Piggyback Mortgage (80/10/10) Math: Blended Rate vs. PMI Costs

When buying a home, the standard advice is to put 20% down. If you put down less, lenders require you to pay Private Mortgage Insurance (PMI)—a monthly fee that protects the lender but provides zero financial benefit to you. Monthly PMI can easily add $100 to $300 to your mortgage payment.

To avoid this cost, some homebuyers use a piggyback mortgage (most commonly structured as an 80/10/10 loan).

Under this setup, you split your home purchase into two loans: a first mortgage covering 80% of the home's value, and a second mortgage covering 10%, leaving you to supply a 10% cash down payment. Because the primary mortgage stays at an 80% Loan-to-Value (LTV) ratio, you do not have to pay PMI.

But second mortgages always carry higher interest rates than first mortgages. To determine if this strategy actually saves you money, you must calculate the blended rate of the two piggyback loans and compare it against the rate and PMI cost of a single 90% LTV loan.

Here is how to run the math.

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The Two Options for a 10% Down Payment

Imagine you are purchasing a home for $400,000 and have a $40,000 (10%) down payment.

Option A: Standard 90% LTV Loan with PMI

You take out one single mortgage for 90% of the purchase price.

• Loan Amount: $360,000
• Interest Rate: 6.25%
• PMI Rate: 0.70% of the loan balance annually (paid monthly)
• Monthly PMI Cost: $210.00

Option B: 80/10/10 Piggyback Mortgage

You split the borrowing into two loans to avoid PMI:

• First Mortgage (80% LTV): $320,000 at 6.25% interest.
• Second Mortgage (10% LTV): $40,000 at 8.00% interest.
• Down Payment (10%): $40,000 cash.

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Step 1: Calculate the Piggyback Blended Rate

Let's find the weighted average interest rate of the 80/10/10 loan combination:

Blended Rate = (First Loan × First Rate + Second Loan × Second Rate) ÷ Total Debt

• First Loan Weight: $$320,000 \times 6.25% = 2,000,000$
• Second Loan Weight: $$40,000 \times 8.00% = 320,000$
• Combined Interest Weight: $2,000,000 + 320,000 = 2,320,000$
• Total Borrowed Amount: $$320,000 + $40,000 = $360,000$
• Blended Interest Rate: $2,320,000 \div $360,000 =$ 6.44%

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Step 2: Calculate the Effective Rate of the PMI Loan

To compare Option A against Option B fairly, we must convert the PMI fee into an interest rate equivalent.

Since PMI costs 0.70% of the loan amount annually, it functions exactly like an additional 0.70% on your interest rate.

• Option A Base Interest Rate: 6.25%
• PMI Equivalent Cost: 0.70%
• Option A Effective Rate: 6.95%

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Step 3: Compare the Total Costs

Let's stack the two options side by side.

By using the 80/10/10 piggyback option, you save $161.00 per month, which translates to $1,932 in annual savings. The blended interest rate of 6.44% is significantly lower than the 6.95% effective rate of the PMI loan.

Check your monthly ratios using our Debt-to-Income Calculator:

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When is a Piggyback Loan a Bad Idea?

While the math above favors the piggyback loan, it is not always the best choice:

1. Tax Considerations: The interest on a primary mortgage is generally tax-deductible. However, the interest on a second mortgage may not be deductible unless the funds are used specifically to improve the home. Consult a tax professional regarding your situation.
2. Refinancing Complexity: If interest rates drop and you want to refinance, having two loans makes the process more complicated. The second mortgage lender must agree to remain in the secondary position (subordination), which can delay or block your refinance.
3. PMI Can Be Cancelled: Under federal law, monthly PMI must automatically terminate once your loan balance reaches 78% of the original purchase price. If home values rise quickly, you can pay for an appraisal and cancel PMI in just a few years. A second mortgage, however, does not go away until you pay off the principal in full.

If you are planning to keep your home long-term and home appreciation is slow, the 80/10/10 blended rate advantage makes a piggyback loan an excellent choice. If you plan to refinance or sell within 3 to 5 years, paying PMI temporarily may be cheaper and simpler.

To evaluate more mortgage structures, check out our Blended Rate Mortgage Calculator to test different 80/15/5 or 80/10/10 configurations.

Frequently Asked Questions

What is an 80/10/10 piggyback mortgage?

An 80/10/10 piggyback mortgage splits a home purchase into three parts: a first mortgage for 80% of the price, a second mortgage (often a HELOC) for 10%, and a 10% cash down payment. Because the first mortgage stays at 80% loan-to-value, you avoid Private Mortgage Insurance (PMI) even though you put down less than 20%.

Is a piggyback loan cheaper than paying PMI?

Often, yes. In the worked example above, the 80/10/10 structure produced a 6.44% blended rate versus a 6.95% effective rate for a single 90% loan with PMI — saving $161 per month. The piggyback wins whenever its blended rate is below the base rate plus the PMI-equivalent cost, but you should run your own numbers because second-mortgage rates vary.

How do I calculate the blended rate on a piggyback mortgage?

Multiply each loan balance by its rate, add those products, then divide by the total borrowed. For $320,000 at 6.25% plus $40,000 at 8.00%: (320,000 × 6.25% + 40,000 × 8.00%) ÷ 360,000 = 6.44%. This weighted average is your true cost of borrowing across both loans.

When is a piggyback mortgage a bad idea?

A piggyback loan is usually worse if you plan to sell or refinance within three to five years, because refinancing requires the second lender to agree to subordination, and second-mortgage interest may not be tax-deductible. PMI, by contrast, cancels automatically once your balance reaches 78% of the original price, so it can be the cheaper short-term option.