🔢 Ahnentafel Number Calculator
Enter an Ahnentafel number (the standard genealogical numbering system where a person's father is 2×n and mother is 2×n+1) to find their generation and direct lineage path back to the root person (number 1).
The arithmetic behind the numbering
Ahnentafel (German for "ancestor table") gives every direct ancestor one number. You are 1; from any person n:
father = 2n mother = 2n + 1
parent of n (up) = floor(n / 2)
generation = floor(log2 n) + 1
generation g holds numbers 2^(g-1) to 2^g - 1Enter 13 and the tool returns generation 4, a woman, and the path 1 → 3 → 6 → 13. Read it outwards: 3 is your mother, 6 is her father, and 13 is his mother — your maternal great-grandmother. Her own parents are 26 and 27. The path is built by halving 13 to 6, 6 to 3, 3 to 1 and reversing the list.
Reading the number itself
- Even is a man, odd is a woman, with 1 as the only exception since the root person can be either. So 12 is a great-grandfather and 13 is a great-grandmother, whatever the intervening lines.
- Generation 1 is you, 2 the parents (2–3), 3 the grandparents (4–7), 4 the great-grandparents (8–15). Add "great" once for every generation past 3.
- The all-paternal line doubles: 1, 2, 4, 8, 16. The all-maternal line is one less than the next power of two: 3, 7, 15, 31.
Frequently asked questions
What is my father's Ahnentafel number?
2, and your mother is 3. Your paternal grandfather is 4, paternal grandmother 5, maternal grandfather 6 and maternal grandmother 7.
How do I work out the generation from an Ahnentafel number?
Count how many times you can halve the number before reaching 1, then add one. Number 100 halves to 50, 25, 12, 6, 3, 1 — six steps, so generation 7, a fourth great-grandparent.
Can two Ahnentafel numbers be the same person?
Yes, when the same ancestor is reached down two different lines. The numbering has no way to say so, which is why software flags duplicate individuals separately from their numbers.