Bulk modulus is the ratio of hydraulic stress to the corresponding hydraulic strain.  It is given by B = -P / (∆v / V) where P is the pressure and (∆v / V) is the volumetric strain.

Also, bulk modulus is reciprocal of compressibility (k). The compressibility is the fractional change in volume per unit increase in pressure. For each atmosphere increase in pressure, the volume of water would decrease by the compressibility value. The value of k for water is 45.8 * 10^-11 m² / N.

In the problem 4, in video 12, Class 11 Mechanical properties of solid, the above value is used for k.