AquariumCalcs: Fin & Figures — for the fish aficionado
Meet the Calcs Family →

Salinity Converter (SG ↔ ppt)

Convert between specific gravity and parts per thousand for reef and marine salinity.

Convert
Your reading

Temperature affects your refractometer reading

Most hobby refractometers are calibrated for a specific temperature -- check yours, since an uncalibrated or temperature-mismatched reading can be off by a meaningful margin.

How this calculator works

Specific gravity (SG) and parts per thousand (ppt) both measure salinity, just on different scales. This calculator uses the common reef-hobby approximation anchored to natural seawater (about 1.025-1.026 SG, 33-35 ppt), which is accurate for the salinity range most reef and marine tanks are kept in.

Where these numbers come from

Natural seawater salinity (33-35 ppt, about 1.025-1.026 SG) is a widely cited oceanographic reference point also used as the standard reef-keeping target range. For precision dosing, a calibrated refractometer or digital salinity probe is more reliable than any calculator.

Frequently asked questions

Why does my refractometer need calibration?

Refractometers can drift out of calibration over time, and most are calibrated for a specific reference temperature -- using distilled water to zero it periodically catches drift before it affects your readings.

What SG should I target for a reef tank?

Most reef keepers target 1.025-1.026 SG (about 35 ppt), matching natural seawater, though some successfully run slightly lower -- consistency matters more than hitting an exact number.

Is this converter accurate for freshwater salinity (like for livebearers)?

This approximation is calibrated for reef/marine-range salinity -- for low-level freshwater aquarium salt dosing, see our Aquarium Salt Dosing Calculator instead.

More Reef & Marine calculators

Salt Mix / Salinity Calculator Aquarium Salt Dosing Calculator

All AquariumCalcs tools