I  am  building  a  chorus  and  want  to  make  pre  and  post
LP  filter  with  just  one  transistor  per  18dB  filter. I  know  that
24dB  is  better,  but  want  to  simplify  the  schematic  and  save  
some  space on  PC board.
I  know  how  to  calculate  12db  filter  and  24dB  are  just  two of
12dB  in  series,  but  I  am  dont  know  how  to  calculate cutt-off
freq  of  one  18dB.
Can  you  help  me  please?

Except in a few special cases you can't write formulas for the -3dB cut-off of 18dB and 24dB filters (and beyond).   Generally you have to simulate the filter or solve complicated equations.

For a Butterworth response you can compute the 'f0' or 'w0' for the filter and this is the same as the -3dB regardless of the filter order/slope.  Butterworth works out easy.