Salinity Conductivity Calculator

Calculator

Calculation direction

Switch direction for reverse estimation.

Model at 25°C

Choose based on water type and instrument practice.

Sample temperature (°C)

Used for temperature compensation to 25°C.

Temperature coefficient α (/°C)

Typical: 0.019–0.021 for NaCl solutions.

TDS factor k

Approximate mg/L from µS/cm: TDS ≈ k·EC(µS/cm).

Output conductivity unit (for reverse mode)

Applied when you estimate conductivity from salinity.

Conductivity value

Used when direction is conductivity → salinity.

Conductivity unit

Converted internally for consistent calculations.

Salinity (ppt / PSU)

Used when direction is salinity → conductivity.

Reset

Example data table

Scenario	Input	Model	Key output (approx.)
Typical seawater	EC = 53 mS/cm, T = 25°C	Seawater anchor	Salinity ≈ 35 ppt, EC25 ≈ 53 mS/cm
Brackish sample	EC = 10 mS/cm, T = 20°C, α=0.019	Power model	Salinity ≈ 4–6 ppt after EC25 correction
Freshwater high minerals	EC = 1500 µS/cm, T = 25°C	Power model	Salinity ≈ 0.8–1.1 ppt (rough)
Reverse estimate	Salinity = 20 ppt, T = 30°C	Seawater anchor	EC increases above EC25 at warmer T

Examples are illustrative; instrument calibration and ion mix can shift real values.

Formulas used

Temperature compensation to 25°C: EC25 = EC / (1 + α·(T − 25))
Salinity model options (at 25°C):
- Seawater anchor: S ≈ (35/53)·EC25 (PSU, EC25 in mS/cm)
- Power model: S ≈ 0.4665·EC25^1.0878 (empirical, EC25 in mS/cm)
- Linear rule: S ≈ 0.64·EC25 (rule‑of‑thumb, EC25 in mS/cm)
Resistivity: ρ(Ω·cm) = 1 / (EC(S/cm)), with EC(S/cm) = EC(mS/cm)/1000
TDS estimate: TDS(mg/L) ≈ k · EC(µS/cm)

Note: Practical Salinity (PSS‑78) uses a more complex conductivity ratio polynomial. This tool provides advanced, configurable approximations useful for quick lab and field estimates.

How to use this calculator

Select the calculation direction (conductivity to salinity, or reverse).
Enter the sample temperature and choose a suitable α for temperature compensation.
Pick a model that matches your water type and reporting needs.
Enter the required input (conductivity or salinity). Submit to view results.
Use Download CSV or Download PDF for records.

FAQs

1) Why does temperature matter for conductivity?

Ion mobility increases with temperature, so conductivity rises. Temperature compensation normalizes readings to a reference, commonly 25°C, improving comparability between samples.

2) Which model should I choose?

Use the seawater anchor for marine samples near typical seawater behavior. Use the power model for general natural waters. Use the linear rule when you need a quick, rough estimate.

3) What does α represent?

α is the temperature coefficient describing how fast conductivity changes per degree Celsius. It depends on ionic composition and concentration, so instrument manuals or standards may specify a preferred value.

4) Is ppt the same as PSU?

They are close for many practical uses, but not strictly identical. PSU is dimensionless practical salinity based on conductivity ratios, while ppt implies mass fraction. For rough reporting, they are often treated similarly.

5) Why is TDS only an estimate?

TDS depends on which ions are present. The factor k compresses that complexity into a single multiplier, so it varies by water chemistry. Use gravimetric methods for definitive TDS when needed.

6) Can I use this for ultra‑pure water?

Not reliably. Very low conductivity requires specialized measurement practices and temperature control. The salinity models here are tuned for natural waters and saline solutions, not high‑purity systems.

7) How accurate is the reverse mode (salinity to conductivity)?

Reverse mode inverts the selected model and applies temperature compensation. Accuracy depends on how well the chosen model matches your sample, plus calibration and measurement uncertainty in your conductivity probe.