Shared DNA & Relationship Calculator

Frequently Asked Questions

• How much DNA do relatives share?+
  On average, you share about 50% of your DNA with a parent, child or full sibling, 25% with a grandparent, half-sibling, aunt or uncle, and 12.5% with a first cousin. The share roughly halves with each additional step of relationship. This calculator shows the expected percentage for the relationship you choose, along with the coefficient and an approximate centimorgan figure.
• What is the coefficient of relationship?+
  The coefficient of relationship, written as r, is the expected proportion of genes two people share because of common ancestry. It is 0.5 for parent–child and full siblings, 0.25 for grandparent–grandchild and half-siblings, 0.125 for first cousins, and so on, halving with each degree of separation. Multiplying it by 100 gives the expected shared-DNA percentage.
• What are centimorgans?+
  Centimorgans (cM) are the unit DNA testing companies like AncestryDNA and 23andMe use to report shared DNA, measuring genetic distance rather than a simple percentage. The total measurable autosomal genome is roughly 6,800 cM, so a parent–child pair shares about 3,400 cM and first cousins around 850 cM. This tool converts the expected percentage into an approximate cM figure on that basis.
• Why do real DNA results vary from these numbers?+
  The percentages here are statistical expectations, but inheritance is random. Full siblings share 50% on average, yet any given pair might share anywhere from roughly 38% to 61% because of how chromosomes recombine. Parent–child is the one fixed case at almost exactly 50%. So treat the figures as the centre of a range, which is exactly how genealogists interpret DNA matches.
• Why might distant cousins share no DNA at all?+
  Because DNA is passed down in chunks and reshuffled each generation, beyond second or third cousins there is a real chance two genuine relatives inherited no overlapping segments from their common ancestors. They remain related on paper, but a DNA test may show no shared segments — which is why genealogists combine DNA evidence with documented family trees.
• What is the inbreeding coefficient shown for the child?+
  If two relatives have a child together, that child’s inbreeding coefficient (F) is half the parents’ coefficient of relationship. For first cousins, whose r is 0.125, a child would have F of about 0.0625. It estimates the chance the child inherits two identical copies of a gene from the shared ancestor, which is why it is relevant to genetics and, in some places, marriage law.
• Is it safe for first cousins to have children?+
  The increase in risk of recessive genetic conditions for first-cousin couples is real but more modest than commonly assumed — studies put it a few percentage points above the baseline population risk. This is a general, educational figure, not personalised medical advice; couples with specific concerns should consult a genetic counsellor.
• Does this work for half relationships?+
  Yes. The calculator includes half-siblings, half first cousins and removed relationships, which share half the DNA of their full equivalents. A half-sibling shares about 25% rather than the 50% of a full sibling, and the tool reflects that directly in the percentage and centimorgan figures.
• Can shared DNA prove exactly how two people are related?+
  Not on its own. Several different relationships can share similar amounts of DNA — a grandparent, half-sibling, aunt and uncle all sit around 25% — so a single shared-DNA figure is consistent with multiple family connections. Genealogists resolve the ambiguity by combining the cM amount with ages, documents and other matches.
• Is the Shared DNA Calculator free?+
  Completely free, with no account or limit. It works offline once the page has loaded and collects no data — your selection never leaves your device.