Most Recent Common Ancestor Calculator

The most recent common ancestor (MRCA) is the closest individual whose descendants include both people in question. Knowing how many generations back the MRCA lived is essential for:

• Tree planning: how far back you need to research to find the common branch
• Relationship verification: confirming someone IS your second cousin requires identifying a shared great-grandparent
• DNA analysis: relationship distance correlates with shared centimorgans, but only relative to MRCA generation

Standard relationship → MRCA generations:

Siblings: MRCA is the parents (1 generation back). Easy.

First cousins: MRCA is the grandparents (2 generations back). Each cousin’s parent is the sibling of the other cousin’s parent.

Second cousins: MRCA is great-grandparents (3 generations back). Each line: cousin → parent → grandparent → great-grandparent (shared) ← grandparent ← parent ← cousin.

Third cousins: MRCA is great-great-grandparents (4 generations back).

Fourth cousins: MRCA is 3rd-great-grandparents (5 generations back).

The pattern: Nth cousin shares (N+1)-greats-grandparents.

“Removed” relationships add generations to ONE side only. First cousins once removed: one person is N generations from MRCA, the other is N+1 generations. Their MRCA is the great-grandparent of one and the grandparent of the other.

Great-aunt and great-niece: MRCA is the great-aunt’s parent (which is the great-niece’s great-grandparent). 1 generation up for one person, 3 generations up for the other.

The “all relatives” formula:

For any two relatives, write down “X generations from common ancestor” for each. Use those two numbers to look up the relationship in a relationship chart (also called a canon law table).

Examples:

• (1, 1): siblings
• (2, 2): first cousins
• (3, 3): second cousins
• (1, 3): great-aunt and great-niece (or great-uncle)
• (2, 3): first cousin once removed
• (3, 4): second cousin once removed
• (4, 4): third cousins

The number of generations to MRCA is the LARGER of the two numbers.

Why this matters for DNA: each generation of separation halves the expected shared DNA on average. Siblings share ~50%, first cousins share ~12.5%, third cousins share ~0.78%. By 5th cousin (6 generations to MRCA), shared DNA averages 0.05% and many actual 5th cousins share zero detectable DNA.

Pedigree collapse: if your tree converges on the same person multiple times (which happens in any tree by ~10 generations back due to mathematical inevitability), the MRCA may be more recent than the canonical relationship suggests. This becomes important in endogamous communities where collapse is severe.

For genealogy databases, MRCA generations help you decide where to research. A confirmed 4th cousin needs a tree extended to 5+ generations back to find the connection. If you have only 3 generations documented on each side, the MRCA is invisible.