Interpreting shared_buffers setting

Excuse me if this is a silly question.  I am trying to fiddle with
shared_buffers setting on postgresql 10.6 on ubuntu 18.04 server.

I have this at bottom of my config file:
shared_buffers = 1GB

Yet when I check the setting from pg_setting I see something quite different:

postgres=# SELECT name, setting FROM pg_settings where name = 'shared_buffers';
      name      | setting
----------------+---------
 shared_buffers | 131072

Is this a question of units?  It looks like 128M.  Note when I change
the setting to 2GB in conf file I see 262144 from pg_setting.  I am
now unsure what the actual shared_buffers allocation is.  I cant see
anything in the docs which tells me how to interpret the integer.

Any clarification welcome.

Regards
Bob