Seawater Density Calculator
Enter temperature, Practical Salinity and pressure (or depth and latitude) to get seawater density with UNESCO EOS-80: 1025.97 kg/m³ at 15 °C, S = 35 at the surface, 1031.43 kg/m³ at 10 °C and 1000 dbar. Also returns sigma-t, potential temperature, sigma-theta and the freezing point; EOS-80 agrees with TEOS-10 to within about 0.04 kg/m³.
Use the Seawater Density Calculator
Input Parameters
Valid range −2 to 40 °C (28.4 to 104 °F), ITS-90.
Dimensionless; often written PSU or ‰. Open ocean ≈ 35.
Sea pressure excludes the atmosphere; 0 = surface. Max 10,000 dbar.
Gravity varies with latitude, so the same depth is a slightly different pressure.
Results
Enter values and click Calculate
In-situ Seawater Density
kg/m³
g/cm³
lb/ft³
lb/US gal
Sigma-t (σt)
ρ(S,T,0) − 1000
Sigma-theta (σθ)
ρ(S,θ,0) − 1000
Typical seawater densities: EOS-80 vs TEOS-10
In-situ density in kg/m³. TEOS-10 uses Reference Salinity (no composition anomaly). σθ is EOS-80 potential density anomaly referenced to the surface.
| Water type | T °C | S | p dbar | EOS-80 | TEOS-10 | σθ |
|---|---|---|---|---|---|---|
| Fresh water (max density) | 4.0 | 0.0 | 0 | 999.975 | 999.976 | −0.025 |
| Brackish estuary | 15.0 | 10.0 | 0 | 1006.783 | 1006.784 | 6.783 |
| Tropical ocean surface | 28.0 | 35.0 | 0 | 1022.395 | 1022.395 | 22.395 |
| Temperate ocean surface | 15.0 | 35.0 | 0 | 1025.972 | 1025.976 | 25.972 |
| Red Sea surface | 26.0 | 40.5 | 0 | 1027.190 | 1027.188 | 27.190 |
| Polar brine | −1.5 | 40.0 | 0 | 1032.237 | 1032.231 | 32.237 |
| Mediterranean outflow | 13.0 | 38.4 | 1,000 | 1033.433 | 1033.435 | 29.060 |
| North Atlantic Deep Water | 2.0 | 34.9 | 3,000 | 1041.571 | 1041.575 | 27.910 |
| Antarctic Bottom Water | −0.5 | 34.7 | 5,000 | 1050.538 | 1050.543 | 27.905 |
| Challenger Deep bottom | 2.5 | 34.7 | 10,900 | 1073.414 | 1073.426 | 27.797 |
Seawater density at depth (S = 35)
EOS-80 in-situ density in kg/m³ by temperature and sea pressure (1 dbar ≈ 1 m).
| T °C | 0 dbar | 1,000 dbar | 2,000 dbar | 4,000 dbar | 6,000 dbar |
|---|---|---|---|---|---|
| 0 | 1028.11 | 1032.82 | 1037.43 | 1046.36 | 1054.91 |
| 2 | 1027.97 | 1032.63 | 1037.19 | 1046.02 | 1054.48 |
| 5 | 1027.68 | 1032.26 | 1036.75 | 1045.44 | 1053.79 |
| 10 | 1026.95 | 1031.43 | 1035.82 | 1044.32 | 1052.50 |
| 15 | 1025.97 | 1030.36 | 1034.67 | 1043.02 | 1051.05 |
| 20 | 1024.76 | 1029.09 | 1033.32 | 1041.55 | 1049.46 |
| 25 | 1023.34 | 1027.61 | 1031.80 | 1039.92 | 1047.74 |
| 30 | 1021.73 | 1025.95 | 1030.10 | 1038.15 | 1045.89 |
Method, worked example and limits
Equation. ρ(S,T,p) = ρ(S,T,0) ÷ (1 − p ÷ K(S,T,p)), with p in bars (dbar ÷ 10). ρ(S,T,0) is the Millero–Poisson (1981) one-atmosphere polynomial and K the secant bulk modulus. EOS-80 was fitted on the IPTS-68 scale, so ITS-90 temperatures are multiplied by 1.00024 first.
Worked example. T = 10 °C, S = 35, p = 1000 dbar (about 990 m at 45°): ρ(S,T,0) = 1026.95 kg/m³ and K = 23,033 bar, so ρ = 1026.95 ÷ (1 − 100 ÷ 23,033) = 1031.43 kg/m³. The potential temperature is 9.88 °C, so σθ = 26.97 while σt = 26.95.
Depth and pressure. Depth = (9.72659·p − 2.2512×10⁻⁵·p² + 2.279×10⁻¹⁰·p³ − 1.82×10⁻¹⁵·p⁴) ÷ (g(φ) + 1.092×10⁻⁶·p), with g(φ) from the 1967 gravity formula (UNESCO 1983). Depth input is converted by solving this for p.
Limits. EOS-80 is valid for −2 to 40 °C, salinity 0 to 42 and 0 to 10,000 dbar. TEOS-10 is the current standard for publication-grade work; differences in the table above come mainly from the newer fit, and real samples can add a composition (Absolute Salinity) anomaly of up to about 0.02 kg/m³.
- UNESCO Technical Papers in Marine Science 44 (Fofonoff and Millard, 1983) — density, θ, freezing point and depth algorithms with check values.
- IOC, SCOR and IAPSO (2010) TEOS-10 Manual
- NIST SP 811 density conversion factors
Summary
Enter temperature, Practical Salinity and pressure (or depth and latitude) to get seawater density with UNESCO EOS-80: 1025.97 kg/m³ at 15 °C, S = 35 at the surface, 1031.43 kg/m³ at 10 °C and 1000 dbar. Also returns sigma-t, potential temperature, sigma-theta and the freezing point; EOS-80 agrees with TEOS-10 to within about 0.04 kg/m³.
How it works
- Enter water temperature in °C or °F (ITS-90; converted to IPTS-68 internally as EOS-80 requires).
- Enter Practical Salinity (PSS-78, often written PSU or ‰), typically 0–42.
- Enter sea pressure in decibars, or a depth in meters or feet plus latitude to convert depth to pressure (UNESCO 1983, Saunders–Fofonoff).
- The tool evaluates the UNESCO EOS-80 surface density and secant bulk modulus, ρ = ρ(S,T,0) ÷ (1 − p/K).
- It also computes sigma-t, potential temperature θ (Bryden 1973 lapse rate, Fofonoff Runge–Kutta), potential density σθ and the freezing point.
Use cases
- Get in-situ density at a given ocean depth for buoyancy, ballast or submersible planning.
- Check CTD readings or lab samples against expected density and sigma-t.
- Compare water masses with potential density (σθ), which removes the pressure effect.
- Convert between depth and pressure for a CTD cast at a known latitude.
- Find whether a cold, salty sample is near its freezing point.
- Support oceanography coursework and estuary stratification studies.