How many GB is a pen drive 983 MB

983MB would be .983GB.

it's likely that this is a 1GB pen drive and two factors are affecting its' size display

1) formatting takes a slight area of space, reducing the size slightly

and

2) Windows incorrectly displays the units 'GiB'/'MiB' as 'MB'/'MiB'. device manufacturers use the latter (base 10) while most operating systems display the former (base 2)

see this page for more info:
http://wintelguy.com/gb2gib.html