Convert specific gravity (SG) to salinity in parts per thousand (ppt) with temperature correction. Check if your reading falls inside the 34–36 ppt reef aquarium range.
Marine and reef aquariums live and die by stable salinity. The hobby uses three interchangeable measurements: specific gravity (SG) at a reference temperature, salinity in parts per thousand (ppt), and conductivity in mS/cm. This calculator converts between SG and ppt with temperature compensation, evaluates whether the resulting salinity is reef-suitable (typically 35 ppt / SG 1.0264) or fish-only-suitable (33-35 ppt), and supports both refractometer and hydrometer workflows.
SG and ppt are related through the seawater equation of state. At 25 degC, 35 ppt corresponds to SG 1.0264; the calculator adjusts for the temperature at which your refractometer or hydrometer is reading. The reef-suitability check assumes a target window of 34-36 ppt (SG 1.0255-1.0270). Fish-only and quarantine tanks tolerate a wider window, sometimes intentionally lowered to 14-16 ppt (hyposalinity) for parasite treatment.
Pick the conversion direction, then enter either a specific gravity (around 1.020–1.030) or a salinity in ppt (around 30–40). Optionally enter the sample temperature in °C if you know it; the calculator uses 25°C by default.
Reef and marine tanks generally sit at 34–36 ppt (SG ≈ 1.025–1.026). The calculator flags readings inside that band as reef-suitable. Outside the band may still be fine for fish-only systems but is risky for corals and invertebrates.
Refractometers with Automatic Temperature Compensation (ATC) report readings as if measured at 25°C. If your hydrometer is uncalibrated for temperature, compare its value against the ATC-corrected reading above and adjust by slowly changing the mix rather than the water.
Natural seawater is 35 ppt (SG ≈ 1.025–1.026 at 25°C). Reef tanks target the 34–36 ppt band. Fish-only saltwater systems can run slightly lower (30–34 ppt) without harm to fish, but corals and most invertebrates need the full reef range.
Water density changes with temperature. The same actual salinity gives a different SG at 20°C vs 25°C. Most refractometers and hydrometers are calibrated to 25°C — readings at other temperatures need correction. Refractometers with ATC (Automatic Temperature Compensation) handle this automatically.
A refractometer is the standard tool — accurate to ±1 ppt and easy to recalibrate with distilled water. Swing-arm hydrometers are cheap but drift over time. Conductivity probes are most accurate of all but require regular calibration with known-salinity standards.
Top off with fresh RO/DI water (no salt mix). Let evaporation dilute the salt over several hours. Avoid huge sudden additions — drop salinity gradually, no more than 1 ppt per hour, especially with corals and invertebrates that are sensitive to osmotic shock.
Yes — saltwater holds less dissolved oxygen than freshwater at the same temperature. A reef tank at 35 ppt and 26°C saturates around 6.7 mg/L vs about 8.1 mg/L in freshwater. This is one reason marine systems use stronger aeration and surface agitation than freshwater tanks.