Dog COI calculator: coefficient of inbreeding from a pedigree
This coefficient of inbreeding calculator works out the COI of a dog — or a planned litter — from a pedigree you type in, up to five generations. It uses Wright’s path method, the standard way to calculate inbreeding by hand, and shows its working: every common ancestor, every path through it, and how much each adds. A pedigree tree highlights the shared ancestors in colour.
The coefficient of inbreeding is the probability that a puppy inherits two identical copies of a gene from the same ancestor on both sides of its pedigree. A COI of 25% means a quarter of its genes are expected to be identical by descent — the same as a puppy from a full brother and sister. Higher COIs are linked with smaller litters, reduced vigour and a higher chance of inherited disease.
Everything runs in your browser; nothing about your dogs is uploaded. It works for any animal with a known pedigree — cats, horses, rabbits or livestock.
How to use the Coefficient of Inbreeding Calculator
- Choose how many generations to enter: 3, 4 or 5.
- Type the sire and dam, then their parents, grandparents and so on in the pedigree grid. Leave unknown animals blank.
- Type each name exactly the same wherever the same animal appears (capital letters and extra spaces are ignored). That is how shared ancestors are found.
- Read the COI, the equivalent mating and the table of paths; common ancestors are coloured in the tree.
- Press Copy result to save the figures, or Clear pedigree to start again.
Coefficient of Inbreeding Calculator features
Wright’s path method
Every path through every common ancestor, with n1 + n2 + 1 and the ancestor’s own COI.
Cross-checked
The path total is checked against the recursive kinship method; both give the same result.
Inbred ancestors handled
If a common ancestor is itself inbred, its COI is calculated from your pedigree and included.
Pedigree tree
An SVG tree up to 5 generations, with shared ancestors highlighted in colour.
Error checks
Flags an animal given two different sires or dams, used as both sire and dam, or as its own ancestor.
The formula and a worked example
COI = Σ (½)n1 + n2 + 1 × (1 + FA), summed over every common ancestor A and every pair of paths from the sire and from the dam to A that do not pass through the same animal twice. n1 and n2 are the number of generations from the sire and from the dam to the ancestor, and FA is the ancestor’s own COI (Sewall Wright, 1922).
Example (the pre-filled pedigree): the sire Ace and dam Bella are half-siblings by Rex: n1 = 1, n2 = 1, so Rex adds (½)3 = 12.5%. Their dams Luna and Daisy are half-sisters by Rose: n1 = 2, n2 = 2, so Rose adds (½)5 = 3.125%. Paths through Rex’s parents or Rose’s parents are not counted again, because they pass through Rex or Rose on both sides. Total COI = 15.625%.
COI of common matings
These assume the shared ancestors are not themselves inbred; if they are, the COI is higher, and the calculator includes that.
| Mating | COI of the puppies |
|---|---|
| Parent × offspring, or full brother × sister | 25% |
| Half-brother × half-sister, or grandparent × grandchild | 12.5% |
| Uncle × niece (full) | 12.5% |
| First cousins | 6.25% |
| Half first cousins | 3.125% |
| Second cousins | 0.78% |
Why the number depends on how many generations you enter
A COI only “sees” ancestors in the pedigree you enter. In many pedigree breeds, shared ancestors go back dozens of generations, so a 5-generation COI is usually lower than one calculated over 10 or more. Compare COIs only when they use the same number of generations, and treat yours as a minimum. Breed clubs and kennel clubs that hold full pedigree databases can give deeper figures and breed averages.
COI is one tool among several: health tests, DNA tests for known conditions and the temperament of both parents matter too. When a litter is on the way, the dog pregnancy calculator works out the due date, and the dog heat cycle calculator helps you plan the mating.
Coefficient of Inbreeding Calculator FAQ
How do I calculate a dog’s COI?
List the pedigree, find every ancestor that appears on both the sire’s and the dam’s side, and add (½)n1+n2+1 × (1 + the ancestor’s COI) for each path. The dog COI calculator does this automatically from names you type.
What is a good COI for a dog?
Lower is better for genetic diversity. Many breeders aim to keep a litter’s COI below the average for their breed; as a reference, 6.25% equals a first-cousin mating and 12.5% a half-sibling mating.
Why is my COI different from my kennel club’s figure?
Mostly because of the number of generations: databases often use many more generations than the 3–5 entered here, which usually gives a higher COI. Spelling differences in names can also hide shared ancestors.
What if an ancestor is unknown?
Leave it blank. Unknown animals are treated as unrelated, so missing information makes the COI lower than the true figure.
Does it work for cats, horses or livestock?
Yes. Wright’s formula is the same for any species; just enter the pedigree.
Is this veterinary advice?
No. Ask your vet — this is general information, not veterinary advice.