Q:

Google Maps: how to calculate the "double" the zoom level?

This answer  to This question
is a good idea for solving my problem.
Now, the problem is: how do I calculate "double" the zoom level? 

EDIT
Meaning:
the map is looking at a square 2x2 km, I want to zoom out to look at a 4x4 km square.

Doing newZoom=oldZoom*2 does not work :-(((

A:

Each increment of the zoom level doubles the scale.  To zoom out by a factor of two, add one to the zoom level.
newZoom=oldZoom-1;

from the documentation:

World coordinates reflect absolute locations on a given projection, but we need to translate these into pixel coordinates to determine the "pixel" offset at a given zoom level. These pixel coordinates are calculated using the following formula:
pixelCoordinate = worldCoordinate * 2  zoomLevel 
From the above equation, note that each increasing zoom level is twice as large in both the x and y directions. Therefore, each higher zoom level contains four times as much resolution as the preceding level. For example, at zoom level 1, the map consists of 4 256x256 pixels tiles, resulting in a pixel space from 512x512. At zoom level 19, each x and y pixel on the map can be referenced using a value between 0 and 256 * 219

