DNA Match Relationship Calculator

These three relationship types are genetically indistinguishable from total shared DNA alone — a grandparent, an aunt/uncle, and a half-sibling all average roughly 1,740-1,760 shared centiMorgans with heavily overlapping ranges, because each is equally distant from you in terms of expected DNA inheritance (all are 'second-degree' relatives). Real genetic genealogists resolve this kind of ambiguity using additional evidence: X-DNA inheritance patterns (which follow different rules for these relationships), comparing shared matches with other known relatives, or documented family history — not centiMorgans in isolation. Approx. Generations to Common Ancestor inherits the same limitation: it assigns one representative number (2) to the whole band, even though a half-sibling's shared parent is genuinely only 1 generation away while a grandparent is 2 generations from a grandchild through an intermediate parent. The same approximation applies to the 1st-cousin-once-removed/half-1st-cousin band (shown as 3), even though a half-1st-cousin's shared grandparent is genuinely 2 generations away, the same distance as a full 1st cousin. Treat Generations Apart as representative of the labeled group, not an exact count for every relationship the label could mean.

The 6,800 cM figure is the standard total autosomal genome length used by genetic genealogy tools like the Shared cM Project — you inherit roughly 3,400 cM from each parent, and 3,400 + 3,400 = 6,800 cM total. The 3,400 cM figure describes only one parent's contribution, not your complete genome, so using it as the denominator for Shared DNA % would roughly double your calculated percentage compared to the standard convention.

It's a supplementary heuristic built from Shared Segments (how many distinct DNA segments you share with the match) and Largest Segment (the size of your single biggest shared segment), not a statistical probability from the Shared cM Project data. More segments and a larger largest segment both generally suggest a more reliable, less coincidental match — a single small shared segment is more likely to be identical-by-chance rather than identical-by-descent from a real recent common ancestor, especially for distant relationships.

Most Likely Relationship is determined by which cM range your Shared centiMorgans falls into, so a change that crosses a range boundary (for example, from 1,290 cM to 1,310 cM, crossing the 1,300 cM boundary into the 2nd-degree-relative range) changes the classification, while a change that stays within the same range does not. Real relationship categories have wide, overlapping cM ranges in practice, so treat a classification near a boundary as less certain than one comfortably in the middle of a range.

It should be broadly similar, since both are built on the same underlying genetic-genealogy science and often reference the same Shared cM Project dataset, but testing companies may use their own proprietary relationship-prediction algorithms that also factor in additional data this calculator doesn't have access to, such as your specific shared segment patterns or a broader network of shared matches. Treat this calculator's output as a general reference for interpreting a cM figure, not a substitute for your testing company's own match report.