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TDS Calculator

Total dissolved solids (TDS) represent the concentration of all ions—both organic and inorganic—suspended in water at sizes smaller than 2 micrometres. Water quality professionals, aquarists, and environmental scientists rely on TDS measurements to assess drinking water safety, salinity, and mineral content. This calculator lets you determine TDS either by entering individual ion concentrations from a lab analysis or by converting electrical conductivity (EC) readings into TDS values.

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Creators Hanna Pamuła, PhD
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Understanding Total Dissolved Solids

Total dissolved solids encompass sodium, calcium, magnesium, potassium, chloride, sulfate, bicarbonate, and other dissolved minerals and salts. In water quality assessment, TDS serves as a proxy for overall ionic strength and directly influences taste, odour, and suitability for different applications.

The standard units for TDS are:

  • mg/L (milligrams per litre) – mass per unit volume
  • ppm (parts per million) – mass-to-mass ratio, which for water approximates mg/L

TDS thresholds vary by context. Drinking water standards typically accept 500–1000 mg/L, while freshwater ecosystems remain below 500 mg/L. Above 1500 mg/L, water becomes brackish or saline and may pose health or taste concerns.

Calculating TDS from Ion Concentrations

When you have a complete water chemistry analysis, sum all cation concentrations and all anion concentrations to arrive at total dissolved solids. Each ion contributes its mass concentration directly:

TDS [mg/L] = (Na⁺ + Ca²⁺ + Mg²⁺ + Fe²⁺ + Ba²⁺ + K⁺ + Sr²⁺ + Mn²⁺ + Al³⁺ + Li⁺) + (Cl⁻ + H₂PO₄⁻ + SO₃²⁻ + SO₄²⁻ + CO₃²⁻ + HCO₃⁻ + F⁻ + NO₂⁻ + NO₃⁻ + SiO₄⁴⁻)

TDS [mg/L] = Cations + Anions

TDS [mg/L] = k × EC [μS/cm]

  • Cations — Sum of all positively charged ions in mg/L (sodium, calcium, magnesium, iron, barium, potassium, strontium, manganese, aluminium, lithium)
  • Anions — Sum of all negatively charged ions in mg/L (chloride, phosphate, sulfite, sulfate, carbonate, bicarbonate, fluoride, nitrite, nitrate, silicate)
  • k — Conversion factor ranging from 0.55 to 0.80; use 0.67 as a standard approximation when unknown
  • EC — Electrical conductivity measured in microsiemens per centimetre (μS/cm) at 25 °C

Converting Electrical Conductivity to TDS

When a direct water analysis is unavailable, electrical conductivity provides a rapid estimate. EC measures the water's ability to conduct electricity, which correlates with dissolved ion concentration. The relationship is linear and expressed as:

TDS [mg/L] = k × EC [μS/cm]

The conversion factor k varies depending on the ionic composition. Natural freshwater typically uses k = 0.67. Agricultural runoff or mineral-rich sources may require k = 0.55–0.75. Where the exact composition is unknown, 0.67 provides a reasonable middle estimate. Higher k values (up to 0.80) apply to waters with more dissolved salts or heavier mineral loads.

TDS Standards and Water Classification

Water quality guidelines classify aquatic systems by TDS levels:

  • Freshwater: 0–500 mg/L – suitable for drinking, agriculture, and most ecosystems
  • Low mineral: Below 100 mg/L – demineralised or distilled water
  • Slightly saline: 500–1500 mg/L – affects taste; marginal for some uses
  • Moderately saline: 1500–5000 mg/L – unsuitable for drinking; useful for industrial cooling
  • Highly saline: Above 5000 mg/L – marine or concentrated brines

Drinking water standards in most countries set acceptable limits between 500 and 1000 mg/L to maintain palatability and safety.

Key Considerations When Measuring TDS

When calculating or interpreting TDS values, keep these practical points in mind:

  1. Ion presence varies with source — Not all ions appear in every water sample. Include only those detected in your analysis. Missing anions or cations will underestimate TDS if they are present but not measured.
  2. Conversion factor uncertainty affects estimates — The standard factor of 0.67 works well for most freshwater and municipal supplies. However, saline, mineral-rich, or industrial waters may have different ionic compositions, yielding k values between 0.55 and 0.80. Request the actual conversion factor from your water supplier if precision is critical.
  3. Temperature influences electrical conductivity — EC readings must be taken at or corrected to 25 °C. Colder or warmer solutions will give different conductivity values, leading to incorrect TDS estimates. Always check the measurement temperature before applying the EC-to-TDS conversion.
  4. Sample storage and handling matter — Dissolved solids can precipitate or gases can escape if samples are exposed to air, sunlight, or temperature fluctuations. Store samples in sealed, cool containers and analyse them promptly to avoid bias in results.

Frequently Asked Questions

What is the difference between TDS and conductivity?

Total dissolved solids measure the mass concentration of all dissolved ions, whereas electrical conductivity measures how readily a solution conducts electricity. They are related: EC increases with TDS, but the exact relationship depends on the type and valence of ions present. A solution high in divalent ions (like Ca²⁺ or SO₄²⁻) will have higher conductivity for the same TDS as a solution dominated by monovalent ions (like Na⁺ or Cl⁻). This is why a conversion factor, rather than a fixed multiplier, bridges the two measurements.

Why do mg/L and ppm values equal each other for water TDS?

The conversion between mg/L and ppm depends on solution density. The formula is [ppm] = [mg/L] × (1000 / density [kg/m³]). Since water has a density very close to 1000 kg/m³ under standard conditions, the denominator becomes 1000, making the conversion ratio equal to 1. Therefore, a TDS of 500 mg/L is equivalent to 500 ppm in water. This convenient equivalence breaks down for non-aqueous solutions or when TDS is so high that solution density deviates significantly from pure water.

How do I choose the correct conversion factor k for EC-to-TDS conversion?

The conversion factor k depends on the ionic composition and minerality of the water. For typical municipal or untreated freshwater, k = 0.67 is the standard approximation. Agricultural runoff or naturally mineral-rich sources may range from 0.55 to 0.75. Seawater or highly saline solutions can reach 0.80. If you have a reference lab analysis, divide the measured TDS (mg/L) by the EC (μS/cm) to calculate your actual k value. Always prefer measured k values over defaults when accuracy is required.

Can TDS alone tell me if water is safe to drink?

TDS is one indicator of water quality but not a complete safety assessment. While 500–1000 mg/L is the typical acceptable range for taste and general safety, TDS doesn't distinguish between harmful contaminants and benign minerals. A high TDS could reflect naturally dissolved minerals (like calcium and magnesium, which may be beneficial) or toxic ions (like lead or arsenic). Always pair TDS measurements with comprehensive microbiological testing and chemical analysis for specific contaminants before determining potability.

What does it mean if my measured anions don't equal my measured cations?

In a perfect analysis, the total equivalents of anions and cations should balance (charge neutrality). Small discrepancies (under 5–10%) reflect measurement uncertainty or missing trace ions. Larger imbalances suggest either incomplete sampling (missing anions or cations not tested), analytical error, or the presence of undocumented ions. Rechecking the analysis, ensuring all major ions are measured, and verifying lab protocols can help identify the source of the discrepancy.

How often should I test TDS in my water supply?

For municipal drinking water, annual testing is typically sufficient unless there is a reported contamination event or water quality complaint. Private wells and small systems should be tested at least once a year, ideally quarterly if the supply serves a facility with strict water quality demands (such as laboratories or aquaculture). Swimming pools, aquariums, and agricultural irrigation systems benefit from weekly or monthly monitoring to maintain optimal conditions and detect drift caused by evaporation, mineral accumulation, or biological growth.

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