Hello sandeep,
You manufacture (mold and cut) a plack in your lab, accurately measure the dimensions and thickness, adjust the dimensions to 20 degrees Celsius and calculate the volume. Then you accurately measure the weight of the plack with an electronic scale and perform the density calculation. Follow all proper laboratory procedures such as not touching the plack with your bare hands.
Do this complete procedure a half dozen times so you know your measurement error.
Sincerely
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com 
