Total Dissolved Solids Calculator
Solve TDS from conductivity, or go back the other way — with step-by-step math, a response chart, a water-classification gauge, and a live parameter diagram.
Strictly linear
TDS is directly proportional to EC. Double the conductivity and you double the TDS — no exponent, no offset, no curvature.
The factor k is empirical
k is not a universal constant. It depends on which ions dominate the water: sodium-dominant waters sit near 0.55, calcium/magnesium-hard waters near 0.75.
Temperature matters
EC rises roughly 2% per °C. Meters normalise to 25 °C; a reading taken without compensation will systematically under- or over-report TDS.
Limits of the estimate
Above ~10,000 mg/L, ion pairing and non-linearity break the simple ratio. Highly saline brines need ion-specific analysis, not a single k value.
How to Use This Calculator
The TDS–EC relation links three quantities: electrical conductivity (EC), the conversion factor (k), and total dissolved solids (TDS). Pick which one you want the tool to solve, then fill in the other two.
1. Choose what to solve for
Pick TDS (from EC and k), EC (from TDS and k), or k (from TDS and EC) — the tool rearranges the formula automatically.
2. Enter the conductivity
Type an EC value and pick its unit — µS/cm, mS/cm, or dS/m. Quick presets cover common fresh, brackish and saline readings.
3. Set the conversion factor
Use 0.64 as a general-purpose default, or choose a factor matched to your water type from the reference table further down the page.
4. Read the result and steps
The result card shows the solved value with engineering notation, and the step-by-step panel shows exactly how the number was reached.
5. Check the chart and water class
The chart plots TDS against EC for three common factors, and the classification gauge places your result on the fresh-to-brine scale.
6. Switch to Advanced for more depth
Advanced mode adds a 1% sensitivity check plus mS/cm, dS/m, ppm and g/L companion metrics, and the full parameter-relationship breakdown.
What This Tool Offers
Built around the standard water-quality relation TDS = EC × k, the workhorse equation behind every conductivity-based salinity estimate.
Solve any variable
Go from conductivity to TDS, from TDS back to conductivity, or derive the conversion factor from a paired lab measurement.
Three EC unit systems
Enter conductivity in µS/cm, mS/cm or dS/m — the tool normalises internally and reports the result in the unit you chose.
Live parameter diagram
A visual EC × k = TDS diagram updates with your inputs, alongside a linear response mini-graph so the relationship is always visible.
Water classification gauge
A five-band fresh → brine scale that instantly places your result in a recognised water-quality category.
Built-in sensitivity check
Advanced mode shows exactly how much a 1% error in EC, TDS or k propagates into the solved value.
Companion metrics
Every result is cross-reported in mS/cm, dS/m, ppm and g/L alongside the salinity classification, so no manual unit maths is needed.
Reference Tables & Conversions
A quick EC → TDS converter, plus lookup tables for conversion factors, conductivity units, and water-quality classification bands.
Quick EC → TDS converter
Standard Conversion Factors (k)
The factor k is the ratio of TDS (mg/L) to EC (µS/cm). It varies with the ionic composition of the water — these are the values most often cited.
| k value | Typical water type | Notes |
|---|---|---|
| 0.50 | Pure NaCl solutions | Sodium-chloride dominated; theoretical lower bound |
| 0.55 | Sodium-dominant waters | Soft, low-hardness supplies |
| 0.64 | General freshwater | Most widely used default (USGS / EPA) |
| 0.67 | Mixed-ion natural waters | Rivers, lakes with balanced ion content |
| 0.75 | Hard, Ca/Mg-rich waters | Limestone and gypsum aquifers |
| 0.85 | Wastewater / high organics | Elevated dissolved organic carbon |
Conductivity Unit Conversions
| Unit | µS/cm | mS/cm | dS/m | mS/m |
|---|---|---|---|---|
| 1 µS/cm | 1 | 0.001 | 0.001 | 0.1 |
| 1 mS/cm | 1,000 | 1 | 1 | 100 |
| 1 dS/m | 1,000 | 1 | 1 | 100 |
| 1 mS/m | 10 | 0.01 | 0.01 | 1 |
TDS Water-Quality Classification
| TDS (mg/L) | Class | Typical use / note |
|---|---|---|
| < 500 | Fresh | Excellent for drinking and irrigation |
| 500 – 1,000 | Low mineral | Good; WHO palatability guideline ceiling ≈ 600–1,000 |
| 1,000 – 3,000 | Brackish | Drinking water becomes objectionable; salt-sensitive crops suffer |
| 3,000 – 10,000 | Saline | Not potable; only salt-tolerant crops and livestock |
| > 10,000 | Brine | Seawater is ≈ 35,000 mg/L; desalination / industrial only |
TDS at Common Conductivity Readings (k = 0.64)
| EC (µS/cm) | EC (mS/cm) | TDS (mg/L) | Class |
|---|---|---|---|
| 100 | 0.10 | 64 | Fresh |
| 500 | 0.50 | 320 | Fresh |
| 1,000 | 1.00 | 640 | Low mineral |
| 2,000 | 2.00 | 1,280 | Brackish |
| 5,000 | 5.00 | 3,200 | Saline |
| 20,000 | 20.0 | 12,800 | Brine |
What Does TDS = EC × k Measure?
Total dissolved solids (TDS) is the combined mass of all dissolved inorganic salts and organic matter in a water sample, expressed in milligrams per litre. Because dissolved ions conduct electricity, the electrical conductivity (EC) of a water sample is a fast, cheap proxy for its TDS content. The conversion factor k bridges the two, and the whole method rests on one linear equation.
The TDS Conversion Formula
The defining relationship is:
Rearranged to solve for the other two variables:
Because the relation is strictly linear, all three forms are exact algebraic rearrangements — there is no quadratic term, no intercept, and no approximation beyond the choice of k itself.
TDS Calculation Variables
| Symbol | Meaning | Typical values |
|---|---|---|
| EC | Electrical conductivity at 25 °C | 50 – 50,000 µS/cm |
| TDS | Total dissolved solids | 0 – 35,000 mg/L (seawater ≈ 35,000) |
| k | Empirical conversion factor | 0.50 – 0.85 |
Why the Conversion Factor k Is Not a Constant
Different ions carry charge with different efficiency. A sodium ion and a calcium ion contribute differently to conductivity per unit mass, and temperature shifts mobility as well. That is why k drifts between roughly 0.50 and 0.85 depending on the water's ion balance. Using 0.64 everywhere is a reasonable engineering default, but for regulatory reporting or process control you should calibrate k against a gravimetric TDS measurement from the same source — which is exactly what the "Solve for k" mode on this page is for.
How Conductivity Affects TDS
Because the relationship is linear, the percentage change in TDS exactly matches the percentage change in EC for a fixed k. A 10% rise in conductivity produces a 10% rise in TDS — a much gentler response than the quadratic behaviour seen in algorithmic complexity, but one that still compounds quickly: a small drift in k from 0.64 to 0.70 is a 9.4% systematic error in every TDS figure you report.
TDS Calculation Examples
TDS = 1,000 × 0.64
TDS = 640 mg/L
EC = 1,280 / 0.64
EC = 2,000 µS/cm = 2.00 mS/cm
k = 480 / 750
k = 0.64
Frequently Asked Questions
It converts a conductivity reading (µS/cm) into an estimate of dissolved solids (mg/L). The factor k encodes the average conductivity-per-milligram of the dissolved ions present, so it is a property of the water's chemistry rather than a physical constant.
0.64 is the standard general-purpose default for natural fresh waters. Use 0.55 for sodium-dominant or very soft waters, 0.67 for mixed-ion surface waters, and 0.75 for hard calcium/magnesium-rich groundwater. If you have a paired lab TDS measurement, solve for k directly and use that site-specific value.
Closely related but not identical. Salinity is usually derived from a different empirical relation (the practical salinity scale) and is reported without units. For most fresh and brackish waters, TDS in mg/L is a good stand-in for salinity in mg/L, but for seawater the two diverge and the dedicated salinity equations should be used.
Ion mobility increases with temperature, so conductivity rises roughly 2% per °C. Meters compensate back to a reference of 25 °C. If your meter does not compensate, or compensates to a different reference, the EC — and therefore the TDS derived from it — will be systematically off.
The WHO palatability guideline suggests TDS below 600 mg/L is good and below 1,000 mg/L is acceptable. Above roughly 1,000–2,000 mg/L taste becomes noticeably objectionable, and above about 3,000 mg/L the water is generally considered unsuitable for drinking regardless of palatability.
Yes. Above roughly 10,000 mg/L, ion–ion interactions reduce the conductivity contribution of each additional ion, so the curve flattens. In that regime k is no longer constant and a single-factor conversion understates TDS. Brines and seawater require ion-specific analysis or a concentration-dependent correction.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| TDS | Total dissolved solids | mg/L |
| EC | Electrical conductivity | µS/cm |
| Conversion Factor | Typical conversion factor (0.55-0.7) |
What it means
Total Dissolved Solids (TDS) is a measure of the concentration of dissolved inorganic salts in water. Electrical conductivity (EC) is easier to measure and correlates strongly with TDS. The conversion factor depends on the water composition, typically ranging from 0.5 to 0.7 for fresh water. A common factor is 0.64 (often used for natural waters). The formula is useful for rapid field assessments, monitoring of water quality trends, and irrigation management. In drinking water, TDS is regulated for aesthetic reasons. Example: A water sample has a conductivity of 500 µS/cm. Using a factor of 0.64, the estimated TDS is 500 × 0.64 = 320 mg/L. This indicates moderate mineral content, which is acceptable for drinking water (typically < 500 mg/L). The factor must be calibrated for specific waters for accurate results.
Worked example
TDS from EC – Two Examples
Real‑World| Parameter | Value |
|---|---|
| EC | 500 µS/cm |
| Conversion factor | 0.65 |
| Parameter | Value |
|---|---|
| EC | 1,500 µS/cm |
| Conversion factor | 0.65 |
Common mistakes
- Conversion factor: Typically ranges from 0.5 to 0.7 for natural waters – depends on ionic composition. A common value is 0.65.
- EC units: Microsiemens per centimetre (µS/cm) – if using mS/cm, multiply by 1000.
- Temperature: EC is temperature‑dependent; standardise to 25°C for consistency.
- Limitation: This is an empirical correlation – for accurate TDS, gravimetric determination is preferred.
- Application: Used for quick estimation in field measurements.
Applications
Total dissolved solids (TDS) can be estimated from electrical conductivity (EC) using a conversion factor, typically between 0.55 and 0.7 (depending on the ionic composition). This relationship allows for rapid, non‑destructive TDS estimation, which is essential for water quality monitoring and irrigation management. Agricultural engineers and environmental scientists use EC‑based TDS to assess salinity risks, to classify water for drinking or irrigation, and to monitor membrane performance in desalination. The conversion factor may be adjusted based on site‑specific water chemistry. By using this proxy, professionals can continuously monitor TDS without complex gravimetric analysis, enabling real‑time decision‑making for water treatment and reuse.
- Routine monitoring of irrigation water quality and salinity
- Assessment of drinking water suitability and compliance with TDS standards
- Monitoring of reverse osmosis and nanofiltration performance
- Field‑based water quality surveys for groundwater and surface water
- Estimating TDS in wastewater and industrial effluents for discharge compliance
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