Salinity and TDS: From Basics to Advanced Concepts

Salinity and total dissolved solids (TDS) are two of the most common ways we describe how much "stuff" is dissolved in water. Whether you work in oceanography, run an aquarium, or care about drinking water quality, you will run into units like PSU, ppm, mg/L, and conductivity. This guide walks you from the basics to advanced concepts and shows how to convert between units. You can use our free Salinity/TDS Converter anytime to convert PSU to ppm, mg/L to salinity, or conductivity to salinity—no sign-up required.

What is Salinity?

Salinity is the concentration of dissolved salts in water. In practice, it usually means the mass of salt per unit mass (or volume) of water. We measure it in the ocean, in estuaries, in aquariums, and in many industrial and environmental settings.

• Ocean water typically has a salinity around 35 grams of salt per kilogram of seawater (often written as 35 PSU or 35 g/kg).
• Brackish water (mix of fresh and seawater) has lower salinity, often between 0.5 and 30 PSU.
• Freshwater has very low salinity, often below 0.5 PSU.

Salinity affects density, freezing point, and the survival of many organisms, so it is a key variable in marine science, aquaculture, and water treatment.

What is TDS?

TDS stands for total dissolved solids. It is the total mass of all dissolved substances in water—not only salts, but also minerals, ions, and other dissolved matter. So TDS is a broader concept than salinity.

• Units: TDS is often reported in ppm (parts per million) or mg/L (milligrams per liter). For dilute solutions in water, 1 ppm ≈ 1 mg/L.
• Typical ranges: Drinking water might be 50–500 ppm TDS; seawater is around 35,000 mg/L (35,000 ppm) of dissolved solids; reef aquariums are often kept in a range that matches natural seawater.

Because TDS includes everything dissolved (salts, carbonates, sulfates, etc.), a single "TDS to salinity" conversion is only approximate and depends on the composition of the water.

Units and Scales

Different fields use different units. Here is a quick reference:

• PSU (Practical Salinity Units): Dimensionless scale based on electrical conductivity. Standard in oceanography. 35 PSU ≈ typical ocean.
• g/kg (grams per kilogram): Mass of salt per kilogram of seawater. Numerically close to PSU for seawater (e.g. 35 g/kg ≈ 35 PSU).
• ppt (parts per thousand): Same idea as g/kg; 35 ppt = 35 g/kg. Sometimes written with the symbol ‰.
• mg/L (milligrams per liter): Mass of dissolved substance per liter of water. Common in water quality and drinking water.
• ppm (parts per million): For dilute aqueous solutions, 1 ppm ≈ 1 mg/L. Widely used in aquariums and environmental monitoring.
• μS/cm (microsiemens per centimeter): Electrical conductivity. Related to salinity and TDS because dissolved ions carry current; conversion to salinity or TDS depends on composition and temperature.

When you see a value in one unit, you often need to convert it to another—our Salinity/TDS Converter supports PSU, g/kg, mg/L, ppm, and conductivity (μS/cm).

Salinity vs TDS

Salinity focuses on salt (mainly sodium chloride and other ions in seawater). TDS includes all dissolved solids. So:

• In seawater, dissolved solids are mostly salts, so salinity and TDS are closely related; 35 PSU corresponds to roughly 35,000 mg/L TDS.
• In freshwater or tap water, TDS can include calcium, magnesium, carbonates, and other ions that are not "salinity" in the oceanographic sense. So a TDS value in ppm does not always translate to a true salinity (PSU) without knowing the composition.

Conversion factors (e.g. PSU to ppm) are therefore approximations that work best for seawater-like composition. For other water types, use conversions as estimates and, when possible, measure salinity or TDS with the method appropriate for your application.

Practical Salinity (PSU)

The Practical Salinity Scale defines salinity from the electrical conductivity of seawater relative to a standard. It is dimensionless and reported in Practical Salinity Units (PSU). Oceanographers use it because conductivity is easy to measure with probes and can be automated.

• PSU is tied to conductivity and temperature: instruments often correct for temperature to report PSU.
• For standard seawater, 35 PSU ≈ 35 g/kg ≈ 35 ppt. So in practice, many people treat PSU and g/kg as numerically similar for seawater.
• PSU is not exactly equal to "grams of salt per kilogram" in a strict chemical sense, but the values are close for natural seawater.

Converting Between Units

Converting between PSU, g/kg, mg/L, ppm, and conductivity is routine in oceanography, aquariums, and water quality work. Common reference: 35 PSU ≈ 35 g/kg ≈ 35,000 mg/L ≈ 35,000 ppm for typical ocean water. Freshwater is 0 PSU, 0 g/kg, 0 mg/L.

Doing these conversions by hand is error-prone. Use our Salinity/TDS Converter to convert PSU to ppm, mg/L to salinity, conductivity to salinity, and more. Enter a value, choose "From" and "To" units, and get instant results—no sign-up required.

Advanced Topics

Temperature and Conductivity

Electrical conductivity of water depends on temperature: warmer water conducts better. Salinity derived from conductivity (e.g. PSU) is usually temperature-corrected so that the same water gives the same salinity at different temperatures. When you use a conductivity-based meter or a converter that goes from μS/cm to salinity, that correction is (or should be) built in.

Absolute Salinity vs Practical Salinity

In the TEOS-10 standard (used in modern oceanography), Absolute Salinity is reported in g/kg and represents the actual mass of dissolved material per kilogram of seawater. Practical Salinity (PSU) remains the conductivity-based, dimensionless scale. For most coastal and open-ocean seawater, the numerical difference is small. For precise work (e.g. density calculations), oceanographers sometimes use Absolute Salinity in g/kg.

Limitations

• TDS vs salinity: TDS (ppm/mg/L) can include non-ionic or non-salt dissolved matter; salinity (PSU/g/kg) is tied to the ionic content that affects conductivity. So one number cannot capture every water type.
• Conversion factors: Rules like "1 PSU ≈ 1,000 mg/L" assume seawater-like composition. Brackish water, river water, or industrial water may have different ratios.
• Conductivity to salinity: Converting μS/cm to PSU or TDS uses empirical relations that are best for seawater. For other waters, treat results as estimates.

Applications

• Oceanography: Salinity (PSU or g/kg) is a core variable; conductivity sensors report PSU.
• Reef and marine aquariums: Hobbyists and professionals match natural seawater (e.g. ~35 PSU) and often use TDS or conductivity meters; converting between PSU, ppm, and μS/cm is common.
• Drinking water: Regulations and guidelines often refer to TDS in ppm or mg/L; salinity is rarely used for tap water.
• Aquaculture and hydroponics: Nutrient solutions and water quality are often expressed in TDS (ppm) or conductivity (μS/cm); converting to or from salinity units helps when mixing with seawater or comparing standards.
• Wastewater and soil salinity: TDS and conductivity are used to monitor dissolved load and soil salinity; conversions to ppm or mg/L are standard.

In all these cases, having a single place to convert between PSU, g/kg, mg/L, ppm, and μS/cm saves time and reduces errors. Our Salinity/TDS Converter supports these units for oceanography, aquariums, and water quality—free and instant.

Conclusion

Salinity (salt content) and TDS (total dissolved solids) describe how much is dissolved in water; we use units like PSU, g/kg, ppt, mg/L, ppm, and conductivity (μS/cm) depending on the field. Understanding the basics—what each term means, how the units relate, and when conversions are approximate—helps you interpret data and choose the right measurements. For quick, accurate conversions between PSU, ppm, mg/L, and conductivity, use our free Salinity/TDS Converter—no sign-up required.