Marine calculator
Salinity Specific Gravity Converter
Convert between salinity in parts per thousand and specific gravity using a practical aquarium approximation at 25 C.
How this calculator works
The converter uses a linear approximation anchored at 35 ppt equaling 1.0264 specific gravity at 25 C. Specific gravity from ppt is 1 plus ppt times 0.0264 divided by 35.
The reverse conversion subtracts 1 from specific gravity and scales the result back to ppt using the same anchor.
Why it matters
Marine aquarium advice may use ppt, specific gravity, or both. Converting between them helps compare recommendations, but the tank should still be managed with a calibrated refractometer or hydrometer.
Worked example
Natural seawater near 35 ppt converts to about 1.0264 SG with this approximation. A reading of 1.025 converts back to about 33.1 ppt.
FAQ
What specific gravity is common for reef tanks?
Many reef aquariums are kept around 1.025-1.026 specific gravity, close to natural seawater.
What specific gravity is common for FOWLR tanks?
Fish-only-with-live-rock systems are often kept around 1.020-1.025, depending on livestock and husbandry goals.
Is this a laboratory conversion?
No. It is a linear aquarium approximation at 25 C, useful for planning but not a substitute for calibrated instruments.
Why does temperature matter?
Specific gravity readings depend on instrument calibration and temperature compensation, so match the scale used by your hydrometer or refractometer.