The birth number of a person is calculated according to the date of birth of a person. For an example if a person has his date of birth on 25-08-1988 then add all these numbers in this way, 2+5+0+8+1+9+8+8 = 41. Now the total is 41 add it within 4+1=5. (You need to continue adding till the time you get any of these: 1 to 9 or 11, 22 or 33. ). Now as the final digit is 5 here so, 5 is the Birth number of this person.