How to use the cousin calculator
There are two ways in. Pick generations if you know who the shared ancestor is: choose what that ancestor is to you (say, your great-grandparent) and what they are to the other person (their grandparent). Describe it in words if you only know the chain, such as “my dad’s cousin’s daughter” or “my grandmother’s brother”. Type it, or tap the steps to build it.
The answer shows the term from both sides, because they are not always the same: your great-aunt sees you as her grandniece or grandnephew. It also names the shared ancestor, explains how the term was worked out and gives the expected share of DNA. The cousin chart below the calculator shows every combination at once; select a box to see it explained, or print the whole chart.
What does “removed” mean?
“Removed” means you and your cousin are in different generations of the family. Count how many generations each of you is below the ancestors you share. If the numbers match, you are plain first, second or third cousins. If they differ, the difference is the number of times removed.
The most common case is a parent’s cousin. Your mom’s first cousin is one generation above you, so she is your first cousin once removed. The relationship works the same in both directions: you are her first cousin once removed too. The term tells you how far apart the generations are, not which of you is in the older one.
How cousin degrees work
Every cousin relationship can be worked out with the same four steps:
- Find the closest common ancestor. Usually this is a couple, such as a shared set of grandparents.
- Count generations down to each person. A child is 1, a grandchild 2, a great-grandchild 3, and so on.
- Take the smaller number and subtract one. That is the cousin degree. Shared grandparents (2) make first cousins; shared great-grandparents (3) make second cousins.
- Subtract the smaller number from the larger. That is how many times removed.
If one of the counts is 1, the person is a sibling of the other person’s ancestor, which makes them an aunt, uncle, great-aunt or great-uncle rather than a cousin. If one count is 0, one person is the ancestor of the other.
Worked example: first cousin once removed, from both sides
Linda is your mother’s first cousin. Linda and your mother share grandparents, which makes them your great-grandparents. Linda is 2 generations below them; you are 3. The smaller number, 2, minus one gives first cousins. The difference, 3 − 2, is 1, so Linda is your first cousin once removed.
Now look from Linda’s side. She counts 2, you count 3, and the arithmetic is identical, so you are her first cousin once removed as well. The same applies to your first cousin’s children: they are your first cousins once removed, and you are theirs.
Worked example: second cousin
Your mother’s first cousin Linda has a son, Sam. Sam and you both count 3 generations down to the same great-grandparents. 3 minus 1 is 2, and there is no difference between the counts, so you are second cousins. This is why second cousins are often described as the children of first cousins.
Worked example: second cousin twice removed
Your grandmother had a second cousin, Ruth. Ruth and your grandmother share great-grandparents, who are your great-great-great-grandparents (3rd great-grandparents). Ruth is 3 generations below them; you are 5. The smaller number, 3, minus one gives second cousins, and 5 − 3 gives twice removed. Ruth is your second cousin twice removed. Your second cousin’s grandchild would be the same relationship, just in the other direction.
Half cousins and double cousins
Half cousins descend from only one shared ancestor rather than a couple. If your father and your cousin’s mother are half-siblings, sharing a father but with different mothers, you share one grandparent instead of two, and you are half first cousins. Half relatives share about half as much DNA as full relatives of the same degree. Tick “Half relationship” in the calculator to see the term and DNA figures.
Double cousins are related through both of their parents. The classic case is two brothers who marry two sisters: their children share all four grandparents and are double first cousins. They are expected to share about 25% of their DNA, twice as much as ordinary first cousins. The calculator handles one line at a time, so for double cousins work out each side separately and add the expected shares together.
Cousins, in-laws and step-relatives
Cousin relationships are relationships by blood. Your cousin’s husband or wife, your spouse’s cousins and your step-parent’s nephews are related to you through a marriage instead, and you share no DNA through that link. Many families call a cousin’s spouse a cousin-in-law and the children of a step-parent’s siblings step-cousins, but these are informal terms.
When you describe a relationship in words, the calculator recognizes husband, wife, spouse, partner, step- and in-law words. It names the common cases (sister-in-law, stepmother, aunt by marriage) and tells you that the path runs through a marriage.
How much DNA do relatives share?
Each child inherits half of each parent’s DNA, picked in random chunks, so the expected share halves with every generation along the path. Testing companies report shared DNA in centimorgans (cM). Across both copies of your chromosomes there are roughly 6,800 cM in total, which is how an expected percentage converts into a cM figure. Real results scatter widely around the average, especially for more distant cousins, which is why a single cM number can fit several relationships.
| Relationship | Expected share | Average cM | Typical range (cM) |
|---|---|---|---|
| Parent or child | 50% | 3,485 | 2,376–3,720 |
| Full sibling | 50% | 2,613 | 1,613–3,488 |
| Half sibling | 25% | 1,759 | 1,160–2,436 |
| Grandparent or grandchild | 25% | 1,754 | 984–2,462 |
| Aunt, uncle, niece or nephew | 25% | 1,741 | 1,201–2,282 |
| Great-grandparent | 12.5% | 887 | 485–1,486 |
| Great-aunt or great-uncle | 12.5% | 850 | 330–1,467 |
| Half aunt or half uncle | 12.5% | 871 | 492–1,315 |
| First cousin | 12.5% | 866 | 396–1,397 |
| Half first cousin | 6.25% | 449 | 156–979 |
| First cousin once removed | 6.25% | 433 | 102–980 |
| First cousin twice removed | 3.13% | 221 | 33–471 |
| Second cousin | 3.13% | 229 | 41–592 |
| Half second cousin | 1.56% | 120 | 10–325 |
| Second cousin once removed | 1.56% | 122 | 14–353 |
| Second cousin twice removed | 0.78% | 71 | 0–244 |
| Third cousin | 0.78% | 73 | 0–234 |
| Third cousin once removed | 0.39% | 48 | 0–192 |
| Fourth cousin | 0.2% | 35 | 0–139 |
| Fifth cousin | 0.05% | 25 | 0–117 |
Source: The Shared cM Project, version 4.0 (March 2020) by Blaine T. Bettinger, which collected tens of thousands of known relationships from testers. Ranges have outliers removed. Full siblings average less than 50% in cM terms because stretches they share on both copies of a chromosome are counted once. From about third cousins outward, some real cousins share no detectable DNA at all, and the averages only count pairs who do.
Using this with your own family tree
The calculator answers one question at a time. If you are sorting out a whole family, it is quicker to enter everyone once. The free family tree maker on this site works out the relationship between any two people in your tree, including half and in-law relationships, and stays on your device. Start with yourself, add parents and grandparents, then fill in their siblings and children. The common ancestor and the generation counts come from the tree, so there is nothing to count by hand.
A tip for DNA matches: when a testing site suggests “second to third cousin”, use the table above to see which relationships fit the shared cM, then look for a common ancestor at that distance. Treat the result as a range of possibilities, not a single answer, until the paper trail confirms it.