Defenses

Table taken from Infolist v3.1 created by Eden Tan.

Planetary/Base Defenses

Defense Posts for n Fighters:   n*(n-1) + 1   
Defense Posts for n Beamtech: 2*n*(n-1) + 1       
Defense Posts for n Beams   : 3*n*(n-1) + 1

Ex: For 5 fighters, 5*4+1 = 21 defense posts (see table below)

NOTE: Base defense is taken into account ONLY when determining the # of beams
      It does NOT affect the # of fighters or the beam tech level.

Defense :   1   3   5   7  13  19  21  25  31  37  41  43  57  61  73  85  91
Fighters:   1   2   2   3   4   4   5   5   6   6   6   7   8   8   9   9  10
Tech    : Las Las XRa XRa Pla Pla Pla Bla Bla Bla Pos Pos Pos Dis Dis HBl HBl
Beams   :   1   1   1   2   2   3   3   3   3   4   4   4   4   5   5   5   6

Defense : 111 113 127 133 145 157 169 181 183 211 217 221 241 265 271 273 307
Fighters:  11  11  11  12  12  13  13  13  14  15  15  15  16  16  16  17  18
Tech    : HBl Phr Phr Phr HDs HDs HDs HPh HPh HPh HPh HPh HPh HPh HPh HPh HPh
Beams   :   6   6   7   7   7   7   8   8   8   8   9   9   9   9  10  10  10

Ground Attack/Defense

For each defense post on a planet, your ground defense goes up by .05 x 
ground defense # against ground attacks.  For each 20 defense posts, the
ratio changes by the ground defense # exactly.

Example with Lizards 30:1 ground attack against Birds 2:1 ground defense:

   Def   Ratio      Ratio Formula: (att #):((def #)*(DP*.05 + 1))
   ---  ------
     0    15:1   1 lizard colonist will kill 15 enemy colonists 
    10    10:1   eg (30:(2*(10*.05 + 1)) = (30:3) = (10:1)
    20   7.5:1      (30:(2*(20*.05 + 1)) = (30:4) = (7.5:1)
    30     6:1      (30:(2*(30*.05 + 1)) = (30:5) = (6:1)
    40     5:1
    60  3.75:1
    80     3:1
   100   2.5:1