📊 Average Shared cM by Relationship Reference Chart

What the ranges in the chart are

Each row gives a typical span of shared centimorgans for a relationship, following the shape of the published Shared cM Project data that genetic genealogy tools draw on. These are observed distributions, not rules: rounded summaries of what testers in that relationship actually turned out to share.

full sibling  ~2,300 - 3,400 cM   (average ~2,600)
1st cousin      ~575 - 1,330 cM   (average ~850)
2nd cousin        ~90 -   480 cM   (average ~230)
3rd cousin         ~0 -   220 cM   (average ~90)

Read the overlap, not just the average. A 230 cM match is the average for a second cousin, yet it also sits inside the first cousin once removed row and at the top of the second cousin once removed row. One number cannot choose between them.

Rows that cannot be told apart

• Grandparent, aunt or uncle, and half sibling share one row at roughly 1,300 to 2,300 cM, all three being expected to share about a quarter of your DNA. Age and known trees separate them; the cM total never will.
• Half relationships run near half the full figure. Half siblings centre around 1,700 cM against about 2,600 for full siblings, and half cousins follow the same halving.
• Parent and child is the one fixed row. Its 3,400 to 3,720 cM span reflects how different companies count, not real variation between families.

Frequently asked questions