325
ppm (mg/L)
0.325
g/L
—
325
ppm (mg/L)
0.325
g/L
—
Total Dissolved Solids (TDS) represents the combined content of all inorganic and organic substances dissolved in water, including minerals, salts, metals, cations, and anions. TDS is a critical parameter in water quality assessment, affecting taste, health, industrial processes, and agricultural irrigation. The most practical method to estimate TDS is from electrical conductivity (EC) using the relationship: TDS = EC × conversion factor.
The conversion factor typically ranges from 0.5 to 0.7, with 0.65 being the most commonly used value for mixed-ion natural water. The exact factor depends on the ionic composition of the water: waters dominated by sodium chloride have a factor closer to 0.5, while waters with higher proportions of calcium and magnesium salts use factors near 0.7. For precise work, the factor should be calibrated against gravimetric TDS measurements.
This calculator converts an EC reading (in μS/cm) to TDS (in ppm or mg/L) and classifies the water quality according to standard guidelines. TDS below 300 mg/L is considered excellent drinking water, while values above 1200 mg/L are generally unacceptable. The World Health Organization recommends TDS below 600 mg/L for good palatability, though water with TDS below 100 mg/L may taste flat.
The TDS estimation from electrical conductivity uses a linear approximation:
TDS (mg/L) = EC (μS/cm) × k
Where:
The conversion factor varies with ionic composition:
The water quality classification follows widely accepted standards:
For accurate TDS measurement, the gravimetric method (evaporating a known volume and weighing the residue at 180°C) is the reference standard. However, EC-based estimation is far more practical for field measurements and continuous monitoring. Most handheld TDS meters are actually EC meters with a built-in conversion factor.
The TDS value reflects the total mineral content of the water. Drinking water with 300-500 mg/L TDS is typical of municipal supplies and tastes normal. Water below 100 mg/L (distilled, reverse osmosis) tastes flat and may leach minerals from distribution pipes. Water above 1000 mg/L has a noticeable salty or mineral taste and may cause scaling in pipes and appliances.
The water quality class provides a quick assessment for drinking water suitability. For irrigation, aquaculture, industrial cooling, or boiler feed, different TDS thresholds apply — consult industry-specific guidelines.
Inputs
Results
Tap water with EC of 450 μS/cm and a standard conversion factor of 0.65 has a TDS of approximately 293 mg/L, classified as excellent quality drinking water.
Inputs
Results
Brackish groundwater with EC of 3000 μS/cm (using 0.55 factor for sulfate-rich water) has TDS of 1650 mg/L. This exceeds all drinking water standards and requires desalination treatment.
For most freshwater applications, use 0.65 as a general-purpose factor. If you know the water is sodium chloride-dominated (coastal areas, road salt runoff), use 0.5. For hard water from limestone regions (high Ca²⁺/Mg²⁺), use 0.7. For the most accurate results, calibrate by comparing EC readings to gravimetric TDS measurements for your specific water source.
TDS meters use their own built-in conversion factor, which is typically 0.5 or 0.7 depending on the manufacturer and model. Check your meter's specifications to find its factor. If it uses 0.5 and you use 0.65, the results will differ by 30%. Some advanced meters allow you to set a custom conversion factor.
No. Water hardness refers specifically to the concentration of calcium and magnesium ions (sometimes also iron and manganese), while TDS includes all dissolved substances — sodium, potassium, chloride, sulfate, bicarbonate, silica, and even dissolved organic matter. Water can have high TDS but low hardness (e.g., soda lakes with high sodium) or low TDS but high relative hardness.
The WHO guidelines state that water with TDS below 600 mg/L is generally considered good quality and palatable. Water with TDS above 1000 mg/L is increasingly unpalatable. There is no formal health-based guideline, as TDS itself is not directly harmful — the concern is with individual constituents (arsenic, lead, nitrate, etc.) that may be elevated in high-TDS water.
The linear EC-to-TDS relationship breaks down at high concentrations (above ~5000 μS/cm). Seawater has an EC of about 50,000 μS/cm and TDS of ~35,000 mg/L. At these concentrations, the relationship is non-linear, and specialized salinity equations (UNESCO Practical Salinity Scale) should be used instead of a simple multiplication factor.
Electrical conductivity increases approximately 2% per degree Celsius. Most EC meters compensate to a reference temperature of 25°C. If your meter does not auto-compensate, apply a correction: EC₂₅ = EC_measured / (1 + 0.02 × (T - 25)). TDS itself (the actual dissolved mass) is not temperature-dependent, but the EC measurement used to estimate it is.
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