The vCalc Bulk Strain equation computes the deformation defined by the fractional change in volume. The Bulk Strain is a ratio of volumes so is a unit-less number.
Pressure creates stress on an object resulting in volume deformation. The corresponding strain is the fractional change in volume. When Hooke's Law applies, the increase in pressure (bulk stress) results in a proportional bulk strain.
Young, Hugh and Freeman, Roger. University Physics With Modern Physics. Addison-Wesley, 2008. 12th Edition, (ISBN-13: 978-0321500625 ISBN-10: 0321500628 ) Pg 366, eq 11.12