Overview
Seawater is an aqueous solution of dissolved salts (primarily sodium chloride) with an average salinity of 3.5% by mass (35 g/kg or 35 PSU — Practical Salinity Units). Its density, viscosity, and other physical properties differ from pure water due to dissolved salts, with significant engineering implications for marine piping, desalination, offshore structures, and ocean thermal systems.
Density Variation with Salinity and Temperature
| Temperature (°C) | Fresh Water (kg/m³) | Seawater 35 PSU (kg/m³) |
|---|---|---|
| 0 | 999.8 | 1028.1 |
| 10 | 999.7 | 1026.9 |
| 20 | 998.2 | 1024.8 |
| 30 | 995.7 | 1021.8 |
| 40 | 992.2 | 1018.1 |
Seawater density increases with salinity (~0.7 kg/m³ per PSU) and decreases with temperature. At typical ocean temperatures of 0-15°C (deep water), density is ~1027-1028 kg/m³.
Physical Properties (20°C, 35 PSU)
| Property | Value | Pure Water Comparison |
|---|---|---|
| Density | 1,025 kg/m³ | +2.5% (1000 kg/m³) |
| Dynamic Viscosity | 1.07 × 10⁻³ Pa·s | +7% (1.00 × 10⁻³) |
| Kinematic Viscosity | 1.04 × 10⁻⁶ m²/s | +4% (1.00 × 10⁻⁶) |
| Freezing Point | -1.9°C | 0°C (depressed by salt) |
| Boiling Point | ~100.6°C | 100°C (slightly elevated) |
| Specific Heat | 3990 J/kg·K | 4182 J/kg·K (slightly lower) |
| Vapor Pressure | 2.3 kPa | 2.34 kPa (negligible) |
| Electrical Conductivity | ~5 S/m | ~0.0001 S/m (huge difference) |
| pH | 7.8-8.3 | 7.0 (buffered by carbonate) |
Specific Gravity
Specific gravity is the density ratio to fresh water at 4°C (1000 kg/m³):
Seawater has SG ≈ 1.025 at 20°C/35 PSU, ranging from ~1.020 in warm, low-salinity coastal water (e.g., Baltic ~1.005) to ~1.028 in cold open ocean or high-salinity basins (e.g., Red Sea up to 1.030). This 0.5-3% range is small for floatation but essential for precise buoyancy, cargo load lines (load line rules use density 1.025 t/m³), and desalination osmotic pressure estimates.
Unit Weight
Unit weight (weight density) is the weight per unit volume: γ = ρ × g.
| Condition | Density (kg/m³) | Unit Weight (kN/m³) | Unit Weight (lb/ft³) |
|---|---|---|---|
| Fresh water (4°C) | 1000 | 9.81 | 62.4 |
| Seawater 35 PSU, 20°C | 1025 | 10.06 | 64.0 |
| Seawater 35 PSU, 0°C | 1028 | 10.08 | 64.2 |
| Deep cold seawater | 1027-1028 | 10.08 | 64.1 |
Hydrostatic pressure at depth: P = ρgh — at 10 m seawater ≈ 100.6 kPa (14.6 psi), about 2.5% higher than fresh water. For a subsea pipeline at 100 m depth the external pressure is ~1.0 MPa (10 bar) of seawater head.
Unit Conversion Reference
| Quantity | Conversion |
|---|---|
| Density | 1025 kg/m³ = 1.025 g/cm³ = 64.0 lb/ft³ |
| 1 kg/m³ | = 0.06243 lb/ft³ |
| 1 lb/ft³ | = 16.018 kg/m³ |
| Salinity | 35 PSU = 35 g/kg = 3.5% by mass |
| Pressure head | 1 m seawater = 10.06 kPa = 1.46 psi |
| 1 m water | = 9.81 kPa = 1.42 psi |
Example: a desalination intake pump lifting 500 m³/h of seawater 20 m requires head = 20 m × 1.025 = 20.5 m fresh-water equivalent — a small but real correction for pump selection. Mass flow = 500 m³/h × 1025 kg/m³ = 512,500 kg/h ≈ 142 kg/s.
Related Calculator
Use the Fluid Density Calculator for saltwater density at different salinities/temperatures, the Density Calculator for general mass-volume conversions, the Pipe Flow Calculator for seawater line sizing, and the Pipe Velocity Calculator to check erosion velocities in copper-nickel and FRP lines.
Frequently Asked Questions
What is the density of seawater? About 1025 kg/m³ at 20°C and 35 PSU salinity — 2.5% denser than fresh water. Cold deep ocean water reaches 1027-1028 kg/m³; warm coastal water can be 1020-1024 kg/m³.
What is the specific gravity of seawater? Approximately 1.025, ranging from 1.005 (Baltic Sea) to 1.030 (Red Sea). SG = density relative to fresh water at 4°C.
What is the unit weight of seawater? About 10.06 kN/m³ (64.0 lb/ft³) — use this instead of 9.81 kN/m³ for hydrostatic pressure, buoyancy, and pipe stress calculations in marine systems.
Why is seawater denser than fresh water? Dissolved salts (mostly NaCl, ~35 g/kg) add mass without significantly changing volume. Each additional PSU of salinity raises density by about 0.7 kg/m³.
How do I convert seawater density to lb/ft³? Multiply kg/m³ by 0.06243: 1025 × 0.06243 = 64.0 lb/ft³. Conversely multiply lb/ft³ by 16.018 to get kg/m³.
What pressure does 100 m of seawater exert? P = ρgh = 1025 × 9.81 × 100 ≈ 1.006 MPa ≈ 10.1 bar ≈ 146 psi — about 2.5% higher than fresh water at the same depth.
Engineering Implications
Marine Piping & Pumping
- Use 1025 kg/m³ density for pump head/power calculations (not 1000)
- Pressure drop ~2-3% higher than fresh water (higher density × slightly higher viscosity)
- Seawater service requires corrosion-resistant materials: Cu-Ni 90/10, 70/30, titanium, duplex stainless steel, or FRP
- Avoid carbon steel without coating/lining — rapid corrosion in aerated seawater
Desalination
- Reverse osmosis operating pressure: 55-65 bar for seawater (vs 10-15 bar for brackish water)
- Osmotic pressure of 35 PSU seawater: ~25 bar
- Distillation (MSF/MED) exploits boiling point elevation
Offshore Structures
- Buoyancy calculations use seawater density (1025 kg/m³) — Archimedes' principle with salt water
- Wave loading and hydrostatic pressure based on saltwater density
- Marine growth on submerged surfaces adds weight and increases hydrodynamic loading
Corrosion Considerations
- Seawater is highly conductive (5 S/m) — galvanic corrosion between dissimilar metals is aggressive
- Severe for steel (~0.1-0.3 mm/yr corrosion rate; up to 1 mm/yr in splash zone)
- Biofouling increases weight and reduces heat transfer in exchangers and piping
- Materials selection critical: 316 stainless only for moderate service; duplex SS, Cu-Ni, titanium for long-term
Summary
Seawater (average density 1025 kg/m³ at 20°C/35 PSU) is 2.5% denser than fresh water with slightly higher viscosity and significantly different thermal/electrical properties. Marine engineering requires these corrected values for pump sizing, pressure drop, buoyancy, and materials selection. Dissolved salts and biological activity make seawater one of the most corrosive natural environments — material selection is critical for long-term reliability.