struct.calcsize problem

[Chandu]

In using the following struct format I get the size as 593. The same C
struct is 590 if packed on byte boundary and 596 when using pragma
pack(4). I am using pack(4) and added 3 spares at the end to get by.
   hdrFormat = '16s 32s 32s B 8s H 8s H 4s H H 20s 64s 64s 64s 32s 32s
64s L L B B B B B 64s 64s'

Any ideas on what I am doing wrong?