Do double first cousins share more DNA?
Yes. Double first cousins share about 25% of their DNA on average — twice the roughly 12.5% of ordinary first cousins — because they are related through both parents and share all four grandparents. DNA services sometimes flag a double first cousin as a possible half-sibling, since the percentages sit in the same range.
Relationship
Double first cousin
- You
- Your double first cousin
- First cousins through both your mother and your father
From the other side, you are their double first cousin.
Double first cousins descend from two shared grandparent couples at once, which a single-ancestor chart can't draw.
The calculator below works out blood relationships through a shared ancestor — use it to explore the rest of your tree.
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This relationship sits outside the shared-ancestor model, so the card above carries the answer and the calculator opens in its default state. Work out any blood relationship with it, or open the full relationship calculator.
FAQs
Why do double first cousins share so much DNA?+
They descend from the same two grandparent couples — related through both their mothers and their fathers.
Can a DNA test tell a double first cousin from a half-sibling?+
Not reliably from percentages alone — both hover around 25%. Tree context and matching patterns settle it.
Does the calculator store my family details?+
No — it runs entirely in your browser. Nothing you enter is uploaded or saved on our servers.
Start with one relative and see what comes back.
Add what you remember and AncestorIQ searches for the rest. Free to build your tree for as long as you like.
No credit card needed. Your tree stays yours.
