经纪人网站用的组合贷款（piggyback）计算器：组合贷款还是单笔贷款加 PMI？

计算器中的定义

Combination (piggyback) mortgage

A purchase financed with two loans closed at the same time: a first mortgage sized to stay at the PMI-free threshold and a smaller second mortgage that covers the rest of the gap above the down payment. This calculator compares that pair against one larger loan with PMI, using the same term for every loan.

PMI

Private mortgage insurance protects the lender, not the borrower, on a conventional loan with less than 20% down. Here it is an annual rate on the single-loan balance, divided by 12 and added to the payment. Under the Homeowners Protection Act of 1998 it can be cancelled on request at 80% of original value and ends automatically at 78%.

First mortgage

The primary loan, entered here as a percent of the home price. Setting it at 80% keeps it at the usual PMI-free line; whatever you borrow beyond it becomes the second mortgage. The first mortgage rate is also used for the single-loan scenario, so the comparison isolates the effect of splitting the debt.

Second mortgage

A loan secured by the same home but subordinate to the first mortgage, so the lender is repaid second after a sale or foreclosure. That extra risk is why its rate is usually higher. In this calculator its amount is the home price minus the down payment minus the first mortgage, and it is amortized over the same term.

Down payment

The cash paid toward the purchase price at closing, entered here as a percent of the price. Everything not covered by it is borrowed, either as one loan with PMI or as a first plus second mortgage. A larger down payment shrinks the second mortgage and, at 20% or more, removes PMI from the single-loan side.

Loan-to-value ratio (LTV)

The loan balance divided by the home’s value, as a percent. A $320,000 loan on a $400,000 home is 80% LTV. Lenders price risk and require PMI based on it. The combination structure keeps the first mortgage’s LTV at the threshold while the combined LTV of both loans stays the same as the single loan.

Principal and interest

The part of a mortgage payment that repays the loan and pays the lender’s interest, excluding taxes and insurance. The combo payment here is the principal and interest on both loans added together; the single-loan payment is principal and interest plus PMI. Neither includes property tax or homeowners insurance, which would be the same in both cases.

Interest rate

The annual percentage the lender charges on the outstanding balance, before fees. This calculator takes two: one for the first mortgage (also used for the single loan) and one for the second. Because the second-mortgage rate is usually higher, the combination only wins when its extra interest costs less than the PMI it avoids.

Loan term

The number of years over which a loan is scheduled to be repaid. A longer term lowers the monthly payment but raises total interest. Here one term applies to the first mortgage, the second mortgage and the single loan alike; in practice second mortgages often have shorter terms or balloon payments, which would change the comparison.