Ahnentafel numbering, explained
Short answerAhnentafel numbering gives every direct ancestor a number. You are 1. Anyone’s father is twice their number (2n) and their mother is twice plus one (2n+1), so your father is 2, your mother 3, and your grandparents 4 to 7. Every father except you is even and every mother is odd, and the number alone tells you the generation and the line.
The rules
- You (the starting person) are 1.
- Father of person n is 2n. Fathers are always even.
- Mother of person n is 2n + 1. Mothers are always odd (except number 1, who can be anyone).
- Child of person n is n ÷ 2, rounded down.
- Generation = the number of times you can halve n before reaching 1, plus one. In symbols, generation = ⌊log₂ n⌋ + 1. Generation g holds the numbers 2g−1 to 2g − 1.
The German word Ahnentafel means “ancestor table”. The system is also called Sosa-Stradonitz numbering, after Jerónimo de Sosa, who used it in 1676, and Stephan Kekulé von Stradonitz, who popularized it in 1896; Michael Eytzinger published an early version in 1590.
A worked table for four generations
| Number | Generation | Who |
|---|---|---|
| 1 | 1 | You |
| 2 | 2 | Father |
| 3 | 2 | Mother |
| 4 | 3 | Father’s father |
| 5 | 3 | Father’s mother |
| 6 | 3 | Mother’s father |
| 7 | 3 | Mother’s mother |
| 8 | 4 | Father’s father’s father |
| 9 | 4 | Father’s father’s mother |
| 10 | 4 | Father’s mother’s father |
| 11 | 4 | Father’s mother’s mother |
| 12 | 4 | Mother’s father’s father |
| 13 | 4 | Mother’s father’s mother |
| 14 | 4 | Mother’s mother’s father |
| 15 | 4 | Mother’s mother’s mother |
Reading a number backwards
Take ancestor 13. It is odd, so 13 is a mother; her child is 13 ÷ 2 rounded down, which is 6. Number 6 is even, a father; his child is 3. Number 3 is odd, a mother; her child is 1, you. Reading back up: 13 is your mother’s father’s mother, one of your great-grandmothers.
A quicker trick: write n in binary and drop the leading 1. Each remaining digit is a step up, 0 for father and 1 for mother. 13 is 1101 in binary; dropping the first 1 leaves 101: mother, father, mother.
Why genealogists use it
- Every ancestor has one fixed number, so lists, files and charts stay in step.
- Gaps are obvious: if 22 is missing you know exactly which great-great-grandfather to look for.
- The number of possible ancestors in generation g is 2g−1: 8 great-grandparents, 16 in the next generation, 32 in the one after.
- When the same person appears twice, because cousins married, they simply hold two numbers. This is called pedigree collapse.
Doing this in Family Trees
Family Trees doesn’t print ahnentafel numbers, but its Ancestors view and Fan chart lay out the same structure: each generation is a row or ring, with the father’s line on one side and the mother’s on the other. Try the printable 4-generation template, whose boxes follow ahnentafel order.
Questions
What does ahnentafel mean?
Ahnentafel is German for “ancestor table”. It is a list of a person’s direct ancestors in which each one has a number that shows their exact position in the family tree.
What is ahnentafel number 1?
Number 1 is the person the table starts from, usually you. Their father is 2 and their mother is 3.
How do I find the generation from an ahnentafel number?
Count how many times you can halve the number (rounding down) before reaching 1, then add one. Numbers 8 to 15 are generation 4, the great-grandparents of person 1.
Why are all fathers even numbers?
Because a father’s number is always twice his child’s number, and any number doubled is even. Mothers are twice plus one, so they are always odd.
Sources
Links go to other sites. Checked 19 October 2026.
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