What Is TDS (Total Dissolved Solids)?
TDS stands for Total Dissolved Solids - a measurement of everything dissolved in your water. This includes calcium, magnesium, sodium, potassium, chloride, sulfate, bicarbonate, and trace amounts of other minerals and organic compounds.
TDS is measured in parts per million (PPM) or milligrams per liter (mg/L) - they're the same unit. A TDS meter works by measuring the electrical conductivity of water: the more dissolved ions present, the better water conducts electricity, and the higher the TDS reading.
Think of TDS as a catch-all number. It tells you how much “stuff” is dissolved in your water, but it doesn't tell you what that stuff is. Water with 300 PPM TDS could be full of calcium (hard water) or full of sodium (soft water) - the TDS reading alone can't distinguish between the two.
What Is Water Hardness?
Water hardness measures specifically the concentration of calcium (Ca) and magnesium (Mg) dissolved in water, expressed as milligrams per liter (PPM) of calcium carbonate equivalent (CaCO3).
Hardness is what causes the problems people associate with “bad” water in the home: scale buildup in pipes and appliances, spots on dishes and shower doors, dry skin and hair, and soap that won't lather properly. These effects are caused specifically by calcium and magnesium - not by TDS in general.
The USGS water hardness scale classifies water into five levels:
Hardness is measured using test strips (quick but approximate), liquid drop titration kits (more accurate), or laboratory analysis (most precise). For details on each method, see our guide to testing water hardness.
Key Differences: TDS vs Hardness
Here's the clearest way to understand the difference:
| TDS | Water Hardness | |
|---|---|---|
| Measures | All dissolved solids | Only calcium + magnesium |
| Units | PPM (mg/L) | PPM (as CaCO3) or GPG |
| Test method | TDS meter (conductivity) | Test strips or titration kit |
| Causes scale? | Not directly | Yes - calcium and magnesium cause scale |
| Removed by softener? | No (sodium replaces Ca/Mg) | Yes |
| Removed by RO? | Yes (90-99% reduction) | Yes |
The most important takeaway: a water softener lowers hardness but not TDS. After softening, your TDS reading may stay the same or even go up slightly because sodium ions replace the calcium and magnesium ions. Your water will stop causing scale, but the TDS meter won't reflect the change.
Can a TDS Meter Measure Hardness?
No. This is one of the most common misconceptions in home water testing.
A TDS meter measures total electrical conductivity, which reflects the sum of all dissolved ions. It cannot distinguish between calcium, magnesium, sodium, or any other mineral. If your water has 200 PPM TDS, that could mean:
- 200 PPM hardness (all calcium/magnesium) - hard water that will cause scale
- 50 PPM hardness + 150 PPM sodium/chloride - slightly hard water that won't cause much scale
- 0 PPM hardness + 200 PPM sodium - perfectly soft water (e.g., softened water)
All three would show the same TDS reading but have completely different hardness levels and completely different effects on your home.
To actually test hardness, use:
- Test strips ($8-15) - quick, approximate, good for a general idea
- Liquid drop titration kit ($15-25) - more precise, counts drops until color change
- Lab test ($25-50) - most accurate, tests for hardness plus other parameters
Or just look up your zip code for free - we have hardness data for 31,000+ US locations.
When to Measure TDS vs Hardness
Both measurements have their place, but they answer different questions:
Measure Hardness When:
- You see white scale on faucets, showerheads, or dishes
- Soap doesn't lather well or leaves a film
- You're considering a water softener
- You're experiencing dry skin or hair problems
- You want to know if your water will damage appliances
Measure TDS When:
- You're monitoring a reverse osmosis filter performance
- You want a general water quality snapshot
- You're checking if your RO membrane needs replacing (TDS creep = time to replace)
- You're comparing water from different sources
For most homeowners concerned about scale, spotting, and appliance damage, hardness is the number that matters. TDS is useful for RO system maintenance and general water quality monitoring, but it won't tell you whether you need a softener. For a full breakdown of testing options, see our complete testing guide. For what the hardness numbers mean, check our hard water vs soft water comparison.
Frequently Asked Questions
Is TDS the same as water hardness?
No. TDS (Total Dissolved Solids) measures everything dissolved in water, including sodium, chloride, sulfate, and other minerals. Water hardness measures only calcium and magnesium. A water sample could have high TDS but low hardness (e.g., softened water with lots of sodium), or moderate TDS with high hardness (e.g., well water dominated by calcium).
Can I use a TDS meter to test for hard water?
A TDS meter cannot accurately measure water hardness. TDS meters measure total electrical conductivity, which reflects all dissolved minerals, not just calcium and magnesium. To test hardness specifically, you need a hardness test strip, a liquid drop titration kit, or a lab test. These methods target the calcium and magnesium ions that define hardness.
Does a water softener lower TDS?
A salt-based water softener does not lower TDS. It swaps calcium and magnesium ions for sodium ions, so the total dissolved solids stay roughly the same or even increase slightly. Your water will be softer (less scale buildup), but a TDS meter reading won't drop. To lower TDS, you need reverse osmosis or distillation.
Does reverse osmosis remove hardness?
Yes, reverse osmosis (RO) removes both hardness minerals and other dissolved solids. An RO system typically reduces TDS by 90-99%, which includes calcium and magnesium. However, RO systems are usually installed at a single faucet for drinking water. For whole-house hard water protection, a water softener is more practical.
What is a safe TDS level for drinking water?
The EPA sets a secondary (non-enforceable) standard of 500 PPM TDS for drinking water. Below 300 PPM is considered excellent, 300-500 PPM is good, and above 500 PPM may taste salty or metallic. TDS below 50 PPM (like distilled water) can taste flat. Note that TDS alone doesn't indicate safety - a water quality test for specific contaminants is more informative than TDS alone.