Inbreeding coefficient (COI) calculator for dogs

What is a coefficient of inbreeding?

Every puppy gets two copies of each gene, one from each parent. If the sire and dam share an ancestor, both copies can be duplicates of the same gene from that ancestor. The coefficient of inbreeding, or COI, is the probability of that happening at any given gene. A COI of 12.5% means that, on average, one in eight of the puppy's genes is expected to carry two copies inherited from the same ancestor.

It is calculated for a litter, not for one parent: two unrelated parents produce puppies with a COI of 0% in the generations you can see, however inbred each parent may be. The more common ancestors there are, and the closer they sit to the litter, the higher the number.

What COI do common matings give?

The calculator's built-in examples double as a check. Each row is computed by the same code that runs the tool:

These match the textbook values, which research on dog breeds uses as reference points: 6.25% for first cousins, 12.5% for half siblings and 25% for full siblings or parent and offspring.

How the calculator works

It uses Sewall Wright's path method from 1922, still the standard way to calculate inbreeding by hand. For every route from the sire up to a shared ancestor and back down to the dam, it adds:

(½)^(n1 + n2 + 1) × (1 + F of the common ancestor)

Here n1 is the number of generations from the sire up to the ancestor and n2 the number from the dam. A route may not pass through the same dog twice, and if the ancestor is itself inbred, its own COI (F) is worked out from its parents and adds a little more.

Worked example: in the line-bred pedigree the calculator opens with, Champion Rex contributes 6.25% and Lady Luna contributes 3.125%, for a total of 9.375%. Champion Rex is the sire's sire (n1 = 1) and the dam's maternal grandsire (n2 = 2), so his route counts (½)^4. Lady Luna sits two generations back on both sides, giving (½)^5.

Why the number of generations matters

Any ancestor you leave blank is treated as unrelated to everyone else, so the COI only reflects the generations you typed in. A litter can show 0% over three generations and several percent over ten. When you compare litters, always compare COIs over the same number of generations.

DNA studies show how much a short pedigree can miss. Analyzing genotypes from 227 breeds, Bannasch and colleagues at UC Davis found an average genomic inbreeding of about 25%, the level of a full-sibling mating. As Kennel Club researchers put it, inbreeding is inevitable in a closed population with a limited number of ancestors.

Does inbreeding affect a dog's health?

Every dog carries some faulty recessive genes that cause no harm as a single copy. Inbreeding raises the chance that a puppy inherits two copies of the same one, which is how many inherited diseases appear. Across breeds, the UC Davis study linked higher inbreeding and larger body size to more insurance claims for illness: smaller, less inbred breeds were the healthiest.

A low COI is not a health guarantee, and a moderate one is not a diagnosis. It is a measure of risk across the whole genome, best used together with the specific DNA and screening tests recommended for your breed.

How to lower the COI of a litter

• Run the calculator on every candidate mate and compare the results over the same number of generations.
• Watch out for popular sires. The Kennel Club's analysis of all 215 UK breeds found popular sires in every breed and named them the main driver of rising inbreeding.
• Look for mates from other lines or countries whose pedigrees share no recent ancestors with your dog.
• For breeds that have it, a DNA diversity test lets a lab compare two dogs before mating, using the whole genome instead of a paper pedigree.

Planning a litter? Our coat color calculator shows the colors to expect, and the heat cycle calculator helps you time the mating.