Combination Mortgage Calculator
A piggyback structure (like 80-10-10) uses a second mortgage to avoid PMI with less than 20% down. Compare it against one loan with PMI.
A worked example
Running this tool on current national benchmarks (Federal Reserve / Freddie Mac via FRED (St. Louis Fed)) — swap in your own numbers to see how the result moves.
- Home price
- $400,000
- Down payment
- 10%
- First mortgage rate
- 6.50%
- Second mortgage rate
- 8.50%
- Loan term (years)
- 30
- PMI rate (single-loan option)
- 0.70%
saves $155.26/mo
- Piggyback payment$2,330.18
- Single loan + PMI$2,485.44
- First / second loans$320,000 / $40,000
The math behind it
This models a piggyback structure like 80-10-10: an 80% first mortgage, a smaller second mortgage covering the gap between your down payment and 20%, and your cash down payment. It adds the payments on both loans to get the piggyback total. It then prices the alternative — a single loan for the same financed amount, plus monthly PMI because you are under 20% down — and shows which monthly payment is lower.
Assumptions & limits
- The first mortgage is fixed at 80% of the home price; the second covers whatever is left after your down payment. Both are amortized over the same term at the rates you enter.
- PMI on the single-loan option is estimated as an annual percentage of the loan, charged monthly, and applies only when your down payment is under 20%.
- The comparison is on monthly payment. It does not model when PMI would eventually cancel on the single loan, which can shift the long-run math.
- Payments are principal and interest (plus PMI on the single option). Taxes and insurance are excluded.
Common piggyback structures
The three numbers are first mortgage, second mortgage, and down payment as percentages of the price.
| Structure | Breakdown |
|---|---|
| 80-10-10 | 80% first, 10% second, 10% down |
| 80-15-5 | 80% first, 15% second, 5% down |
| 80-20 | 80% first, 20% second, nothing down |
Source: Freddie Mac PMMS via FRED. Annual averages; latest weekly reading shown.
Common questions
What is an 80-10-10 loan?
It is a piggyback: an 80% first mortgage, a 10% second mortgage, and 10% down. Because the first loan is exactly 80% of the price, you avoid PMI without putting a full 20% down. Variations like 80-15-5 shift how much comes from the second loan versus your cash.
Does a piggyback always beat a single loan with PMI?
Not always. The piggyback wins when the second mortgage's cost is less than the PMI it replaces. Second mortgages carry higher rates, so if that rate is steep or the PMI would be cheap and cancel soon, a single loan with PMI can cost less. This calculator shows which is lower at your inputs.
Can the second mortgage's rate change?
Often, yes — piggyback seconds are frequently HELOCs with variable rates. The calculator treats the rate you enter as fixed, so if your second loan is variable, test a higher rate to see how the comparison holds up if rates rise.