TDS Calculator

Welcome to Omni's TDS calculator, the tool that allows you to determine water's total dissolved solids (TDS) from its chemistry analysis or from its electrical conductivity EC.

We invite you to keep reading if you're wondering what total dissolved solids are and how to calculate them. In the text below, you'll also find more detailed information related to the calculation of TDS, such as:

• How to calculate TDS from water chemistry analysis;
• How to calculate TDS from conductivity;
• How to convert EC to TDS;
• How to convert TDS in mg/L to ppm; and
• Levels of TDS and water classification.

The term total dissolved solids or TDS refers to the sum of all ionic particles, smaller than 2 μm, dissolved in water. These include organic and inorganic substances. Special attention is given to the ions that form part of water's salinity, water hardness, and alkalinity since these are often used as water quality indicators.

It is expressed in units of concentration as parts per million ppm to refer to mass/mass concentration or as milligrams per liter (mg/L), indicating a mass/volume concentration. These units are usually interchangeable for water since, under standard conditions, water's density is 1 mg/L or 1000 kg/m³.

All the dissolved ions in water affect its conductivity. The more ions, the higher the conductivity of water; the fewer, the less it conducts. We can see then that TDS is associated with water conductivity, and it depends on the ionic strength and the type of ion. In the following table, you can find the most common ions present in water:

We can obtain the presence of these ions and their ionic strength through a laboratory's water analysis.

TDS analysis is used in the study of water quality. TDS are the primary determinant of tap water's taste. Acceptable TDS levels in drinking water are set between 500 to 1000 mg/L. Lower than this, we find low mineral water. Higher values can affect water's taste or even become unsafe.

Total dissolved solids in water can also affect water salinity. This is why some standards classify water in terms of its salinity levels. Below you can find a water classification by TDS levels. Notice in this classification that freshwater levels match those for drinking water.

To determine the TDS (total dissolved solids) from water chemistry analysis:

1. Sum the concentrations of all the cations in mg/L or ppm reported in the water analysis. i.e:

   Cations [mg/L] = Na⁺ + Ca²⁺ + Mg²⁺ + Fe²⁺ + Ba²⁺ + K⁺ + Sr²⁺ + Mn²⁺ + Al³⁺ + Li⁺

2. Add up all the anions in mg/L or ppm:

   Anions [mg/L] = Cl⁻ + H₂PO₄⁻ + SO₃²⁻ + SO₄²⁻ + CO₃²⁻ + HCO₃⁻ + F⁻ + NO₃⁻ + NO₂⁻ + SiO₄⁴⁻

   Note that not all of these ions are always present. Include only those that apply.

3. Sum the total of cations and anions:
   TDS [mg/L] = Cations [mg/L] + Anions [mg/L]

To calculate TDS (total dissolved solids) from electrical conductivity (EC), use the formula:

TDS [mg/L] = kₑ × EC [μS/cm]

where:

• EC – Electrical conductivity of the water at 25 °C; and
• kₑ – Multiplier or conversion factor.

The value of kₑ increases along with the increase of ions in water. It ranges from 0.5 to 0.8, but usually, 0.67 is used.

Electrical conductivity and total dissolved solids are frequently used as water quality parameters, and they're correlated. The electrical conductivity EC measures a liquid's ability to conduct an electric charge.

This capacity to conduct electricity depends on dissolved ion concentrations, ionic strength, and the temperature of the solution. If EC is measured, an approximate value of TDS can be easily obtained using the equation above.

EC is usually measured in microsiemens (µS/cm) with a portable water quality checker known as EC or TDS meter.

Notice that this method allows determining the TDS of the ions that can conduct electricity. Not all dissolved solids increase the EC readings and won't be picked up in the measurements; thus, they won't be shown as part of the TDS.

We have a dedicated ionic strength calculator that determines the concentration of ions in a solution in real time!

The TDS calculator will determine the total ion concentration from water analysis data or an electrical conductivity read.

From water analysis:

1. From the Calculation method field, choose Water analysis.

2. Below, you'll find the section to input the concentration for the anions. In this section, enter the concentrations for all that anions that apply.

3. Similarly, in the cations section, enter the concentrations for the different cations present in your water analysis report.

4. In the next section, Total dissolved solids, you'll find the results for the total ions concentration.

From electrical conductivity:

1. From the Calculation method field, select the Electrical conductivity option.

2. Enter the value for the Conversion factor (k). It ranges from 0.55 to 0.8; the higher the value of k, the higher the electrical conductivity. The value 0.67 is used as a good approximation if the actual number is unknown.

3. Proceed to input the Electrical conductivity (EC) read from the EC or TDS meter.

4. Finally, the calculator will display the result for an approximated TDS amount in the Total dissolved solids (TDS) field.

From a measure of electrical conductivity EC of your water solution, you can determine an approximate value of the TDS using the following correlation:

TDS [mg/L] = kₑ × EC [μS/cm]

The conversion factor kₑ ranges from 0.5 to 0.8. Usually, 0.67 is used when the actual factor is unknown.

To convert a TDS value from mg/L to ppm:

1. Use the formula to go from a concentration in mg/L to ppm:

   [ppm] = [mg/L] × 1000 / density [kg/m³]

2. Since TDS is a quality parameter commonly associated with water, we'll see that the conversion becomes fairly easy assuming water density as 1000 kg/m³.

3. By substituting this value of density in the formula from step 1, we get:

   [ppm] = [mg/L]

   This means that for water, the TDS concentration value in mg/L is precisely the same as that of ppm.