help needed! on hard disc space

[need help]

I bought an external hard disc. The product specified 160G. But it actually showed 149G free space in my computer. Is it normal due to disk format? or because it has damaged places. Thanks

 

[Caohaoying]

cause 1 G has 1024 M. and 1M has 1024 k. 1k has 1024 bytes.
therefore 160,000,000,000 bytes = 149x1024x1024x1024 bytes = 149Gbytes.