Once the loan reaches 80% of the original LTV you have to terminate PMI if the borrower requests it and there are certain conditions met.
Automatic termination of PMI occurs at 78% LTV based on the amortization schedule.
The midpoint of the amortization schedule triggers the "final termination" requirement if the loan is not already at 78% LTV based on its amortization schedule at that time.
Reference chapters 4, 5, 6 of the ACB/MBAA "Implementation Guide to PMI"
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Opinion expressed are my own and not necessarily those of my employer.