Saltwater Salinity Calculator
Convert between specific gravity, salinity (ppt) and conductivity, correct a hydrometer reading for temperature, and work out how much salt to mix or fresh water to add.
How to use this calculator
- Pick what you measured. Seawater refractometers and most digital meters already correct for temperature. A swing-arm or floating hydrometer does not, so enter the water temperature and the temperature the hydrometer was calibrated at.
- Read the salinity in ppt. It is the number that matters to your livestock, because it does not change with temperature. Specific gravity and conductivity are shown on the standard 25 °C basis for comparison.
- Mixing new saltwater? Enter the batch size and target. The calculator gives the weight of salt mix to start with. Add most of it, let it dissolve fully with a pump and heater running, measure, then top up to the target.
- Correcting the tank? Enter the real water volume (not the tank size) and the calculator tells you how much to swap or add. Spread big corrections over several days.
Formulas used
Salinity is the mass of dissolved salts in a kilogram of seawater, written in parts per thousand (ppt). Specific gravity and conductivity both depend on temperature, so converting between them needs the physics of seawater, not a simple ratio.
- Density and specific gravity use the UNESCO 1981 international equation of state for seawater (EOS-80) at atmospheric pressure. Specific gravity is the seawater density divided by the density of pure water.
- Refractometer SG is taken on the 25 °C / 25 °C basis that seawater refractometers are calibrated to: seawater density at 25 °C ÷ pure water density at 25 °C.
- Hydrometer SG is modeled as the sample's density at your water temperature ÷ pure water density at the calibration temperature. The small expansion of the hydrometer itself is ignored.
- Conductivity uses the Practical Salinity Scale 1978 (PSS-78), with 35 ppt seawater at 15 °C defined as 42.914 mS/cm. Readings are assumed to be temperature-compensated to 25 °C, as most aquarium meters do.
Lower salinity by adding fresh water only: fresh = volume × (current ÷ target − 1)
Raise salinity by swapping with stronger saltwater: fraction = (target − current) ÷ (new − current)
The adjustment formulas are simple salt balances. They are accurate to within a few percent, which is well inside the precision of hobby test equipment.
Worked examples
Refractometer reading 1.024: that is 31.9 ppt, about 3 ppt below natural seawater. Conductivity would be about 48.8 mS/cm.
Hydrometer reading 1.023 at 78 °F, calibrated at 77 °F: the warmer water reads slightly low, and the true salinity is 30.8 ppt.
Mixing 20 gallons at 35 ppt: 20 gallons is 75.7 liters. Seawater at 35 ppt weighs about 1.023 kg per liter, so 75.7 × 35 × 1.023 ≈ 2,712 g of pure dissolved salts. With 20% extra for a commercial mix, start with about 3,254 g (7.2 lb).
Lowering a 50 gallon tank from 37 to 35 ppt: 1 − 35 ÷ 37 = 5.4%, so remove 2.7 gallons of tank water and replace it with fresh RO/DI water.
Salinity, specific gravity and conductivity
Values at 25 °C, calculated with the formulas above.
| Salinity (ppt) | Specific gravity | Conductivity (mS/cm) |
|---|---|---|
| 30 | 1.0226 | 46.2 |
| 31 | 1.0233 | 47.6 |
| 32 | 1.0241 | 49.0 |
| 33 | 1.0249 | 50.4 |
| 34 | 1.0256 | 51.7 |
| 35 | 1.0264 | 53.1 |
| 36 | 1.0271 | 54.4 |
Frequently asked questions
What specific gravity is 35 ppt?
Seawater at 35 ppt has a specific gravity of about 1.0264 when measured at 25 °C (77 °F) against pure water at the same temperature, which is the basis seawater refractometers use. Its conductivity is about 53 mS/cm at 25 °C.
Is 1.025 the same as 35 ppt?
Not quite. On the usual 25 °C basis, 1.025 is about 33.2 ppt. Natural seawater of 35 ppt reads about 1.026. Many older guides quote 1.025 because they used a different temperature basis.
Why does my hydrometer reading change with temperature?
A hydrometer measures density, and warm water is less dense than cool water. The same salt water reads lower when it is warm. Salinity in ppt does not change with temperature, which is why this calculator converts your reading to ppt using the water temperature.
How much salt do I need per gallon of saltwater?
Seawater at 35 ppt contains about 136 grams of dissolved salts per US gallon (about 36 grams per liter). Commercial salt mixes contain some bound water, so you usually need more by weight. Follow your brand's chart, mix, then measure and adjust.
How fast can I change salinity in a tank with livestock?
Change it gradually. A common rule of thumb is no more than about 1 ppt per day for a reef tank. Spread larger corrections over several days with small water changes or top-offs.
Related calculators
- Aquarium volume calculator: find the real water volume of your tank
- Water change calculator: plan changes around nitrate
- Aquarium weight calculator: saltwater is heavier than fresh
Sources: Seawater density (EOS-80) and conductivity (PSS-78) from Fofonoff & Millard, UNESCO Technical Papers in Marine Science 44 (1983). Reef target of 35 ppt (SG 1.0264, 53 mS/cm) from Randy Holmes-Farley's salinity articles in Reefkeeping Magazine. Refractometer calibration from Bulk Reef Supply. Instant Ocean mixing rate from LiveAquaria.
Last updated: October 2026