Scott,

MOI stands the mass moment of inertia. In baseball and softball bats, Mass moment of inertia is the force required to swing a bat. It’s known more often by its name swing weight. The heavier the swing weight, the more ‘end-loaded’ it feels, and the higher the MOI.

We have an entire article on the ideas of MOI, swing weight and scale weight. Based on your question, you will likely find it helpful.

In terms of bat sizing, we think our bat size chart would answer most of your questions for bat sizes by age.

How can you use MOI to help fit bats to players? We aren’t hitting coaches, so not sure we can answer that entirely. But, it seems natural that you can use the MOI of one bat to see where it falls compared to others. If a hitting coach thinks a lighter or heavier bat might be useful then they could recommend based on the rankings of that list?

At least that is one idea.
Thanks,
Brian