COI Calculator
Enter ancestor names in the pedigree table below. Any name that appears on both the sire's side and the dam's side is a common ancestor and contributes to the inbreeding coefficient. The COI updates live as you type.
Click any cell to enter a name. Common ancestors are highlighted automatically.
| Parents | Grandparents | Great-Grandparents |
|---|---|---|
| Sire | Sire's Sire | SS Sire |
| SS Dam | ||
| Sire's Dam | SD Sire | |
| SD Dam | ||
| Dam | Dam's Sire | DS Sire |
| DS Dam | ||
| Dam's Dam | DD Sire | |
| DD Dam | ||
COI Result
Enter ancestor names in the pedigree table to calculate COI.
The same name on both sire's and dam's side = a common ancestor = inbreeding.
COI Reference:
| 0% | Outcross |
| ~3.1% | 4th cousins |
| ~6.25% | 1st cousins (target) |
| ~12.5% | Half-siblings (high) |
| ~25% | Full siblings (very high) |
This calculator uses Wright's formula on entered names only and reflects a 3-generation window.
What is COI?
The Coefficient of Inbreeding (COI) is a number between 0 and 1 (usually expressed as a percentage) that represents the probability that a dog inherited two identical copies of any given gene from a common ancestor on both sides of its pedigree. The higher the COI, the more inbred the dog.
A COI of 0% means no shared ancestors were found in the pedigree window you entered. It does not rule out inbreeding from deeper generations.
How is COI Calculated?
This calculator uses Wright's path coefficient formula, developed by geneticist Sewall Wright in 1922. For each path connecting a common ancestor through both the sire's side and the dam's side, the contribution is:
(0.5)^(n1 + n2 + 1)
Where n1 is the number of steps from the sire to the common ancestor and n2 is the number of steps from the dam to the common ancestor. The total COI is the sum of all such paths.
COI and Breed Health
Most canine geneticists recommend keeping COI below 6.25% (equivalent to first cousins) as a target for line-breeding. Above 12.5% (equivalent to half-siblings), the risk of expressing recessive genetic conditions increases substantially.
However, COI is only one factor. Health testing, structural soundness, and documented ancestry all matter alongside the COI number. A dog with a higher COI from health-tested lines may be a better breeding choice than an untested outcross.
American Bulldog Pedigrees
Every American Bulldog on pedigreedatabase.ca has a COI score calculated across all available generations -- not just 4. Search by registered name for a complete picture.