Each hexadecimal digit is equivalent to four binary digits..
FFF = 1111 1111 1111 in binary.
convert now binary to decimal.the LSD raised to 0, 2nd LSD raised to 1 and so forth in ascending manner,raised in increment of 1.
FFF=4095 in decimal
. The process is quite long specially for the number of digits you had given in your posted message..If you have a scientific calculator with HEX OCT BIN and DEC functionality you can convert it directly..