Total Dissolved Solids Calculator
ChemistryCalculate Total Dissolved Solids (TDS) in water from electrical conductivity (EC) in μS/cm. Convert to mg/L, ppm and classify drinking water quality by WHO and BIS standards.
500 μS/cm
μS/cm
TDS (mg/L)
320
TDS (ppm)
320
Water Quality Classification
Acceptable (WHO guideline)
What is a TDS?
The Total Dissolved Solids (TDS) Calculator converts an electrical conductivity (EC) reading in microsiemens per centimetre (μS/cm) into Total Dissolved Solids concentration in mg/L and ppm, and classifies the result against WHO and BIS IS 10500:2012 drinking water standards. TDS is the single most-checked water quality parameter in India — used by householders assessing RO purifier performance, water utilities monitoring distribution networks, environmental engineers evaluating groundwater, and agricultural planners assessing irrigation suitability.
TDS represents the sum of all dissolved ions in water — primarily calcium, magnesium, sodium, potassium, bicarbonates, chlorides, sulphates, and nitrates. High TDS results from water passing through mineral-rich geological formations (limestone, gypsum, evaporite deposits) or from contamination with agricultural runoff, industrial effluents, or seawater intrusion. In India, groundwater TDS ranges from below 100 mg/L in parts of the Western Ghats to above 5,000 mg/L in arid Rajasthan, Haryana, and coastal Tamil Nadu — a variation driven almost entirely by geology and depth of the water table.
The conversion from EC to TDS is the basis for every portable TDS meter on the market. These instruments measure electrical conductivity and multiply by a fixed conversion factor (k) to display a TDS reading — they do not measure TDS directly. Understanding this k factor — its default values, when it needs adjustment, and how it affects accuracy — is the key to interpreting TDS meter readings correctly. For ion-specific analysis beyond total TDS, the PPM to Molarity Calculator can convert individual ion concentrations from lab reports into molar concentrations for further analysis.
How to use this TDS calculator
- Use a handheld EC meter or TDS/EC pen (available at most hardware and laboratory supply stores in India) to measure the electrical conductivity of your water sample. Note the reading in μS/cm (not mS/cm — multiply mS/cm values by 1,000 before entry).
- Enter the EC reading in the Electrical Conductivity (EC) field.
- Adjust the TDS Conversion Factor (k) slider. Use 0.64 for general-purpose Indian tap or borewell water; use 0.67 for natural river or reservoir water following ISO standards; use 0.50 for sodium chloride-rich coastal or brackish water.
- Read TDS (mg/L) — compare this to the BIS IS 10500:2012 acceptable limit of 500 mg/L. Values between 500 and 2,000 mg/L are permissible if no alternative is available.
- Read the Water Quality Classification — this provides the regulatory context for the TDS value immediately.
- For a detailed ion analysis, request a full water chemistry report from a certified lab and use the Water Hardness Calculator for the hardness component and the Normality Calculator for equivalent-based dosing calculations.
Formula & Methodology
TDS from electrical conductivity:TDS (mg/L) = EC (μS/cm) × kWhere: -EC= electrical conductivity in microsiemens per centimetre (μS/cm) -k= conversion factor (0.5–0.9, default 0.64) -TDS (ppm) = TDS (mg/L)for dilute aqueous solutions (ρ ≈ 1 g/mL) WHO/BIS classification thresholds: | TDS (mg/L) | Classification | |---|---| | < 50 | Excellent (very pure) | | 50–150 | Good (WHO acceptable) | | 150–300 | Good (BIS acceptable) | | 300–500 | Acceptable (WHO guideline) | | 500–600 | Fair (BIS permissible limit) | | 600–1,000 | Poor — requires treatment | | > 1,000 | Not suitable for drinking | Worked example — household RO performance check: A resident in Jaipur measures tap water EC = 780 μS/cm and RO output EC = 95 μS/cm using a TDS pen set to k = 0.64.Inlet TDS = 780 × 0.64 = 499.2 mg/L → Acceptable (WHO guideline) Output TDS = 95 × 0.64 = 60.8 mg/L → Good (WHO acceptable) TDS rejection = (499.2 − 60.8) / 499.2 × 100 = 87.8%The RO system is removing approximately 88% of the dissolved solids — within normal operating range (85–95%) for a well-functioning membrane. A rejection below 80% on a mature system suggests the membrane may need replacement or the pre-filter requires servicing. For irrigation water assessment: EC = 1,200 μS/cm → TDS ≈ 768 mg/L. The FAO classifies EC above 700 μS/cm as causing slight-to-moderate restriction for sensitive crops — this source requires blending with lower-EC water for irrigation use.
Frequently Asked Questions
Total Dissolved Solids (TDS) is the total concentration of all dissolved inorganic and organic substances in water, expressed in milligrams per litre (mg/L) or parts per million (ppm). These substances include minerals such as calcium, magnesium, sodium, potassium, bicarbonates, chlorides, and sulphates, as well as traces of organic matter. TDS is a key water quality parameter used by regulators, water utilities, bottled water producers, RO system manufacturers, and environmental monitoring agencies.
TDS (mg/L) = EC (μS/cm) × k, where EC is the electrical conductivity in microsiemens per centimetre and k is an empirically determined conversion factor between 0.5 and 0.9. This relationship works because dissolved ions carry electrical charge — more ions means higher conductivity and higher TDS. The exact value of k depends on the composition of the dissolved salts and must be calibrated for a given water source.
The conversion factor k (typically 0.5–0.9) accounts for the fact that different ionic species have different equivalent conductances per mole. A sodium chloride solution has a different k than a calcium bicarbonate solution with the same TDS, because Na⁺, Cl⁻, Ca²⁺, and HCO₃⁻ all have different molar conductivities. The ISO standard recommends k = 0.67 for most natural waters; k = 0.64 is widely used for general-purpose TDS meters. For accurate results, calibrate k using a gravimetric TDS measurement and EC reading from the same sample.
EC measures the ability of a water sample to conduct electricity — a direct physical measurement made by a conductivity meter in μS/cm or mS/cm. TDS is a calculated estimate of the total dissolved ion mass derived from EC using the k factor. EC can be measured directly and instantaneously; TDS is always derived (unless measured gravimetrically by evaporating a known volume and weighing the residue). TDS meters in the field are really EC meters reporting a derived TDS value.
TDS includes all dissolved ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻, SO₄²⁻, NO₃⁻, and others), while water hardness is specifically the concentration of calcium and magnesium ions expressed as CaCO₃ equivalents. Hard water always contributes to TDS, but high-TDS water is not necessarily hard — it may be dominated by sodium chloride (as in brackish water) which contributes to TDS without adding hardness. Use the [Water Hardness Calculator](/water-hardness-calculator/) to analyse the hardness component separately.
TDS measures dissolved substances — ions and molecules that pass through a 0.45 μm filter and cannot be removed by settling or simple filtration. Turbidity measures suspended and colloidal particles — undissolved matter that scatters light. A water sample can have high TDS and low turbidity (clear brackish water) or low TDS and high turbidity (silty river water). RO membranes reduce TDS; microfiltration and coagulation-flocculation reduce turbidity.
The Bureau of Indian Standards IS 10500:2012 — Drinking Water Specification — sets the acceptable TDS limit at 500 mg/L and the permissible limit (in the absence of an alternative source) at 2,000 mg/L. Water above 500 mg/L may have a noticeable salty or bitter taste; above 1,000 mg/L it is generally unsuitable for regular daily consumption. Most RO purifiers sold in India are designed to bring TDS below 150–300 mg/L for the treated output.
TDS levels in Indian municipal water vary widely by source. Mumbai's Bhandup and Vihar reservoir-fed piped supply typically measures 100–200 mg/L — low TDS, soft water. Delhi's treated Yamuna water is 200–500 mg/L. Chennai's desalinated seawater blend can reach 300–500 mg/L. Rajasthan borewells frequently exceed 1,000–3,000 mg/L due to dissolved fluorides, nitrates, and sodium chloride from geological formations. Most urban RO systems are sized to handle inlet TDS up to 2,000 mg/L.
Measure the electrical conductivity of your water sample using a handheld EC or TDS meter and note the reading in μS/cm. Enter this value in the 'Electrical Conductivity (EC)' field. Adjust the 'TDS Conversion Factor (k)' slider if you know the k value for your water source — use 0.64 as the default for most tap or borewell water. The calculator gives TDS in mg/L, ppm, and a WHO/BIS quality classification.
Use k = 0.64 as a general starting point for most Indian groundwater and municipal water sources. Use k = 0.67 for natural waters following ISO 11348. For sodium chloride-dominated waters (coastal wells, brackish water), k ≈ 0.50–0.55 is more accurate. For calcium bicarbonate-dominated waters (limestone aquifers, inland groundwater), k ≈ 0.70–0.75 is appropriate. If you have a gravimetric TDS measurement from a certified lab, divide it by the EC reading to find your sample-specific k and enter that.
Yes — reverse osmosis membranes are highly effective at removing dissolved ions, typically achieving 90–95% TDS rejection. A water sample with 500 mg/L TDS entering an RO system will produce permeate (purified water) at 25–50 mg/L TDS. Very low TDS water (below 50 mg/L) is considered very pure and is used in pharmaceutical injections, semiconductor manufacturing, and laboratory reagent preparation. However, extremely low TDS water (below 10 mg/L) can be mildly corrosive to metal pipes and may leach minerals.
Yes — TDS is one of the key parameters for assessing irrigation water suitability. The FAO guidelines classify irrigation water by EC: below 700 μS/cm (no restriction), 700–3,000 μS/cm (slight to moderate), above 3,000 μS/cm (severe restriction). Indian agricultural departments and NABARD-funded projects use EC for soil salinity and water quality assessments. Enter the EC reading from a portable meter and use the TDS output alongside salinity classification for irrigation planning.