About the Aquarium Salinity Converter
This calculator converts a hydrometer specific gravity reading into salinity expressed in parts per thousand, the unit most saltwater and reef aquarium guidelines are written in. It also flags whether the reading falls within the typical reef target range, useful for dialing in water changes and salt mixes.
How It Works
You enter a specific gravity reading from a hydrometer or refractometer, a decimal value just above 1.000 that reflects how much denser the saltwater is than pure fresh water. The calculator converts that reading into parts per thousand using a standard approximation formula, then compares the result against the commonly cited 32-35 ppt reef target range to report a status.
Formula & Methodology
By hand, subtract 1 from the specific gravity reading, then multiply the result by 1305 to get salinity in parts per thousand. For example, a reading of 1.023 gives (0.023 x 1305), or about 30.0 ppt.
Examples
A Typical Reef Tank Reading
A specific gravity reading of 1.025 converts to (1.025 - 1) x 1305 = 32.6 ppt, which falls within the typical reef target range of 32-35 ppt.
A Fish-Only Tank Running Lower
A specific gravity reading of 1.021, more typical of a fish-only marine setup, converts to (1.021 - 1) x 1305 = 27.4 ppt, which the calculator marks as outside the typical reef target range since that band is calibrated for reef tanks with corals.
Advantages
- Translates a hydrometer's specific gravity scale into the ppt units most stocking and dosing guidelines actually use.
- Gives an instant status check against the reef target range, useful when mixing new saltwater or topping off after evaporation.
- Works with either a swing-arm hydrometer or a refractometer reading, since both instruments report specific gravity.
Common Mistakes
- Assuming every marine tank should target the 32-35 ppt reef range, when fish-only systems are commonly and appropriately run lower.
- Using an old or uncalibrated swing-arm hydrometer, which can drift out of accuracy over time and feed the calculator a misleading specific gravity reading.
- Ignoring water temperature when taking the reading, since hydrometers are typically calibrated for a specific temperature and readings taken well outside that range skew the result.
Edge Cases to Watch For
- Entering a specific gravity of 1.000 or lower returns an error, since the formula assumes saltwater denser than fresh water and a reading at or below 1.000 is not valid saltwater.
- The conversion uses a fixed approximation rather than a temperature-compensated formula, so a hydrometer reading taken at a very different temperature than it was calibrated for can produce a less accurate ppt result.
- Fish-only marine tanks are commonly run lower, around 1.020-1.024 SG, than the 32-35 ppt reef range this calculator checks against, so an 'outside typical reef target range' status does not necessarily mean a fish-only tank's salinity is wrong for its purpose.
Common Use Cases
- Reef and saltwater aquarium owners checking a new hydrometer or refractometer reading against target salinity.
- Hobbyists mixing new saltwater batches for water changes and verifying salinity before adding it to the tank.
- Fishkeepers troubleshooting coral or fish stress who want to rule out salinity drift as a contributing factor.