Why a TDS meter can't tell you if your water is safe

What a TDS meter actually measures

Total dissolved solids is the total weight of everything dissolved in water. A lab measures it by filtering a sample, evaporating it and weighing what's left. A handheld TDS meter doesn't do that. It measures electrical conductivity and converts it to an estimate.^[source]

The Water Quality Association's own technical glossary explains the catch: conductivity is only an estimate of TDS, because it isn't precisely proportional to the weight of what's dissolved, and substances that don't conduct electricity can't be measured by electrical tests at all.^[source] In practice, most of what a meter is reading is ordinary dissolved minerals like calcium, magnesium and sodium.

Local TDS numbers, in context

Charlotte Water's 2025 report lists average TDS of 59 ppm (52–66).^[source] EPA's secondary guideline is 500 mg/L, and above it the noticeable effects EPA lists are hardness, deposits, colored water, staining and a salty taste, all aesthetic.^[source] EPA describes secondary contaminants as not considered a health risk at those guideline levels.^[source]

Other local systems vary. Greenville Water's finished water averaged 29.5 mg/L,^[source] Spartanburg Water reported 62 ppm,^[source] and the Catawba River water Union County Water buys averaged 149 ppm, a figure its report notes was "measured thru conductivity", the same principle as a handheld meter.^[source] For perspective, FDA requires bottled water labeled "mineral water" to contain at least 250 ppm of dissolved solids.^[source] A higher number isn't a contamination signal; it often just means more minerals.

How in-home demos mislead

Door-to-door water demos often rely on tests that produce a dramatic result regardless of whether anything is wrong. The Water Quality Association has warned about salespeople who run a free test, then claim a hardness test shows the water is unsafe.^[source]

• Precipitation (color-change) tests. WQA explains these use chemicals or electricity to make hardness minerals turn dark brown or black. It's a valid way to measure hardness, but it does not show pollution or indicate a health concern.^[source]
• TDS meter before-and-after. The salesperson reads your tap, then reads water from their reverse osmosis unit and shows a much lower number. RO does remove dissolved inorganics,^[source] so the drop is real, but it only shows that minerals went down. It says nothing about whether your tap water had a problem in the first place.
• The carbon-filter trick in reverse. Carbon filters generally don't remove dissolved minerals,^[source] so a certified carbon filter that reduces chlorine taste may barely change a TDS reading. That doesn't mean it isn't working; it means the meter can't see what the filter removes.

WQA's advice is to have water tested by a water treatment professional or a certified lab, and to choose products certified to industry standards.^[source] We agree, and we don't run these demos at all. It's on our list of things we won't sell you.

When a TDS meter is useful

A TDS meter is a fine tool for one job: checking that a reverse osmosis system is still working. Compare the reading before and after the RO membrane over time. If the treated water's TDS creeps up, the membrane or filters may need service. That's how a reverse osmosis system can be monitored between lab tests, and it's part of what a Care Plan visit checks.

What to use instead

1. Step 1

   On city water, read your utility's report

   Your utility tests far more than a meter can. Find yours with our water lookup tool or browse our Charlotte-area water report.

2. Step 2

   On a well, use a certified lab

   A certified-lab panel measures specific contaminants by name. Our water testing service arranges one.

3. Step 3

   Match treatment to a named result

   Choose a product certified for the specific contaminant you want reduced. Our NSF/ANSI standards guide shows how to check.