Cousin Relationship Calculator
How this cousin relationship calculator works
This calculator identifies the standard genealogical “cousin degree” and any “removed” amount between two people based on how many generations each person is from their nearest shared (common) ancestor. It’s the same naming system used in most family history charts and genealogy software.
What to enter (how to count generations)
For each person, count the number of steps up the family tree to the shared ancestor you’re using:
- 1 = parent
- 2 = grandparent
- 3 = great-grandparent
- 4 = 2× great-grandparent, and so on
Tip: The most common mistake is choosing the wrong common ancestor. If two people share multiple ancestors (common in endogamy, pedigree collapse, or double-cousin situations), you’ll get different results depending on which shared ancestor you select.
Definitions: “cousin degree” vs. “removed”
Two values determine the label:
- Cousin degree (1st, 2nd, 3rd, …): how far down from the common ancestor the closer person is, minus one.
- Removed (once, twice, …): the difference in generations between the two people.
Formulas (genealogical standard)
Let gA be the generations from Person A to the common ancestor, and gB be the generations from Person B to the same ancestor.
Cousin degree:
Removed:
Special case: If gA = gB = 1, the two people share a parent, so they are siblings (not cousins).
How to interpret the result
The calculator returns one of these patterns:
- Siblings (when both are 1 generation from the same parent)
- kth cousins (when both are the same number of generations from the ancestor and that number is 2 or more)
- kth cousins, r times removed (when the generation counts differ)
“Removed” does not mean “not related.” It only indicates the two people are in different generations relative to the shared ancestor.
Worked example (step-by-step)
Scenario: You and another person are both descended from the same great-grandparent.
- From you to that great-grandparent is: you → parent (1) → grandparent (2) → great-grandparent (3). So gA = 3.
- From the other person to the same great-grandparent is also 3 steps. So gB = 3.
- Cousin degree: min(3, 3) − 1 = 2 → 2nd cousins.
- Removed: |3 − 3| = 0 → no “removed” term is added.
Quick lookup table (common inputs)
| Person A generations (gA) | Person B generations (gB) | Result | Typical relationship scenario |
|---|---|---|---|
| 1 | 1 | Siblings | Same parent |
| 2 | 2 | 1st cousins | Same grandparent |
| 3 | 3 | 2nd cousins | Same great-grandparent |
| 3 | 4 | 2nd cousin once removed | One person is a great-grandchild; the other is a 2× great-grandchild of the same ancestor |
| 2 | 3 | 1st cousin once removed | Your cousin’s child (or your parent’s cousin) |
| 4 | 6 | 3rd cousin twice removed | More distant cousins in different generations |
Assumptions and limitations (important)
- Nearest shared ancestor: The result depends on which common ancestor you choose. If the pair shares multiple ancestors, you may get multiple valid labels.
- Half relationships: This tool labels relationships by generation counts only and does not distinguish half-cousins, half-siblings, or step-relations.
- Aunt/uncle vs. cousin wording: Some inputs correspond to relationships commonly described as aunt/uncle or niece/nephew (e.g., 1 and 2). This calculator will still output a cousin-style label unless you interpret that case separately.
- Minimum value: Generations must be whole numbers of 1 or more. Entering 0 (the person is the ancestor) is not supported in this version.
- Real-world genealogy edge cases: Adoption, donor conception, and cultural naming conventions can change how families describe relationships even when the generation math is the same.
FAQ
- What does “once removed” mean?
- It means the two people are one generation apart relative to their shared ancestor (for example, your parent’s 1st cousin is your 1st cousin once removed).
- Why aren’t siblings considered cousins?
- Siblings share a parent (gA = gB = 1). In genealogy naming, “cousin” starts when the shared ancestor is a grandparent or older.
- How do I find the “common ancestor” to use?
- Look for the most recent ancestor both people descend from (often the nearest shared grandparent, great-grandparent, etc.). Then count each person’s steps up to that ancestor.
- Can two people have more than one cousin relationship?
- Yes. If they share more than one ancestral line (for example, double cousins), the cousin label can differ depending on which shared ancestor you use.
Source note: The rules used here follow standard cousin-degree and removal conventions commonly used in genealogy.