TDS Calculator
Calculate total dissolved solids from electrical conductivity with a factor, from gravimetric dried-residue data, or from summed ion concentrations when you already know the major dissolved species.
Edited by Gail Joyce
This calculator page is maintained by the Chemistry Calculators editorial team. The conductivity-factor estimate, gravimetric definition, and ion-sum workflow on this page are reviewed against standard water-analysis reference material before major updates.
TDS Calculator
Choose the method that matches the data you actually have: electrical conductivity with an estimate factor, dried residue and sample volume, or a direct sum of known ion concentrations.
Scope: conductivity gives an estimate, gravimetric residue gives the most direct formal TDS result, and the ion-sum path helps when you already have separate concentration measurements.
Quick fills
How to Use the TDS Calculator
Choose the method that matches your measurement source instead of forcing every water-quality workflow into the same formula.
Use conductivity when you only have EC
This is the common field or meter workflow. The TDS result is an estimate that depends on the factor you choose.
Use gravimetric mode for a direct mass-based result
If you dried the sample and weighed the residue, divide the residue mass by the sample volume in liters.
Use ion-sum mode when the major ions are already known
Add the reported mg/L values for the ions you want included in the total dissolved solids estimate.
Interpret the result in context
The calculator gives a broad quality label for context, but actual water-quality decisions still depend on source, standards, and target use.
Table of Contents
Understanding TDS
Total dissolved solids describe the total mass of dissolved material in water, typically reported in mg/L and, for dilute samples, approximately the same numeric value in ppm.
Why EC-based TDS is only an estimate
Conductivity depends on the ionic makeup of the water, not just the total mass dissolved. That is why the EC-to-TDS factor changes with water type.
Why gravimetric TDS is different
Gravimetric TDS comes from dried residue and sample volume, so it is the direct mass-based path rather than a conductivity estimate.
Formula and Equation
Conductivity estimate: TDS = k × EC
Here EC is electrical conductivity and k is the conversion factor chosen for the water type or instrument basis.
Gravimetric method: TDS = residue mass / sample volume
Use residue mass in mg and sample volume in liters to get the result directly in mg/L.
Ion-sum method: TDS = Σ ion concentrations
Worked Examples
Step-by-step solutions demonstrating how to calculate total dissolved solids by conductivity estimate, gravimetric residue, and ion-sum methods. These examples show you how to use the TDS Calculator effectively for the most common lab and water-quality workflows.
Example 1: Conductivity estimate
Scenario: A conductivity meter reads 500 µS/cm, and you want a quick TDS estimate using a 0.50 conversion factor.
Solution:
Use the conductivity estimate formula TDS = k × EC.
TDS = 0.50 × 500 = 250 mg/L.
Answer: TDS = 250 mg/L.
Example 2: Gravimetric TDS
Scenario: A 250 mL water sample leaves 145 mg of dried residue after evaporation. You need the direct gravimetric TDS value.
Solution:
First convert the sample volume to liters: 250 mL = 0.250 L.
Then divide residue mass by sample volume: 145 mg / 0.250 L = 580 mg/L.
Answer: TDS = 580 mg/L.
Example 3: Ion-sum estimate
Scenario: A water report lists four major ions at 80, 45, 25, and 12 mg/L, and you want a quick summed TDS estimate.
Solution:
Add the ion concentrations directly when they are all reported on the same mg/L basis.
TDS = 80 + 45 + 25 + 12 = 162 mg/L.
Answer: TDS = 162 mg/L.
Common Mistakes
Most TDS mistakes come from treating every method as equivalent or from using the wrong factor without noting what it represents.
Treating EC-based TDS as exact
Conductivity with a factor is a practical estimate, not a universal direct measurement of dissolved mass.
Forgetting to convert mS/cm to µS/cm
A conductivity unit mismatch can throw the estimate off by a factor of 1000.
Using raw residue mass without dividing by volume
Gravimetric TDS is mass per liter, not just the residue mass itself.
Summing ions with mixed units
Ion-sum mode assumes all listed concentrations are already on the same mg/L basis.
Frequently Asked Questions (FAQs)
Is ppm the same as mg/L?
For dilute aqueous samples, the numeric values are usually treated as equivalent, which is why many TDS meters show ppm while labs often report mg/L.
Which factor should I use for conductivity?
Use the factor that matches the water type or the instrument basis. If you do not know it, 0.50 is a common quick estimate, but it is still only an estimate.
When is gravimetric TDS better?
Use it when you have dried-residue data and want the direct mass-based TDS result instead of a conductivity-based shortcut.
What if I only need EC-to-TDS conversion?
Use the narrower TDS-to-conductivity or conductivity-based pages if you do not need the broader method comparison this page provides.
References
| Resource | Description |
|---|---|
| EPA: Secondary Drinking Water Standards | Useful context for dissolved-solids interpretation and common drinking-water guideline ranges. |
| USGS: Specific Conductance and Water | Background on why conductivity is related to dissolved ions but still not identical to direct dissolved-solids mass. |