Bulk Modulus of Water
| Value | 2,180,000,000 Pa |
| Status | Measured: ± 30,000,000 Pa (0.014 relative) |
| Source | IAPWS / Crane TP-410 |
| Categories | Material PropertiesEngineering & Tradefluids |
| pascal | 2,180,000,000 Pa |
| kilopascal | 2,180,000 kPa |
| megapascal | 2,180 MPa |
| bar | 21,800 bar |
| atmosphere | 21,514.927 atm |
| millimeter of mercury | 16,351,342 mmHg |
| pound per square inch | 316,182.27 psi |
| foot of water column | 729,324.59 ft H₂O |
| gigapascal | 2.18 GPa |
| kip per square inch | 316.18227 ksi |
| inch of mercury | 643,753.64 inHg |
| inch of water column | 8,751,895.1 in w.g. |
Learning zone
Water is not incompressible, only nearly so: squeezing it to 100 bar reduces its volume by about 0.45 %. That is enough to matter in three places — hydraulic system stiffness and response, water hammer, and deep-ocean density. The sound speed follows directly from it, c = √(K/ρ) = 1480 m/s, which is why the surge pressure from stopping a 1 m/s flow instantly is around 1.5 MPa.
The trap in hydraulics is entrained air. Just 1 % of undissolved air by volume can cut the effective bulk modulus of the mixture by an order of magnitude, because the gas takes up almost all the compression. That is spongy brakes, a hydraulic actuator that will not hold position, and an accumulator working without being installed. Water's bulk modulus also rises with temperature to a maximum near 50 °C, unlike most liquids, and hydraulic oils are considerably softer at 1.5–2.0 GPa — which is why servo-hydraulic system stiffness calculations use the fluid's actual modulus rather than treating the oil as rigid.