So I recently wrote my loan provider on whether the extra online payments I was making were applied directly to principal and it turns out they were not, they were just treating it like extra pre-payments that applied toward principal and interest.
It looks like this answer provides the formula for extra payments that directly apply toward the principal.
But what is the formula for extra payments (those pre-payments that were not directly paying off principal) that pay off both principal and interest? I'm trying to calculate how much money I would have saved between the 2 methods.