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.
Want the why behind the numbers? Read the reef parameters guide. More reef guides, species, and dosing tools live in the Saltwater & Reef hub.
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.