Ahnentafel Number Calculator

Enter an ahnentafel number to see which ancestor it is, or build the number from your line of fathers and mothers. Load a GEDCOM file to number your own ancestors.

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The Ahnentafel Number Calculator tells you who an ahnentafel number stands for. In this numbering, also called Sosa-Stradonitz, you are number 1, your father is 2, your mother is 3, and every father is twice his child’s number and every mother twice plus one. Type a number and you get the relationship, such as “great-grandmother”, the line of fathers and mothers from you to that person, the generation and the numbers of their parents. A path builder works the other way round and turns a line of parents into a number. A second tab numbers the ancestors in a GEDCOM file, and a third converts Henry and d’Aboville numbers for descendants. All of it runs in your browser.
Ahnentafel example

How to use the ahnentafel number calculator

  1. Open the tab “Number to relationship”. Type a whole number into “Ahnentafel number”, or press one of the example buttons.
  2. Read the first tile, “This person is your”. Below it you see “Generation”, “Line from you”, the steps as F and M, “Father’s number”, “Mother’s number”, “Partner’s number” and “Child in your line”.
  3. To build a number, press “Add father” or “Add mother” once for each step from yourself. The number field and the tiles follow every press. “Remove last step” goes one step back and “Start again” returns to 1.
  4. Open “Generations and GEDCOM” to see the number ranges of the first six generations and the relationship of every number from 1 to 31.
  5. To number real ancestors, press “Choose File” or drop a GEDCOM file on the box, or press “Try the sample family”. The file stays in your browser.
  6. Under “Person (number 1)” choose the person to start from and under “Generations to list” how far back to look. The table lists the ancestors found, with copy and “Download CSV” buttons.
  7. Open “Descendant numbers” to convert a number between d’Aboville and Henry. Choose the system, type the number and read the other form.

What is an ahnentafel number?

The ahnentafel list gives every ancestor one number. You are 1, your father is 2 and your mother is 3. Your father’s parents are 4 and 5, your mother’s parents are 6 and 7. The rule is the same at every step: a father has twice the number of his child and a mother has that number plus one. So 13 is the mother of 6, 6 is the father of 3 and 3 is the mother of you. Read from you, 13 is your mother’s father’s mother, a great-grandmother.

What does the generation number tell me?

The generation is the number of steps from you, and it is the position of the leading digit of the number written in binary. Generation g holds the numbers from 2 to the power g up to 2 to the power g plus 1, minus one. Generation 3, the great-grandparents, holds the numbers 8 to 15. An even number is always a man and an odd number a woman, because only fathers are doubled.

How are the relationship names chosen?

Generations 1 to 3 are parents, grandparents and great-grandparents. From the fourth generation the name counts the greats with an ordinal: 16 is a 2nd great-grandfather, 37 a 3rd great-grandmother. The calculator also shows the older spelling with repeated greats, as in great-great-grandfather, for up to five greats.

What are the Henry and d’Aboville systems?

They number descendants instead of ancestors. The first person is 1 and each child adds a position. In d’Aboville numbers the positions are separated by dots, so 1.2.10 is the tenth child of the second child of 1. In Henry numbers they are written together, and the tenth child is X, the 11th to 33rd are A to W: 1.2.10 becomes 12X.

What is pedigree collapse?

When the same person is an ancestor by two lines, the GEDCOM list shows one person under two numbers and marks both.

Frequently asked questions

What does ahnentafel mean?

It is German for ancestor table. The list numbers the ancestors of one person, called the proband, so that the number tells you the line.

Is Sosa-Stradonitz the same as ahnentafel?

Yes. The numbering is also called the Sosa or Sosa-Stradonitz method and, in Wikipedia, the Eytzinger method. The father is 2n and the mother 2n+1 in all of them.

How do I find the father or mother of a number?

Double it for the father. Double it and add one for the mother. The calculator shows both numbers in the tiles “Father’s number” and “Mother’s number”.

How do I find someone’s husband or wife in the numbering?

The partner of an even number is that number plus one, and the partner of an odd number is that number minus one. Number 6 and number 7 are a couple.

How big a number can I enter?

Numbers up to generation 100 are accepted, which is about 1.27 times 10 to the 30th power. The number is handled exactly, not rounded.

Does the GEDCOM list change my file?

No. The file is read in your browser and nothing is saved or uploaded. The father is taken from the HUSB line and the mother from the WIFE line of the family the person was born into.