germs and some chlorine, with dissolved minerals and chemicals still in it. Distilled water is the condensed steam, so most minerals, metals and salts are left behind in the distiller. Filtered water depends entirely on the filter: a carbon pitcher mainly improves taste, while a certified reverse osmosis system reduces a long list of dissolved contaminants.

	Boiled	Distilled	Filtered (certified)
Kills germs	Yes	Yes	Only with a cyst or microbial claim, or UV
Removes lead	No	Yes	Yes, if certified to NSF/ANSI 53 for lead
Removes PFAS	No	Varies; check the claim	Yes, if the listing shows a PFAS claim
Removes fluoride	No	Yes	Only RO or special media with a tested claim
Removes minerals	Some hardness	Nearly all	RO removes most; carbon keeps them
Cost and effort	Energy, time	Slow, uses power	Filter price plus replacements

For a deeper look at the standards in the last column, see water filter certifications explained. And if you use a pitcher today, does Brita remove lead, PFAS and fluoride lists exactly what each Brita filter is certified for.

What to use instead of boiling

Match the fix to the problem. Here are three common situations.

Not sure which applies to you? Answer a few quick questions in the 15-second filter finder and it will point you to the right type and certification.

A simple plan, in 4 steps

1. Find out what is in your water. Read your utility’s annual report, or test a private well through a certified lab.
2. Boil only for germs. Use the CDC’s 1-minute rule during a boil notice, and stop once it is lifted.
3. Filter for chemicals. Pick the certification that names your contaminant, then confirm the model in the certifier’s database.
4. Replace cartridges on time. A certified claim only holds up to the rated capacity on the performance data sheet.

Sources and how we checked this page

Every fact on this page comes from the US EPA, the CDC and its ATSDR, NSF, state health departments, a water utility, and a peer-reviewed 2024 study, checked on October 5, 2026. We do not test water or filters in a lab. The lead example is a calculation to show the effect, not a measurement.

• US EPA: Does boiling water remove lead?
• US EPA: Basic information about lead in drinking water
• CDC: Lead in drinking water
• Massachusetts DPH: Lead in drinking water FAQ
• US EPA: National Primary Drinking Water Regulations (limits for lead, nitrate, fluoride, arsenic)
• Louisiana Department of Health: Lead and Copper Rule Improvements FAQ (2026)
• CDC: Boil water advisory
• New Mexico Department of Health: PFAS fact sheet
• Congressional Research Service: PFAS drinking water regulation (June 2026)
• CDC ATSDR: Health consultation stating boiling does not remove fluoride (PDF)
• Kansas Department of Health and Environment: Nitrate in private wells
• Virginia Department of Health: Arsenic in private wells (PDF)
• Fayette County (GA) Health Department: Chlorination (PDF)
• City of Englewood, CO: Chloramine FAQs
• Yu et al., Drinking boiled tap water reduces human intake of nanoplastics and microplastics, Environ. Sci. Technol. Lett. (2024)
• Smithsonian Magazine: Boiling tap water could help remove microplastics
• NSF: Standards for water treatment systems
• NSF listings: NSF/ANSI 53 systems with lead reduction