Ahnentafel Number Calculator
Turn an Ahnentafel ancestor number into the path back from the subject, and a path such as mother’s father’s mother back into its number.
A numbering system for ancestors
Ahnentafel, German for “ancestor table”, numbers every ancestor in a pedigree so that the number alone says who they are. The subject is 1. Anyone’s father is twice their number and their mother twice their number plus one. So the subject’s father is 2 and mother 3; the paternal grandparents are 4 and 5, the maternal grandparents 6 and 7; the great-grandparents are 8 to 15. Fathers are always even and mothers odd, except the subject. Each generation g holds the numbers from 2^g to 2^(g+1) − 1: the 16 great-great-grandparents are 16 to 31.
Reading a number
Write the number in binary and drop the leading 1; each remaining digit is one step back, 0 for father and 1 for mother. Number 13 is 1101 in binary. Dropping the first 1 leaves 1, 0, 1: mother, father, mother. Ancestor 13 is the subject’s mother’s father’s mother, a great-grandmother on the maternal side. Her husband is 12, her parents are 26 and 27, and her child in the chart is 6, the subject’s maternal grandfather. Going the other way, the path FMF, father’s mother’s father, is 1 → 2 → 5 → 10: ancestor 10. Genealogy software uses the same scheme in Ahnentafel reports, often called ancestor lists or register-style reports.
Why it is useful
Ahnentafel numbers make it easy to refer to ancestors in notes, correspondence and filing systems without long descriptions. They also show the shape of research at a glance: if you have names for all of 8 to 15 but only a few of 16 to 31, you know which lines need work. The numbers are stable as long as the subject stays the same, so a folder labelled 13 always belongs to the same woman. A common mistake is to renumber when the subject changes; instead, keep one subject per chart, or note which chart a number belongs to.
Pedigree collapse
When cousins marry, the same person fills more than one slot. If the subject’s parents were second cousins, a pair of great-great-grandparents appears twice, for example as both 18 and 19 and 28 and 29. The numbering still works: the person simply has two numbers, and good practice is to record both and cross-reference them. Over many generations, collapse is universal, which is why the number of distinct ancestors is always far smaller than 2^g.
How to use it
- Choose whether you are starting from a number or a path.
- Enter the Ahnentafel number, or the path as F for father and M for mother, starting from the subject.
- Read the relationship, the generation and whether the line is male or female.
- Use the parents’ and child’s numbers to move around the chart.
Questions from the record office
How do Ahnentafel numbers work?
The subject is 1. A person’s father is twice their number and their mother twice their number plus one. So 2 and 3 are the parents, 4 to 7 the grandparents and 8 to 15 the great-grandparents. Men have even numbers and women odd numbers, except the subject.
Who is ancestor number 13?
The subject’s mother’s father’s mother, a great-grandmother on the maternal side. In binary, 13 is 1101; dropping the leading 1 leaves 101, which reads mother, father, mother, with 1 for mother and 0 for father.
What generation is an Ahnentafel number in?
Find the largest power of 2 that is not greater than the number: that exponent is the generation. Numbers 16 to 31 are generation 4, the great-great-grandparents, and numbers 1,024 to 2,047 are generation 10.
What happens to Ahnentafel numbers when cousins married?
The same ancestor appears in more than one position and therefore has more than one number. This is pedigree collapse. Record every number the person holds and cross-reference them, rather than leaving the second position blank.
Disclaimer: Genealogy results apply published conventions and inheritance averages to the values you enter. They are not proof of a relationship, parentage or descent, and not legal, medical or genetic advice. Several relationships share each DNA figure, and calendars and spellings varied by place and time: check every conclusion against original records, and consider the privacy of living relatives before sharing it.