Using this tool
This calculator will help you calculate the compatibility between characters, hopefully allowing you to get your double O setups easier. If you have no idea what compatibility is or how exactly it works, I recommend reading the inheritance guide first.
The tool should be pretty intuitive - first, select the character you're training in the "Main Char" box. After that, select at least one of her parents, and the tool will automatically start calculating the compatibility.
"GP" refers to "grandparent", or the parents' parents. 1-1 and 1-2 are Parent 1's parents, 2-1 and 2-2 are Parent's 2 parents.
By the way, if a character has multiple versions, it doesn't matter which one you're using, they all provide the same compatibility. So if you have Tokai Teio from the anime collab gacha banner, just select "Tokai Teio" from the character list, they're interchangeable.
How is compatibility calculated?
If you've already tried playing around with the compatibility tool, you might have noticed that there are several parts the overall compatibility calculation consists of.
To start from the beginning, there are several groups of characters, and every group has an assigned compatibility rating. You can imagine those as indicators of how compatible the girls are, or how much they like each other, whatever you want.
Now, let's assume there's a group consisting of characters A, B, and C, and the rating of the group is 2. Let's also assume that you're training character A that has parent B, and grandparents C and D on that side.
The game will check all the groups for the combination of characters that you're using.
First of all is compatibility between your main character and her parents. In our example, both the main character A and her parent B are in the group. That means that the game will add +2 (the group rating) to the overall compatibility counter. If either A or B wasn't in the group, you'd get nothing from that group.
After checking both parents, grandparents are also examined. In their case, all of the [main character, parent, grandparent] have to be in the group.
In our example, there are two grandparents to check - C and D. We already know that main character A and the B are there, so we only have to look for her grandparents. C is there, so that's another two points! D is not there, so no points from her in this group.
The last thing that gets checked is the compatibility of the two parents together. That one is simple - if both of the main character's parents are in the group, you get the points. So if the other parent of the girl from our example was C, the game would check if both B and C are there.
Note that there are a lot of groups, and the same characters can be in several of them. In that case, all groups that apply get added together.
All of this is calculated by this tool, and I've personally checked that it's correct. However, according to some research posted on Japanese Twitter, there is another "hidden" factor, and that one can't really be included here.
That factor is supposed to be a point added for races won by both a parent and the corresponding grandparent. In our example, if both parent B and grandparent C won Arima Kinen, you'd get an extra point. If the grandparent D won the same race, you'd get another one. Only G1, G2, and G3 races are supposed to count.