Aquarium Salinity Converter

Salinity measurement is essential for maintaining a healthy saltwater or brackish aquarium. There are several different units used to express salt concentration in water, and converting between them is a common task for aquarists.

Common salinity units:

• Specific Gravity (SG): The density of saltwater compared to pure water. Pure water has an SG of 1.000. Typical reef aquarium SG is 1.025-1.026 on the 25°C reference this page uses, and there is a section further down on why you will also see 1.023 quoted for the same water.
• Parts per Thousand (PPT or ppt): The grams of dissolved salt per kilogram of water. Also written as “per mille” (using the symbol similar to percent but with two zeros).
• Practical Salinity Units (PSU): Numerically almost identical to PPT for seawater. Based on conductivity measurements.
• Conductivity (mS/cm): Millisiemens per centimeter. Measures the electrical conductivity of the water, which correlates with salt content.

Conversion formulas (approximate, at 25 degrees Celsius): The relationship between specific gravity and salinity is not perfectly linear, but a widely used approximation is:

• PPT = (SG - 1) x 1328.57
• SG = 1 + (PPT / 1328.57)
• PSU is approximately equal to PPT for seawater in the normal range
• Conductivity (mS/cm) = PPT x 1.5 (rough approximation at 25 degrees Celsius)

Target salinity ranges for aquariums:

Why you will see natural seawater quoted as both 1.023 and 1.026

This is the single most confusing thing about aquarium salinity, and it is not a disagreement about the water. Specific gravity is a ratio of densities, and the answer depends on which temperature the reference water is at.

• Referenced to pure water at 25°C, seawater at 35 ppt reads 1.0264
• Referenced to pure water at 4°C, the same water reads 1.0234

Both are correct. The 25°C/25°C convention is the one refractometers and the hobby use, and it is the one this calculator runs on, so if a chart hands you 1.023 for ocean water it is quoting the 4°C reference and the two are not interchangeable. Mixing them up is a three-parts-per- thousand error, which is more than the entire safe range for a reef tank.

Working from a table you do not trust? Use PPT. It has no reference temperature to get wrong.

Measuring salinity accurately:

• Refractometers measure specific gravity directly and are the most popular tool among reef hobbyists. They should be calibrated with a reference solution (not just freshwater) for best accuracy.
• Hydrometers (floating or swing-arm) also measure specific gravity but are less accurate than refractometers. Temperature affects readings significantly.
• Digital salinity meters measure conductivity and convert to PSU or PPT internally. They are the most accurate when properly calibrated.

Temperature effects: Specific gravity changes with temperature because water density changes. Most conversion tables and refractometers are calibrated for 25 degrees Celsius (77 degrees Fahrenheit). If your water temperature is significantly different, readings may need correction. A 5-degree Celsius difference can shift SG readings by approximately 0.001.

Why stable salinity matters: Marine fish and invertebrates are sensitive to salinity changes. Rapid changes can cause osmotic stress, leading to illness or death. When topping off evaporated water, always use freshwater (not saltwater) because salt does not evaporate. When performing water changes, match the new saltwater to the tank’s salinity before adding it. Aim to keep salinity stable within 0.001 SG (about 1 PPT) from day to day.