Your Kua number is a single digit between 1 and 9 (excluding 5) calculated from your birth year and gender. It identifies your auspicious and inauspicious compass directions in the Ba Zhai (Eight Mansions) school of Feng Shui.
Short answer
Your Kua number is a single digit between 1 and 9 (excluding 5) calculated from your birth year and gender. It identifies your auspicious and inauspicious compass directions in the Ba Zhai (Eight Mansions) school of Feng Shui.
In depth
For people born up to and including 1999: males subtract the last two digits of birth year from 100, then divide by 9 and use the remainder. Females add 5 to the same last two digits, then divide by 9 and use the remainder. For 2000+: males use 9 minus (last-two-digits sum reduced mod 9); females use 6 plus the same.
A simpler shortcut: use a free Kua calculator that handles the formula and the boundary cases (year 1 = 9, Kua 5 substitution rules) for you.
Kua 5 doesn't exist as a personal number — males with a calculated Kua 5 use Kua 2; females with calculated Kua 5 use Kua 8. Kua numbers split into two groups: East Group (1, 3, 4, 9) and West Group (2, 6, 7, 8). East Group people have auspicious East-facing directions; West Group people have auspicious West-facing directions.