TDS Meter for Water: What It Tells You (And What It Doesn't)
Number copied. Text it from your phone: 443-277-2204.
TDS stands for total dissolved solids: the minerals and salts dissolved in your water, measured in parts per million (ppm). A TDS meter is a small pen that estimates that amount in a few seconds. It cannot tell you what is dissolved, and when the question is whether your water is safe to drink, what is dissolved is the whole question.
The lab tests my customers send in show why. Water under 300 ppm, a low and normal-looking number, still came back with coliform bacteria, lead, arsenic or nitrate. A TDS meter is a good tool for checking a reverse osmosis faucet against your tap, and a poor one for deciding whether water is safe.
Quick Answer
A TDS meter measures how easily a tiny electric current passes through your water, then converts that into an estimate of dissolved minerals and salts in parts per million (ppm). It tells you roughly how much is dissolved, never what it is.
- How it gets the number. It reads conductivity and multiplies by a set factor, so the number is an estimate. See What a TDS meter measures.
- What it is good for. Comparing a reverse osmosis faucet with your tap to see whether the membrane is still working. See Checking your RO with a TDS meter.
- What it cannot tell you. Bacteria, lead, arsenic or nitrate. Low-TDS water in my customers' lab tests had all four. See What a TDS reading cannot tell you.
- What number is normal. Under 500 ppm meets the EPA's guideline, which is about taste, scale and staining, not health. See What your TDS number means.
- What to use for safety. A certified lab test. See When you need a lab test instead.
We do not sell TDS meters, and none comes in the box with our reverse osmosis systems; a basic pen meter is all the RO check needs.
What's in This Guide
What a TDS meter measures
A TDS meter measures electrical conductivity, then converts it into an estimate of total dissolved solids in parts per million. It measures how much is dissolved, not what it is. Two metal probes at the tip pass a tiny current through the water. Dissolved minerals carry an electric charge, so more of them lets more current through, and the meter multiplies that reading by a set factor to show ppm.
The factor is the meter's guess about your water. The HM Digital TDS-3, one of the common pens, uses 0.5, set with a salt (sodium chloride) solution. The U.S. Geological Survey puts the real ratio in natural water between 0.55 and 0.75. On 369 lab reports my customers got back, the lab's TDS figure is almost exactly 0.58 times its conductivity reading. So a pen set to 0.5 reads about 15 percent lower than the lab on the same water.
The pen's own limits matter too. The TDS-3 is rated at plus or minus 2 percent, and the smallest step it shows is 1 ppm, far bigger than the amounts of lead or arsenic that matter.
What Drives a TDS Reading
- Calcium and magnesium (the hardness minerals)
- Sodium and potassium
- Bicarbonate and carbonate (alkalinity)
- Chloride and sulfate
- Dissolved iron, which is why the number drops a little when an iron filter takes it out
In other words: mostly the ordinary minerals in well water and city water.
What a TDS reading cannot tell you
A high TDS reading does not mean your water is unsafe, and a low one does not mean it is safe. Lead and arsenic matter at parts per billion (ppb), a thousand times smaller than the parts per million a TDS meter reads. The EPA's action level for lead is 15 ppb (0.015 ppm) today and drops to 10 ppb on November 1, 2027, and the EPA's health goal for lead is zero. The limit for arsenic is 10 ppb.
What a TDS Meter Cannot Detect
- Lead (matters at 15 ppb, far below meter resolution)
- Arsenic (matters at 10 ppb)
- Nitrate (contributes a little conductivity, but the meter cannot distinguish it from minerals)
- Bacteria, E. coli, viruses (not dissolved solids, no conductivity)
- Pesticides, VOCs (non-ionic, essentially invisible)
Low-TDS water that still had a problem
These are the certified lab tests my customers sent in through September 2026 where the TDS came back under 300 ppm. Each row counts only the tests that checked that item.
| What the lab found | Tests under 300 ppm TDS | Lowest TDS with this problem |
|---|---|---|
| Coliform bacteria | 94 of 227 tests | 14.8 ppm |
| E. coli | 11 of 227 tests | 56.9 ppm |
| Lead, 15 ppb or more | 13 of 170 tests | 22.3 ppm |
| Arsenic, over 10 ppb | 6 of 108 tests | 103 ppm |
| Nitrate, over 10 ppm | 3 of 175 tests | 63.5 ppm |
Put together, 104 of those 261 low-TDS tests had at least one of these. A caller replacing a 27-year-old system read me his report in July: TDS 142, and a "heavy presence of iron bacteria."
What your TDS number means
There is no health limit for TDS. The EPA's 500 ppm line is a secondary standard: non-mandatory guidance about hardness, deposits, colored water, staining and salty taste, and the EPA does not enforce it. The World Health Organization proposes no health-based value for TDS either and rates water by taste instead. What I tell callers is the same: "Anything below 500 on total dissolved solids is acceptable by EPA standards."
| TDS reading | What it usually means | My customers' lab tests |
|---|---|---|
| Under 50 ppm | Very few minerals. The WHO notes very low TDS can taste flat. | 22 of 411 tests |
| 50 to 300 ppm | The WHO rates under 300 as excellent for taste. | 239 of 411 tests |
| 300 to 500 ppm | More minerals. Still under the EPA's 500 line. | 83 of 411 tests |
| 500 to 900 ppm | Over the EPA line: expect scale, spots, staining or a salty taste. | 46 of 411 tests |
| 900 to 1,200 ppm | The WHO rates this poor for taste. | 6 of 411 tests |
| Over 1,200 ppm | The WHO rates this unacceptable for taste. Past 1,000, the USGS no longer counts it as fresh water. | 15 of 411 tests |
The middle of my customers' tests is 218 ppm. Over 500, the only thing that brings the number down at the kitchen tap is reverse osmosis. When a caller read me a TDS of 513, I told him: "the only way you can remove that is with an RO under the sink." The 500 line is about drinking water, not bathing, showers or laundry, so the fix goes at the sink, not on the whole house. One iron filter buyer wrote that she was dealing with "high TDS (1500) with some salinity encroachment," and I pointed her to sediment filters, a softener and an under-sink RO.
Send me your TDS number
Text a photo of the meter or the report and tell me where the water came from: the tap, the well or the RO faucet. I will tell you what the number means for your water.
8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.What changes a TDS reading
Only reverse osmosis drops a TDS reading by a lot. Most other treatment barely moves it, and an acid neutralizer raises it. A caller in August put a TDS meter on the whole-house filter he had just installed and got almost the same number as his tap, while his RO faucet read "barely anything." Nothing was wrong with his filter. I told him: "Only reverse osmosis is gonna change the TDS. Nothing else will."
| Water treatment you have | What it does to TDS | Why |
|---|---|---|
| Reverse osmosis at the sink | Drops the reading a lot | The membrane holds back dissolved minerals and salts. |
| Whole-house or carbon filter | Barely changes the reading | Carbon handles chlorine, tastes and odors, not dissolved minerals. |
| Water softener | Reading stays about the same | A softener swaps calcium and magnesium for sodium instead of removing them. |
| Acid neutralizer | The reading goes up | Calcite dissolves into the water as it raises the pH. |
| Iron filter | Can drop the reading slightly | Dissolved iron counts in the TDS, so taking it out lowers the number. |
One acid neutralizer buyer wrote that his well went from pH 5.5 to 7.2 and "It took my tds from 25 to 135 so no worries there either." That is the calcite doing its job. To know whether a softener or an iron filter is working, test the water for hardness or iron, not TDS.
Checking your RO with a TDS meter
To check a reverse osmosis unit, read the tap, read the RO faucet, and divide. The same pen reads both glasses, so its factor cancels out when you divide one reading by the other. That is why this is the job a TDS meter does best.
- Run the RO faucet for a few minutes first. Water that sat in the unit overnight reads higher until it flushes; the Water Quality Association says it should improve after 5 to 10 minutes of use.
- Read your tap water.
- Read the RO faucet.
- Divide the RO number by the tap number. Tap 300 and RO 15 is 5 percent, meaning 95 percent came out.
| RO reading compared to tap | What it means |
|---|---|
| Under 10 percent of tap | The membrane is working. FilmTec rates its new 50-gallon-per-day membrane at 98 to 99 percent removal in its own test. |
| 10 to 25 percent of tap | The membrane is fading. Plan a new one. |
| Over 25 percent of tap | Time for a new membrane. Watts, which makes reverse osmosis units, recommends one when the reduction falls below 75 percent. |
For the MAW-50, the membrane is the TFC-50 replacement membrane ($135). Write each month's two numbers on the filter housing, and the trend tells you when it is due.
Does a higher TDS mean dirtier water?
A higher TDS number means more dissolved minerals, not dirtier water. If bottled or filtered water reads lower than your tap, all that shows is that it has fewer minerals in it. A low reading says nothing about bacteria, lead or arsenic, as the lab tests above show.
A demo you may see online runs a current through two glasses with an "electrolysis" gadget, and the tap water turns brown or green. Full Fact, a fact-checking organization, looked into it: the color comes from the gadget's own metal electrodes corroding. In their words, "When current is passed through the system the iron from the electrode will dissolve in the water causing it to turn brownish." Distilled water barely conducts, so its glass stays clear.
When you need a lab test instead
When the question is whether your water is safe to drink, you need a certified lab test, not a meter. A lab measures lead, arsenic, nitrate and bacteria one by one, at the levels where they matter. On a private well, the CDC says to test at least once every year for total coliform bacteria, nitrates, total dissolved solids and pH.
One test shows what the meter cannot
The Well Water Test Kit covers 53 contaminants through certified labs in the SimpleLab network: lead, arsenic, nitrate, bacteria, uranium, fluoride, plus the pH, iron, hardness, and TDS numbers needed to size any treatment. Aidan reviews every report personally.
Mail-in kit, run by an accredited lab.On city water, the City Water Test Kit ($199, 47 contaminants) looks for what can get into the water between the treatment plant and your tap, including lead and copper from plumbing and chlorine or chloramine. Once you have a report, how to read well water test results walks through every line.
Frequently asked questions
What does a TDS meter actually measure?
Electrical conductivity. Dissolved mineral ions carry electrical charge, so the meter passes a small current through the water, measures how easily it flows, and multiplies by an assumed conversion factor (typically 0.5 to 0.7) to estimate total dissolved solids in ppm. It is an indirect estimate: the factor is a guess about your specific mineral mix, and anything non-conductive (bacteria, pesticides, most organics) is invisible to it.
Is TDS the same as water hardness?
No. Hardness counts only calcium and magnesium, the minerals that cause scale and soap problems. TDS counts every dissolved ion, including sodium, chloride, sulfate, and bicarbonate. Water can have high TDS and low hardness (coastal or road-salt-influenced water heavy in sodium chloride) or moderate TDS that is mostly hardness. A TDS meter cannot size a water softener; that requires an actual hardness number in grains per gallon from a titration test or lab report.
Are TDS meters worth it?
For checking a reverse osmosis faucet against your tap, yes; a basic pen is all that job needs. For deciding whether water is safe to drink, no, because it cannot see bacteria, lead, arsenic or nitrate at the levels that matter.
Is TDS in water good for you?
TDS is not good or bad for you by itself. It is a measure of how much is dissolved, and the WHO proposes no health-based limit for it. Very low TDS water, like reverse osmosis water, is healthy to drink: it is very clean, very safe water, though it can taste flat.
Is a TDS level of 400 safe for drinking water?
A TDS of 400 is under the EPA's 500 ppm guideline, and the WHO rates water between 300 and 600 as good for taste. Whether it is safe depends on what the 400 is made of, which a TDS meter cannot tell you and a certified lab test can.
How do I know if there's bacteria in my water?
Only a lab test for total coliform and E. coli will tell you. A TDS meter cannot, and low-TDS water in my customers' lab tests still had coliform.
Aidan Walsh
Co-owner & Residential Water Treatment Specialist, Mid Atlantic WaterAidan Walsh has spent 32 years in residential water treatment, the first stretch of it installing and servicing well systems before Mid Atlantic Water became an online supplier. Mid Atlantic Water is a family company; every recommendation on this site is based on what actually failed in the field and what fixed it. If you have a question this page did not answer, call 800-460-5810 or text him at 443-277-2204.
Aidan answers calls and texts 8 AM to 5 PM Eastern, every day.Number copied. Text it from your phone: 443-277-2204.