Free breeder tool
COI calculator for a planned mating
Type the names in the sire’s and dam’s pedigrees, three generations back. The calculator finds the ancestors they share and works out the coefficient of inbreeding (COI) of the kittens or puppies from that pairing.
Identical names are treated as the same animal. Leave unknown ancestors blank.
COI of the offspring
0.00%
LowUnder 3%. No close relationship within the generations you entered.
What the coefficient of inbreeding tells you
The coefficient of inbreeding is the probability that a kitten or puppy inherits two identical copies of a gene from the same ancestor, one through its sire and one through its dam. A COI of 6.25% means that, on average, 6.25% of the offspring’s genes are expected to be identical by descent. Higher figures raise the chance that hidden recessive problems pair up, and are linked to smaller litters, lower fertility and weaker immune systems (“inbreeding depression”).
How this calculator works
It uses Sewall Wright’s path method, the same approach registries and BreederPro use. For each ancestor that appears on both sides, every loop from the sire up to that ancestor and back down to the dam adds (½)n1+n2+1 × (1 + FA), where n1 and n2 are the generations from each parent to the ancestor and FA is the ancestor’s own inbreeding. Within the form, FA is worked out from that ancestor’s entered parents. Anything further back is unknown and counted as 0. As a guide: half-siblings give 12.5%, first cousins 6.25%, and a single shared great-grandparent 3.125%.
Why three generations is only a starting point
COI is only as good as the pedigree behind it. Missing or misspelled ancestors hide shared lines, so a short pedigree always understates the true figure. Most breed communities quote COI over five or ten generations. Read pedigree completeness and COI to see how gaps distort the number and how to fill them.
Skip the typing
BreederPro stores each animal’s pedigree once and calculates COI for any pairing from everything you have recorded, with a completeness score alongside so you know how far to trust it. Connect Claude and you can ask, in plain English, what the COI of a planned mating would be.
Frequently asked questions
What is a good COI for a litter?
Many breeders aim to keep a planned litter under about 5–6% on a five- or ten-generation pedigree, and below the average for their breed. What is reasonable depends on the breed’s gene pool. Use COI alongside health testing, not instead of it.
Why is my COI 0% when the cats are related?
COI can only see ancestors you enter. If the shared ancestor sits further back than three generations, or is spelled differently on each side, this calculator cannot find it. Identical names are treated as the same animal, so check the spelling.
How is COI calculated?
With Wright’s path method: for every common ancestor, each pair of independent paths from the sire and the dam to that ancestor adds (½) raised to the number of animals in the loop, multiplied by 1 plus the ancestor’s own COI. The contributions are added together.
Is a three-generation COI accurate?
It is a minimum. Three generations miss most of the inbreeding in a pedigree, so registry figures calculated over 5 or 10 generations are usually higher. Treat this result as a quick check for close relationships.
Last reviewed October 8, 2026. For information only. Not veterinary advice.
Let BreederPro do this for you
BreederPro calculates COI from the pedigrees you have already stored, as many generations deep as you have entered, and shows how complete each pedigree is so you know how far to trust the number. Ask Claude about a planned pairing and it checks COI, carrier status and blood type in one go. Free to start, no card needed.