When you test water quality — especially in drinking water, hydroponics, aquariums, and pools — you’ll often see TDS, PPM, and EC on meters and reports. These terms are connected but not the same.
This guide explains each one clearly so you always choose the right measurement.
TDS vs PPM vs EC
| Term | Full Form | What It Measures | Unit | Used In |
|---|---|---|---|---|
| TDS | Total Dissolved Solids | All minerals, salts & impurities dissolved in water | mg/L or ppm | Drinking water, RO filters |
| PPM | Parts Per Million | Concentration scale for TDS in water | ppm (mg/L) | Water testing, chemistry |
| EC | Electrical Conductivity | Ability to conduct electricity based on ions | mS/cm (or µS/cm) | Hydroponics, aquariums |
👉 For water testing:
1 ppm ≈ 1 mg/L
What is TDS?
TDS = Total Dissolved Solids
The total amount of minerals + salts such as:
- Calcium
- Magnesium
- Sodium
- Chloride
- Nitrate
- Phosphate
- Metals
TDS gives a broad view of water purity but does not identify each substance.
| TDS Range | Water Quality |
|---|---|
| 0–50 ppm | Very pure (RO water) |
| 50–150 ppm | Good for drinking |
| 150–300 ppm | Acceptable |
| 300–500 ppm | Hard |
| > 500 ppm | Not recommended |
What is EC?
EC = Electrical Conductivity
It measures how well water conducts electricity:
More dissolved ions → Higher EC
Typical units:
| Unit | Meaning |
|---|---|
| µS/cm | Microsiemens per cm (weakly conductive water) |
| mS/cm | Millisiemens per cm (stronger conductivity) |
EC is used heavily in:
- Hydroponic nutrient control
- Aquaculture water balancing
- Wastewater treatment
How EC converts to PPM
Meters convert EC → PPM using conversion factors:
| Factor Scale | Used Where |
|---|---|
| 500 scale | USA hydroponics |
| 640 scale | EU/Aquariums |
| 700 scale | Australia hydroponics |
Formula:
PPM = EC (mS/cm) × Factor
Example:
EC = 1.0 →
500 scale → 500 ppm
700 scale → 700 ppm
So readings change depending on your meter’s factor.
What is PPM?
PPM = Parts Per Million
It is a unit expressing concentration of dissolved solids in water.
If 1 liter of water contains 1 mg of dissolved minerals:
1 mg/L = 1 PPM
PPM is mostly used for:
- Drinking water standards
- Aquarium chemistry
- Lab concentration calculations
Which Should You Use?
| Situation | Best Measurement | Why |
|---|---|---|
| Drinking water quality | TDS / PPM | Measures impurities |
| Hydroponics | EC → converted to PPM | Controls nutrients |
| Aquariums | EC + PPM | pH stability + mineral balance |
| Wastewater | EC | Monitoring contamination changes |
| RO purifier testing | TDS (PPM) | Purification effectiveness |
❌ Misunderstanding to Avoid
TDS meters don’t actually measure TDS —
they measure EC, then convert to PPM.
So two TDS meters can give different readings because:
- One uses 500 scale
- Another uses 700 scale
Example:
Same water →
EC = 1.0 mS/cm
Meter A (500 scale): 500 ppm
Meter B (700 scale): 700 ppm
Both are correct — just different scales.
Recommended Water Ranges
| Usage | Ideal Value |
|---|---|
| Drinking Water | 50 – 150 ppm |
| Hydroponics Nutrient Water | EC 1.0 – 2.4 |
| Freshwater Aquarium | 100 – 250 ppm |
| RO Purifier Output | < 50 ppm |
✔ Key Takeaways
- TDS = Total minerals in water
- EC = Conductivity of dissolved ions
- PPM = Concentration unit for TDS
- EC meters estimate PPM using different conversion factors
- Always know which scale your meter uses
Useful Tools for This Topic
Use these free online calculators for accurate water quality measurement:
Final Notes
Understanding TDS, PPM, and EC helps you:
✔ Make drinking water safer
✔ Prevent algae in aquariums
✔ Optimize hydroponic nutrient delivery
✔ Monitor RO filter performance
Better water = better health + better growth + better balance 💧🌱🐠
