🌳 Generations to Common Ancestor Calculator

Enter a cousin degree and how many times removed to see how many generations back the shared common ancestor is for each person.

Cousin degree (1 = 1st cousin, 2 = 2nd cousin, etc.)

Times removed

Reading a cousin label backwards

Give it the degree and the removal and it returns the two generation counts that produce that label, measuring from the couple both people descend from:

closer person  = degree + 1
farther person = degree + 1 + removed

The defaults, degree 2 and one removal, return 3 and 4. The closer of the two is a great-grandchild of the shared couple and the other is a great-great-grandchild. Third cousins with no removal return 4 and 4 — both are great-great-grandchildren of the same couple, which is what "third cousin" means.

What to do with the two numbers

Climb your own tree by the closer figure to find the couple to look for. For third cousins that is generation 4, so the link lies among your great-great-grandparents.
Count the candidate couples before you start. Generation g holds 2^(g-1) ancestral couples: 4 sets of great-grandparents, 8 sets of great-great-grandparents, 16 the step after. A distant label means a long list to work through.
Expected DNA falls out of the same pair. The share is 2^(1 − A − B), so 3 and 4 gives 1.56%, near 106 cM on a 6,800 cM basis. Real second cousins once removed have been recorded from about 45 to 235 cM.

Degree must be 1 or more, so aunts, uncles, nieces, nephews and siblings cannot be expressed here — they are the degree-0 cases. Entering 0 silently produces first cousins, so read the label on the card rather than assuming your entry was accepted.

Frequently asked questions

How far back is the common ancestor of third cousins?

Four generations for each of them, meaning a shared set of great-great-grandparents. You have eight such sets, so a documented third cousin match narrows a DNA link to one of eight couples.

What does once removed do to the generation counts?

It adds one to the farther person only. Second cousins are 3 and 3; second cousins once removed are 3 and 4; twice removed, 3 and 5. The closer person never moves.

Can I use the answer to confirm a DNA match?

Only as a prediction to test. The expected share for a given pair of counts is a single figure, while the observed range around it is wide and overlaps neighbouring relationships, so a total consistent with your label is support, not proof.