I think what you are looking for is
du -s <directory> shows you much disk space a directory's contents use up.
du can also count the sizes of each file in the directory individually and tell you how big their total content is:
du -s --apparent-size <directory>
If you want to know the amount of "wasted" space resulting from allocation in blocks, just subtract the second command's result from the first's.
--apparent-size reports the size in kilobytes. You can use the
-b flag instead, if you want to know the exact number of bytes a file contains. This is useful if you want to know how many bytes (without headers and such) you would need to send over the network to deliver the file, for example.