Will your egg actually get the passives you want?
Species is only half the plan. List what each parent is carrying, pick what you want the child to end up with, and see the real odds, not a guess.
How the math works
Community datamining converges on this model: an egg first rolls how many passives it inherits from the combined parent pool, either 1, 2, 3, or 4, with roughly 40%, 30%, 20%, and 10% odds. It then draws that many passives at random from the pool (duplicates between parents count once). Any leftover slots can get filled with unrelated random passives, but that doesn't push out the ones already drawn. Fewer total passives in the pool means each one you want has better odds of being among the ones drawn.
Why this isn't in most calculators
Most breeding tools stop at predicting the species. But the actual Steam and Reddit complaints about breeding "feeling random" are almost always about passives, not species, getting an unexpected pal is rare; getting the right pal with the wrong traits is the norm. This calculator treats that as the real question.