How the Seawater Converter Works
The Seawater Converter divides seawater mass by a density set by salinity and temperature. The default 1.025 kg/L matches average ocean water at 35 practical salinity units (PSU) and about 17°C.
Know the salinity
Use 35 PSU for the open ocean, 7 PSU for the Baltic Sea or a refractometer reading for an aquarium.
Find the density
Read the density for your salinity at 20°C in the table below.
Enter the mass
Type the seawater mass in kilograms.
Read the volume
The liters shown include the extra mass of dissolved salts.
Seawater Converter Formula
The seawater conversion formula is V = m ÷ ρsw(S, T). Near 20°C, density climbs by about 0.00077 kg/L for every 1 PSU of salinity.
- S
- Salinity in PSU (≈ grams of salt per kg of seawater)
- T
- Water temperature, °C
- ρsw
- Seawater density from the UNESCO equation of state
Worked Examples
- Ocean water at 35 PSU, 20°C: 100 kg ÷ 1.0248 = 97.58 L.
- Reef aquarium at 1.025 SG: 300 kg ÷ 1.025 = 292.7 L (77.3 US gal).
- Dead Sea (≈1.24 kg/L): 100 kg ÷ 1.24 = 80.65 L.
Seawater Converter Conversion Table
Seawater at 35 PSU and 20°C has a density of 1.0248 kg/L, so 1 kg fills 0.976 L. The table lists density by salinity at 20°C (68°F).
| Salinity | Density (kg/L) | Liters per 1 kg | Liters per 1 tonne | Typical Water Body |
|---|---|---|---|---|
| 0 PSU (fresh water) | 0.9982 | 1.002 | 1,001.8 | Rivers, lakes |
| 7 PSU | 1.0036 | 0.996 | 996.4 | Baltic Sea |
| 18 PSU | 1.0118 | 0.988 | 988.3 | Black Sea |
| 30 PSU | 1.0209 | 0.980 | 979.5 | Coastal water |
| 35 PSU | 1.0248 | 0.976 | 975.8 | Open ocean average |
| 38 PSU | 1.0271 | 0.974 | 973.6 | Mediterranean |
| 40 PSU | 1.0286 | 0.972 | 972.2 | Red Sea |
| ≈ 340 g/kg | 1.2400 | 0.806 | 806.5 | Dead Sea |
At 35 PSU, density reaches 1.0281 kg/L at 0°C and drops to 1.0217 kg/L at 30°C.
What is a Seawater Converter?
A Seawater Converter is a calculator that converts seawater mass to volume using a density corrected for salt content and temperature.
Dissolved salts add mass without adding much volume. Average seawater holds 35 g of salt per kg, mostly sodium chloride (NaCl), which makes seawater about 2.5% denser than fresh water.
Oceanographers compute seawater density with the Thermodynamic Equation of Seawater 2010 (TEOS-10). Cold, salty polar water sinks and drives the global thermohaline circulation.
What are the Uses of a Seawater Converter?
There are 5 main uses of a Seawater Converter.
Reef aquariums
Hobbyists convert mixed saltwater weight into tank volume.
Ship ballast
Crews convert ballast tonnage into tank liters.
Desalination
Plant operators convert feedwater mass flow into volume flow.
Diving and buoyancy
Divers calculate displacement in salt water versus fresh water.
Marine research
Scientists convert sample masses at sea into liters.