Calculate the Coefficient of Inbreeding for any dog in the archive. Analyzes up to 10 generations using Wright's formula.
The Coefficient of Inbreeding (COI) is a number that measures how closely related a dog's parents are. It is expressed as a percentage. A higher COI means more inbreeding. A lower COI means more genetic diversity.
This calculator uses Wright's Coefficient of Inbreeding -- the original and most widely accepted formula for calculating inbreeding, developed by geneticist Sewall Wright in 1922. It remains the standard used by breed registries, genetic researchers, and responsible breeders worldwide.
Wright's formula calculates COI by tracing every path through the pedigree where the sire and dam share a common ancestor. For each common ancestor found, the formula calculates that ancestor's genetic contribution based on how many generations back they appear on both the sire's and dam's side.
The formula is:
F = Σ [(1/2)(n1 + n2 + 1) × (1 + Fa)]
Where n1 and n2 are the number of generations from sire and dam to the common ancestor, and Fa is the inbreeding coefficient of that ancestor.
Some breeders have heard of Hardiman's method (sometimes written as Hardman's), which is a simplified lookup approach that estimates COI from known relationship types (half-sibling, uncle-niece, etc.). It can give a quick approximation but does not account for the full complexity of a multi-generation pedigree where the same ancestor appears multiple times through different paths.
Wright's formula is more accurate because it traces every single path through the pedigree. This is especially important in American Bulldogs, where popular sires appear repeatedly across many generations and the true level of inbreeding is often higher than a simple relationship-based estimate would suggest.
The depth of the pedigree analysis matters. A 5-generation COI will miss common ancestors that appear further back. In a breed like the American Bulldog -- where the modern population descends from a small number of foundation dogs -- a 10-generation analysis captures a much more complete and accurate picture of genetic diversity.
Our calculator analyzes up to 10 generations of pedigree data from the American Bulldog Archive -- over 1,000 ancestor positions per dog.
For an American Bulldog, a coefficient of inbreeding below 6.25% is generally treated as low, and that is the range most responsible breeding programs aim for. The 6.25% figure is not an arbitrary cutoff. It is the COI produced by mating two first cousins, so it marks the point at which a pairing begins carrying roughly as much shared ancestry as a first-cousin breeding.
Treat that as a guideline rather than a universal standard, because the figure quoted varies by organisation. Lower ceilings, commonly around 5%, are advised by some breed clubs and registries. Others avoid a fixed number altogether and advise breeders to stay below their own breed's average instead. There is no single agreed threshold, so the honest reading is that 6.25% is the point where shared ancestry becomes worth a deliberate decision, not a line dividing safe from unsafe.
The American Bulldog carries a specific complication behind that number. The breed passed through a severe population bottleneck after the Second World War, and most modern dogs trace back to the same small set of foundation animals through the Johnson, Scott, Painter and Williamson lines. Shared ancestry deep in a pedigree is therefore common in this breed rather than automatically a sign of careless breeding, and a figure that would alarm you in a large outbred population has to be read against that history.
| COI Range | Rating | What It Means |
|---|---|---|
| 0 -- 6.25% | Low | Excellent genetic diversity. This is the target range for most responsible breeding programs. |
| 6.25 -- 12.5% | Moderate | Equivalent to the offspring of first cousins. Acceptable in some breeding strategies but should be monitored. |
| 12.5 -- 25% | High | Equivalent to a half-sibling or grandparent/grandchild mating. Increased risk of genetic health problems. |
| 25%+ | Very High | Equivalent to a parent/offspring or full sibling mating. Significant inbreeding depression risk. Not recommended. |
The American Bulldog went through a severe population bottleneck after World War II. The entire modern breed descends from a relatively small number of foundation dogs preserved by John D. Johnson, Alan Scott, and a handful of other dedicated breeders. This means that even dogs that appear unrelated on a 3 or 4 generation pedigree may share significant common ancestry further back.
High COI is associated with:
Responsible breeders use COI as one tool in their breeding decisions -- not the only factor, but an important one. The goal is not to eliminate all inbreeding (which is impossible in a closed breed), but to make informed decisions that maintain breed type while preserving genetic diversity for future generations.
Want to check the COI of a planned breeding before you commit? Use the Sire x Dam COI Tool to calculate what the offspring's COI would be.
American Bulldog owners and breeders worldwide -- add your dog's pedigree to the most comprehensive American Bulldog database in existence.