Understanding water quality begins with three key measurements: PPM (parts per million), TDS (total dissolved solids), and EC (electrical conductivity). While each is related, they measure different aspects of the same idea — how many substances are dissolved in water.
This guide breaks down what each term means, how they’re measured, and when to use one over the other. Whether you’re testing water in a lab, managing a hydroponic system, or treating a pool, getting these numbers right matters.
What Do PPM, TDS, and EC Actually Measure?
PPM (Parts Per Million)
PPM is a unit of measurement, not a tool. It expresses how much of one substance is present in a million parts of another. In most water testing contexts, 1 PPM equals 1 mg of solute per liter of water (mg/L).
It’s widely used in:
- Water treatment
- Environmental testing
- Fertilizer solutions
- Pool chemistry
To calculate PPM directly, you can use the PPM Solution Calculator or learn from our guide: How to Calculate PPM in Water.
TDS (Total Dissolved Solids)
TDS refers to the total concentration of all dissolved substances in water — minerals, salts, metals, and even organic matter. It’s usually measured in PPM but represents a broader chemical picture than a single compound.
High TDS doesn’t always mean water is unsafe, but it can affect:
- Taste and odor
- Conductivity
- Health (if levels exceed safety standards)
You can convert or estimate TDS with the TDS to PPM Calculator if you have EC values from a meter.
EC (Electrical Conductivity)
EC measures how well water conducts electricity, which increases with more dissolved ions. It’s expressed in µS/cm (microsiemens per centimeter) or mS/cm (millisiemens per centimeter).
Because ions like calcium, magnesium, or chloride carry charge, EC is a fast way to estimate dissolved solids indirectly.
Typical EC readings:
- <500 µS/cm — Clean water
- 500–1500 µS/cm — Fertilizer solutions
- >2000 µS/cm — Industrial or contaminated water
For conversions, try the EC to PPM Calculator or PPM to Microsiemens Calculator.
How Are These Values Measured?
💧 Instruments Commonly Used
| Parameter | Tool | Units |
|---|---|---|
| PPM | Digital calculators, lab analysis | mg/L (PPM) |
| TDS | TDS meter | PPM |
| EC | EC meter | µS/cm or mS/cm |
Important note:
Most TDS meters are actually EC meters with a built-in conversion. They multiply EC by a factor (commonly 0.5 or 0.7) to estimate TDS in PPM. This leads to different readings depending on the calibration.
PPM vs. TDS vs. EC: What’s the Difference?
Key Comparison Table
| Feature | PPM | TDS | EC |
|---|---|---|---|
| Type | Unit | Value/estimate | Value/measurement |
| What it represents | Ratio of solute to solvent | Total dissolved solids | Conductivity |
| Unit | mg/L (PPM) | mg/L (usually in PPM) | µS/cm or mS/cm |
| Measures | One substance or total content | Total solids (measured or estimated) | Ion concentration (indirect) |
| Used in | Labs, pools, hydroponics | Water filtration, drinking water | Hydroponics, agriculture, labs |
| Direct measurement? | Sometimes | Yes | Yes |
Example:
You test a hydroponic solution:
- EC: 1.4 mS/cm
- Using a 0.5 factor: TDS ≈ 700 PPM
- But if the TDS meter uses 0.7: TDS ≈ 980 PPM
So even though the EC is fixed, your PPM can vary by the meter used. That’s why EC is often considered more scientifically accurate.
Which Should You Use?
For Home Use or Hydroponics
- TDS (PPM) is easier to understand and interpret
- Many growers prefer TDS for feeding schedules
See our Fertilizer PPM Calculator or Nutrient PPM Calculator to fine-tune concentrations.
For Scientific or Industrial Labs
- EC is preferred because it’s directly measurable and doesn’t rely on conversion factors
- PPM is used in formal documentation when referencing standards or compliance (e.g., Safe PPM Levels in Drinking Water)
Conclusion
While PPM, TDS, and EC are often used together, they’re not interchangeable. PPM is a unit of concentration, TDS tells you how much is dissolved, and EC reflects conductivity.
Understanding how they differ — and when to use each — helps ensure more accurate readings, better system control, and more confident decision-making.