Salinity Converter
Convert between specific gravity and ppt in either direction. Most reef tanks target 1.025–1.026 SG (33–35 ppt); fish-only systems often run slightly lower.
Uses the standard aquarium approximation ppt ≈ (SG − 1) × 1325, accurate for marine-range readings near 77°F / 25°C. Calibrate refractometers with 35 ppt solution, not RO water, for best accuracy.
SG vs. ppt
Specific gravity compares your water's density to pure water, which is what a swing-arm hydrometer or glass hydrometer reads. Parts per thousand measures dissolved salt directly, which is what a refractometer or digital tester typically reports. At typical aquarium temperatures each 0.001 of SG above 1.000 corresponds to roughly 1.3 ppt.
Stability matters more than the exact number — corals tolerate 34 ppt held steady far better than water swinging between 30 and 36. The AquaLens app tracks salinity over time and flags drift before it becomes a problem.