Do you mean 5 digit BCD to hex?
if this was the case then 5 digit bcd max = 99999 decimal using windows calculator to convert it to hex you get 1869F. So you need 17 bits to store it in.
for simplicity you need 3 bytes for each 5 digit BCD input to store it.
If you are pushed for space you can utilize the 6 bits free of the last byte to hold data and save space.
Mahmood