Apparent porosity defines the relationship between the volume of the mass absorbed to the volume of the water absorbed. Here is the Apparent porosity formula to calculate the apparent porosity. To compute, divide the weight of dry piece by the weight of the soaked piece and subtract the obtained value from the weight of the soaked piece. Again subtract the weight of the piece soaked and immersed from the resultant value. Multiply with 100 to get the result in percentage.
The difference between apparent porosity and true porosity is the former does not include the volume of the sealed pores while the later includes the volume of the closed pores. After substituting the values in the Apparent porosity formula, do the calculations as per the BODMAS rule.