Font PX size in Ems
 
  50px = 3.13em
40 = 2.5 
32 = 2    
30 = 1.88
24 = 1.50
20 = 1.25
16 = 1     
15 = 0.94
14 = 0.88
12 = 0.75
10 = 0.63
  9 = 0.56
  8 = 0.50
  7 = 0.44
  5 = 0.31
  3 = 0.19
  1 = 0.06
  .5 = 0.03 
 

For more details see:     http://riddle.pl/emcalc/