Unit Weight of Water
Geotechnical calculations rarely use density directly: they want unit weight, the force per volume that turns depths into pressures.
9.81 kN per cubic meter
| Fresh water | 9.81 kN per cubic meter |
| Common rounded value | 10 kN per cubic meter |
| Imperial | 62.4 lb per cubic foot |
| Seawater | about 10.05 kN per cubic meter |
| Pressure per meter of depth | 9.81 kPa |
Notes for design and site
Every effective stress line on every geotechnical section is total stress minus this number times the head. The lazy 10 overstates pore pressure by two percent, conservative in some checks and not in others, so codes that care say 9.81. Behind a retaining wall, this is also the number that usually wins the fight: a meter of trapped water pushes harder than most of the soil it soaked.
Related Calculators and Reference
- Groundwater Flow Simulator: watch these properties drive an aquifer.
- Effective Stress Equation: where the unit weight of water earns its keep.
- USGS Water Science School: authoritative water properties and explanations.