# TDS meter reading: what the number means, what is too high, and what the pen cannot see > A TDS pen reads conductivity times a factor, not contaminants. EPA's secondary standard is 500 mg/L; NHMRC rates 0 to 600 mg/L good for taste. | | | |---|---| | Source | https://ownerspec.com/water/guides/tds-meter-reading-what-it-means/ | | Publisher | OwnerSpec (https://ownerspec.com/) | | Published | 2026-09-14 | | Updated | 2026-09-14 | | Facts verified | 2026-09-14 | | Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. | --- ## Short answer A TDS meter reading is an estimate of total dissolved solids in mg/L, also written ppm, made by measuring the water's electrical conductivity and multiplying it by a factor: the HM Digital TDS-3 uses a NaCl factor averaging 0.5 per its spec page. Total dissolved solids are the minerals and salts dissolved in water, mostly calcium, magnesium, sodium, chloride, sulfate and bicarbonate. EPA's secondary standard sets a non-enforced aesthetic ceiling of 500 mg/L, and the WHO 2004 palatability bands reproduced by Australia's NHMRC rate 0 to 600 mg/L good and only above 1,200 mg/L unacceptable. A reading under 500 mg/L says nothing about lead, PFAS, nitrate or bacteria, because a pen cannot resolve any of them at the levels that matter. Use the pen for the two jobs it does well: spotting a change in tap water over time, and checking a reverse osmosis membrane by comparing feed and product readings against the maker's published 90 to 99 percent rejection. ## Fact sheet - **Answer:** A TDS meter converts conductivity to an mg/L estimate with a fixed factor, about 0.5 for NaCl on the HM Digital TDS-3. EPA's secondary standard sets a non-enforced ceiling of 500 mg/L for total dissolved solids, and the WHO 2004 bands reproduced by Australia's NHMRC rate 0 to 600 mg/L good and above 1,200 mg/L unacceptable. The pen cannot resolve lead, PFAS, nitrate or bacteria. - **Basis:** EPA National Secondary Drinking Water Standards: total dissolved solids 500 mg/L, a non-enforced aesthetic guideline; palatability bands from WHO 2004 as reproduced by the Australian NHMRC guidelines. - **Assumptions:** The reading comes from a pen with a NaCl conversion factor, the default on consumer meters; Feed and product readings for the rejection check are taken with the same meter on the same day; Meter ranges, accuracies and factors are the makers' published figures, not measured - **Method:** Limits from EPA's secondary standards page and the Lead and Copper Rule page, palatability bands from the NHMRC guideline, meter specs from HM Digital's product pages, the rejection figure from APEC's FAQ, all read on 2026-09-14; nothing was tested. - **Primary source:** [EPA, Secondary Drinking Water Standards: Guidance for Nuisance Chemicals](https://www.epa.gov/sdwa/secondary-drinking-water-standards-guidance-nuisance-chemicals) (government or regulatory source) - **Facts verified:** 2026-09-14 - **Last updated:** 2026-09-14 - **Canonical URL (cite this):** https://ownerspec.com/water/guides/tds-meter-reading-what-it-means/ If you bought the pen to check for lead, you can stop here: EPA's action level of 0.015 mg/L sits below the 1 ppm step the TDS-3 resolves, so a low reading proves nothing about lead and the [lead guide](/water/guides/lead-in-tap-water-service-line/) is the page you need. ## What does the reading tell you? The number is a sum, not a list: it says how much conductive material is dissolved in the glass, not which ions make it up. | Reading | Likely cause | Confirm with | |---|---|---| | 0 to 50 mg/L | Reverse osmosis, distilled or deionized product water | Nothing further for TDS; decide on remineralization separately | | 50 to 300 mg/L | Typical municipal water or a shallow well | No action from the TDS number alone | | 300 to 600 mg/L | Higher mineral content, still inside the NHMRC "good" band | A certified lab test if you notice taste, deposits or staining | | 600 to 1,200 mg/L | Above EPA's 500 mg/L ceiling; NHMRC rates it fair to poor for taste | A full lab panel to learn which ions are elevated | | Above 1,200 mg/L | NHMRC "unacceptable" palatability band | A lab panel before choosing any treatment; TDS alone does not name the contaminant | The bands are taste bands. A 700 mg/L well can pass every primary standard, and a 120 mg/L tap can carry lead from a service line, so the reading decides what to test, never what to filter. ## What does the number mean? EPA's Secondary Drinking Water Standards list total dissolved solids at 500 mg/L and state that EPA does not enforce these secondary maximum contaminant levels; they are guidelines for aesthetic effects, which the table records as hardness, deposits, staining and a salty taste. The full list of primary and secondary limits is on the [EPA limits table](/water/guides/epa-drinking-water-limits-table/). The Australian NHMRC guideline reproduces the WHO 2004 palatability table: 0 to 600 mg/L good, 600 to 900 mg/L fair, 900 to 1,200 mg/L poor, above 1,200 mg/L unacceptable, and it puts the guideline value for good palatability at 600 mg/L. Conductivity is the ease with which water carries an electric current, and it rises with the amount of dissolved ions. USGS describes specific conductance as the real-time surrogate for total dissolved solids, and every consumer pen works that way: it measures conductance and applies a conversion factor. The factor is where two pens disagree on the same glass. HM Digital's TDS-3 page lists a fixed NaCl factor averaging 0.5, while its COM-100 page lists NaCl, KCl and 442 conversions the user selects, with a range of 0 to 5,000 ppm on the 0.5 scale and 0 to 8,560 ppm on the 0.7 scale. A lab reports TDS by evaporation and weighing, so expect the pen and the lab to differ by the factor, not by a fault. ## What can a TDS pen not see? | The pen reads | The pen cannot resolve | |---|---| | Calcium, magnesium, sodium, chloride, sulfate and bicarbonate, the bulk of tap and well TDS | Lead: EPA's Lead and Copper Rule action level is 15 ppb, which is 0.015 mg/L, below the 1 ppm resolution HM Digital lists for the TDS-3 | | Any ion present at high enough concentration to shift conductivity | Bacteria: a coliform result is a lab culture, not a conductivity reading, so a low TDS number says nothing about it | | A change over time, if you read the same tap with the same meter | PFAS and most organic contaminants: non-ionic or present at parts-per-trillion levels, far under a ppm-scale pen | | A remineralization cartridge adding calcium or magnesium back into RO water | Nitrate at EPA's 10 mg/L maximum contaminant level: a small addition on a 100 to 500 ppm baseline, masked by the meter's plus or minus 2 percent accuracy | Each of those needs a lab panel. The [well water test panel](/water/guides/well-water-test-panel/) lists what to order and the [water test kit review](/water/reviews/best-water-test-kit-well-and-city/) compares the kits; the guides on [PFAS](/water/guides/pfas-in-tap-water-what-filter/) and [nitrate](/water/guides/nitrate-in-well-water/) cover each contaminant's limit and treatment class. ## How do you check an RO membrane with it? Read the tap water that feeds the system, then the water from the RO faucet, with the same meter on the same day. Rejection in percent is (feed TDS minus product TDS) divided by feed TDS, times 100. APEC's FAQ states that its high-rejection membranes remove 90 to 99 percent of dissolved solids. A 300 ppm feed should therefore read 3 to 30 ppm at the faucet; a 300 ppm feed reading 90 ppm is 70 percent rejection, well below the published figure, and points at a fouled or failing membrane rather than a wrong meter. Two things move the product reading without a fault. Water that sat in the tank for hours reads higher than a fresh draw, so run the faucet first. And a remineralization stage adds calcium or magnesium back on purpose, so on those systems read the product line before the mineral cartridge, or accept the higher number; the [remineralization guide](/water/guides/reverse-osmosis-remineralization-do-you-need-it/) covers why a maker adds it. The [RO contaminant table](/water/guides/does-reverse-osmosis-remove-contaminant-table/) lists what the membrane rejects by contaminant, and the [1812 membrane cross-reference](/water/parts/reverse-osmosis-membrane-cross-reference-1812/) covers the replacement once the check fails. ## Which meter fits the job? | Meter | Range | Accuracy | Conversion factor | Skip it if | |---|---|---|---|---| | HM Digital TDS-3 | 0 to 9,990 ppm, 1 ppm steps to 999 then 10 ppm steps | Plus or minus 2 percent | Fixed, NaCl averaging 0.5 | You need to switch scales to match a lab method | | HM Digital COM-100 | 0 to 5,000 ppm on the 0.5 scale, 0 to 8,560 ppm on the 0.7 scale | Plus or minus 2 percent | User-selectable NaCl, KCl or 442 | You want a live feed-versus-product check plumbed in | | HM Digital DM-2 inline dual monitor | 0 to 9,990 ppm per channel, one probe on feed and one on product | Plus or minus 2 percent | Not published on the DM-2 page read for this article | You only want an occasional tap or well spot-check; it stays plumbed into one system | Every figure is the maker's published spec from HM Digital's product pages; nothing was tested. The COM-100 page does not publish a resolution step, and no HM Digital DM-2 listing could be confirmed on Amazon, so it is named without a link. **Before you buy anything** Do not buy a filter because the pen read high, and do not skip a lab test because it read low. The HM Digital TDS-3 covers 0 to 9,990 ppm at plus or minus 2 percent per HM Digital and is enough for the two jobs on this page: a tap reading logged over time and a feed-versus-product membrane check. If the reading is above 600 mg/L, or you have any reason to suspect lead, PFAS, nitrate or bacteria, the next purchase is a lab panel from the [test kit review](/water/reviews/best-water-test-kit-well-and-city/), because it is the only thing that names the contaminant. The HM Digital COM-100 earns its extra cost only if you need to match a lab's conductivity method by choosing the factor. ## What should you do first? 1. Rinse the probe, read the tap twice, and log the number with the date; the trend is worth more than any single reading. 2. Compare the number with the bands above: under 600 mg/L needs no action from TDS alone. 3. If you own an RO system, read the feed and the faucet the same day and compute the rejection percentage. 4. If the reading is above 600 mg/L, or above EPA's 500 mg/L ceiling with a salty taste or deposits, order a lab panel to learn which ions are elevated. 5. Filter for the contaminant the lab names, never for the TDS number; the [test report guide](/water/guides/how-to-read-a-water-test-report/) explains the panel line by line. ## Frequently asked questions ### Is EPA's 500 mg/L TDS limit enforceable? No. EPA lists total dissolved solids at 500 mg/L in its National Secondary Drinking Water Standards and states that it does not enforce these secondary maximum contaminant levels. They are guidelines for public water systems to manage aesthetic effects, which for TDS the table records as hardness, deposits, colored water, staining and a salty taste. A reading above 500 mg/L breaks no federal rule. ### Can a TDS meter detect lead in tap water? No. EPA's Lead and Copper Rule sets an action level of 15 parts per billion, which is 0.015 mg/L, while the HM Digital TDS-3 resolves 1 ppm steps against a mineral background that typically reads 100 to 500 ppm. Lead at the action level is invisible inside that number. Only a lab test resolves lead, and the same applies to PFAS and bacteria. ### What TDS reading is good for taste? The WHO 2004 palatability bands, reproduced in the Australian NHMRC drinking water guidelines, rate 0 to 600 mg/L good, 600 to 900 mg/L fair, 900 to 1,200 mg/L poor and above 1,200 mg/L unacceptable, and the NHMRC guideline says TDS should not exceed 600 mg/L for good palatability. EPA's own aesthetic ceiling is lower at 500 mg/L. Both are taste figures, not health figures. ### How do I check my RO membrane's rejection with a TDS meter? Read the tap water feeding the system and the water from the RO faucet with the same meter, then compute (feed minus product) divided by feed, times 100. APEC's FAQ states its high-rejection membranes remove 90 to 99 percent of dissolved solids, so a 300 ppm feed should read around 15 ppm at the faucet. A result far below the maker's published figure points at a fouled or failing membrane. ### What does a TDS meter's conversion factor do? The meter measures electrical conductivity and multiplies it by a factor from a reference-salt curve to estimate mg/L. HM Digital's TDS-3 page lists a fixed NaCl factor averaging 0.5; the COM-100 page lists user-selectable NaCl, KCl or 442 conversions, with 0 to 5,000 ppm on the 0.5 scale and 0 to 8,560 ppm on the 0.7 scale. Two pens with different factors give different numbers on the same glass. ## Sources - [EPA, Secondary Drinking Water Standards: Guidance for Nuisance Chemicals](https://www.epa.gov/sdwa/secondary-drinking-water-standards-guidance-nuisance-chemicals) (government or regulatory source) - [EPA, Lead and Copper Rule](https://www.epa.gov/dwreginfo/lead-and-copper-rule) (government or regulatory source) - [Australian NHMRC, Australian Drinking Water Guidelines, Total Dissolved Solids](https://guidelines.nhmrc.gov.au/australian-drinking-water-guidelines/part-5/physical-chemical-characteristics/total-dissolved-solids) (manufacturer or vendor document) - [USGS, Chloride, Salinity, and Dissolved Solids](https://www.usgs.gov/mission-areas/water-resources/science/chloride-salinity-and-dissolved-solids) (government or regulatory source) - [HM Digital, TDS-3 product page](https://hmdigital.com/tds-3/) (manufacturer or vendor document) - [HM Digital, COM-100 product page](https://hmdigital.com/com-100/) (manufacturer or vendor document) - [HM Digital, DM-2 product page](https://hmdigital.com/dm-2/) (manufacturer or vendor document) - [APEC Water Systems, FAQ (membrane rejection)](https://www.apecwater.com/pages/faq) (manufacturer or vendor document) --- Citation: OwnerSpec, "TDS meter reading: what the number means, what is too high, and what the pen cannot see", https://ownerspec.com/water/guides/tds-meter-reading-what-it-means/, facts verified 14 September 2026. 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