Private mortgage insurance (PMI) is typically required for conventional first-lien loans when the loan-to-value (LTV) ratio exceeds 80%. That is, when the borrower puts down less than 20% as a down payment.

“PMI is required by lenders on conventional loans with a loan-to-value ratio greater than 80 percent.”

— Homeowners Protection Act of 1998; CFPB PMI Guidance