Natural seawater at 35 ppt reads 1.0243 on a hydrometer calibrated at 60°F when the water is at 77°F, 1.0264 on one calibrated at 77°F, and conducts 53.1 mS/cm. The three numbers describe the same water, which is why a reading of 1.025 can mean 33.2 ppt or 35.9 ppt depending on the instrument. The calculator converts between specific gravity, salinity in ppt and conductivity at your water temperature, for the calibration printed on your hydrometer, and works out the salt mix for a water change.
How specific gravity, ppt and conductivity relate
Salinity in parts per thousand is grams of salt per kilogram of water. Density follows from salinity and temperature through the UNESCO 1981 equation of state, and conductivity through the Practical Salinity Scale 1978. Both are published polynomials: 35 ppt at 25°C has a density of 1023.34 kg per cubic metre, and 42.914 mS/cm at 15°C is the defining conductivity of 35 ppt.
A floating hydrometer compares the sample with pure water at the temperature it was calibrated for, so:
Reading = density of the sample at its temperature ÷ density of pure water at the calibration temperature
For 35 ppt at 77°F (25°C): 1023.34 ÷ 999.02 (pure water at 60°F) gives 1.0243, ÷ 997.05 (77°F) gives 1.0264, and the 68°F figure is 1.0252. A hydrometer that compares with water at the same temperature, or a refractometer with temperature compensation, avoids the correction: use the 77°F column and the same temperature.
Hydrometer temperature correction
The three calibration choices, 60°F, 68°F and 77°F, cover the printed values on common hydrometers. The example reading of 1.025 with the water at 77°F means:
| Hydrometer calibrated at | Salinity for a reading of 1.025 at 77°F |
|---|---|
| 60°F (15.6°C) | 35.9 ppt |
| 68°F (20°C) | 34.8 ppt |
| 77°F (25°C) | 33.2 ppt |
The spread is 2.7 ppt, so find the calibration temperature on the box or the instructions. Fritz Aquatics gives natural seawater as 34 to 37 ppt, 1.026 at 35 ppt, and Instant Ocean says the same.
Conductivity to salinity
Type the value from a meter in mS/cm (divide microsiemens by 1,000). A meter that shows 53 mS/cm at 25°C reads 34.9 ppt, and 35 ppt at 20°C is 47.9 mS/cm. The scale is defined from about 2 to 42 ppt, so the page warns outside that range. If your meter compensates its reading to 25°C, enter 77°F as the water temperature.
Brackish tanks
Wikipedia gives brackish aquarium readings of 1.005 to 1.010, which at 77°F against water at 77°F is 6.6 to 13.3 ppt. The presets in the salt mix section use those two readings.
Salt mix for a water change
Salt mass is salinity times the mass of the water. For 11 gallons, a 20% change of a 55 gallon tank, at 35 ppt and 77°F: 35 ÷ 1000 × 1023.34 kg per cubic metre × 41.6 L is 1.49 kg, or 3.29 lb, which is 5.5 cups. Instant Ocean says 1.5 lb makes 5 gallons and, for small batches, 1/2 cup per gallon. Together those give 0.3 lb per gallon and 0.6 lb per cup, and the physics gives 1.49 lb for 5 gallons, so the manufacturer's dose is a 35 ppt mix. Tropic Marin gives 1/2 cup per gallon too, targeting 1.025 to 1.027 (34 to 35 ppt). Different brands and their bags settle differently, so weigh salt rather than measuring cups when you can, and start about 5% under the target.
Add the salt to the water, stir until clear, then measure. For the size of the change, use the tank volume from the aquarium volume calculator or the fish tank calculator.
Refractometers
A refractometer reads salinity or specific gravity from a drop of water and is more accurate than a swing-arm hydrometer, according to Fritz Aquatics, who advise rinsing and calibrating whichever you use. Calibrate it with a reference solution or pure water as its instructions say, at the temperature it was made for.
Next steps
Work out how much water you are changing with the aquarium volume calculator, check how many fish the tank holds with the aquarium stocking calculator, and cover the bottom with the aquarium substrate calculator. The rest of the tank tools are on the aquarium hub.