What is the relationship between salinity and specific gravity?

Salinity (ppt) and specific gravity (SG) are both measures of dissolved salts in water. SG is the density of the solution relative to pure water. As salinity increases, SG increases proportionally. The relationship is also affected by temperature.

What is the typical SG for saltwater aquariums?

Most marine aquariums target SG 1.020–1.026, corresponding to roughly 26–35 ppt salinity. Reef aquariums typically aim for 1.025–1.026 (35 ppt).

Why does temperature matter for SG readings?

Water density changes with temperature — warmer water is less dense. A hydrometer calibrated at 25°C will give inaccurate readings at other temperatures unless corrected.

How do I measure salinity in my aquaculture system?

Common methods include refractometers (most popular), hydrometers (floating glass), and digital salinity meters. Refractometers are preferred for accuracy and ease of use.

ToolsConductScience tool

AquacultureFree in-browser calculator

Salinity / SG Converter.

Convert between salinity (ppt) and specific gravity with temperature adjustment. Two-way instant conversion for aquaculture and marine science.

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Validated2026-04-08

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Conversion Parameters

Direction

Salinity: 35 ppt

0153045

Temperature: 25\u00B0C

0\u00B0C10\u00B0C20\u00B0C30\u00B0C40\u00B0C

Results

Salinity

35.0 ppt

parts per thousand

Specific Gravity

1.0273

at 25°C

Temperature

25°C

reference temp

When to use

Converting between salinity and SG for water preparation
Adjusting hydrometer readings for temperature
Preparing brackish water at a target salinity
Cross-referencing salinity measurements from different instruments

Do not use for

For extremely high salinities (>45 ppt) — the simplified equation loses accuracy
As a substitute for instrument calibration

Calibrate your refractometer

Use RO/DI water or a calibration fluid. An uncalibrated refractometer can be off by 1–2 ppt.

Temperature compensation

Most modern refractometers have automatic temperature compensation (ATC), but hydrometers do not. Always note the temperature when using a hydrometer.

Freshwater is not always 0 ppt

Municipal tap water can have measurable TDS (0.1–0.5 ppt). Use RO/DI water for precise dilutions.

Method

Conversion uses the simplified UNESCO 1983 equation for the relationship between salinity, specific gravity, and temperature. Accurate for 0–45 ppt at 0–40°C.

Validated

Last validated 2026-04-08. Calculations are designed for planning and documentation support; verify procurement decisions against manufacturer specifications or institutional SOPs.

How to cite

How to Cite

ConductScience Salinity/SG Converter (v1.0). ConductScience, Inc. 2026. Available at: https://conductscience.com/tools/salinity-sg-converter

UNESCO. Algorithms for computation of fundamental properties of seawater. UNESCO Technical Papers in Marine Science 44. 1983.

Salinity and Specific Gravity in Aquaculture

Salinity is a critical parameter in marine and brackish-water aquaculture. It directly affects osmoregulation, growth rate, and disease susceptibility. Specific gravity (SG) is often used as a proxy for salinity because it is easy to measure with a hydrometer or refractometer. The UNESCO 1983 equation provides the standard conversion between these units.

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