I couldn’t find this information today when I needed, so I thought I’d share. If it’s in the Google maps documentation, then I missed it.

If you want to zoom a map to fit a particular distance as tightly as possible, you need to know the scale of each of the 19 levels. In metres per pixel, I worked them out to be as follows:

zoom m/px
  19  0.19
  18  0.37
  17  0.74
  16  1.48
  15  3
  14  6
  13  12
  12  24
  11  48
  10  95
   9  190
   8  378
   7  752
   6  1,485
   5  2,909
   4  5,540
   3  10,064
   2  16,355
   1  21,282

So if you needed to calculate the zoom level to fit an area of a given radius you could use a function like this:

function radiusToZoom( r ){
    var w = myMapInstance.getSize().width;
    var d = r * 2;
    var zooms = [,21282,16355,10064,5540,2909,1485,752,378,190,95,48,24,12,6,3,1.48,0.74,0.37,0.19];
    var z = 20, m;
    while( zooms[--z] ){
        m = zooms[z] * w;
        if( d < m ){
            break;
        }
    }
    return z;
}