You can calculate it by multiplying the length of the boat by the width and by draft (in feet) and corr. factor and by dividing this amount authorised with 100.
BT Bavaria 37 Cruiser= (37.45ft × 12.54ft × 6.43ft × 0.57)/100= cca 17.21 BT
If you want to calculate BT of catamaran Lagoon 450 in charter than you can calculate:
BT Lagoon 450= (46.06ft × 25.87ft × 4.29ft × 0.57)/100= cca 30 BT
For power vessels you can use the same formula, but only by using value 0.67 instead 0.57 in the formula above. This formula works for vessels up to cca 75ft (cca 23m) and it is only informative. You need to check your license for each boat! This is very important.