How to Calculate Savings on an Assumable Mortgage

The math on assumable mortgages is what makes them so compelling. Once you see the numbers, it's hard to look at a 7% mortgage the same way. Let me walk you through exactly how to calculate your savings.

The Basic Formula

Your savings come from the rate difference between the assumable loan and what you'd pay on a new mortgage today.

Monthly payment formula: P = L[c(1+c)^n]/[(1+c)^n - 1]

Where:

• P = monthly payment
• L = loan amount
• c = monthly interest rate (annual rate / 12)
• n = number of monthly payments remaining

Don't worry about memorizing that. I built a calculator that does it for you. But understanding the concept matters.

A Real Example

Let's say you're looking at a home listed at $420,000 with an existing VA loan:

• Remaining loan balance: $340,000
• Assumable rate: 2.75%
• Remaining term: 26 years (312 months)
• Equity gap: $80,000

Step 1: Calculate the assumable payment

$340,000 at 2.75% for 312 months = $1,427/month

Step 2: Calculate what you'd pay at market rate

$420,000 at 7% for 360 months = $2,794/month

Step 3: Find your monthly savings

$2,794 - $1,427 = $1,367/month

Step 4: Calculate total savings over the loan life

$1,367 x 312 months = $426,504 total savings

That number isn't a typo. On this single property, you'd save over $400,000 compared to getting a new mortgage at current rates.

But What About the Equity Gap?

Right. You need to cover that $80,000 equity gap. Let's say you put $30,000 cash down and get a second mortgage for $50,000 at 9% for 15 years.

Second mortgage payment: $507/month

Your total monthly payment: $1,427 + $507 = $1,934

Compare that to the market rate payment: $2,794

You're still saving $860/month. And after 15 years when the second mortgage is paid off, your payment drops back to just $1,427.

This is the blended rate strategy in action. Your effective rate on the total purchase price works out to about 4.3%. Way better than 7%.

The Blended Rate Calculation

When you combine the assumed first mortgage with a second mortgage, your blended rate tells you the true effective cost:

Blended rate = (First mortgage balance x first rate + second mortgage balance x second rate) / total borrowed

Using our example: ($340,000 x 2.75% + $50,000 x 9%) / $390,000 = ($9,350 + $4,500) / $390,000 = 3.55%

Your blended rate is 3.55%. Compare that to 7% on a new mortgage. The gap is enormous.

Variables That Affect Your Savings

The assumable rate. Lower is obviously better. Rates from 2020-2022 vintage loans range from about 2% to 4%. Every half-percent matters. On $400,000, the difference between 2.5% and 3.5% is about $225/month.

The remaining term. More years remaining means more months of savings. A loan with 27 years left saves you more total dollars than one with 20 years left.

The equity gap. Larger gaps require more cash or a bigger second mortgage, which eats into (but rarely eliminates) the savings.

The second mortgage rate. If you need one, the rate matters. Current second mortgage rates for assumptions run 8-10%. Even at these rates, the blended rate is still well below market.

Home price. Higher-priced homes have larger absolute dollar savings. The percentage savings are the same, but $975/month on a $400,000 home becomes $1,950/month on an $800,000 home.

Quick Reference Table

| Home Price | Assumable Rate | Market Rate | Monthly Savings | 25-Year Savings | |-----------|---------------|-------------|-----------------|-----------------| | $300,000 | 2.5% | 7% | $806 | $241,800 | | $400,000 | 3.0% | 7% | $975 | $292,500 | | $500,000 | 2.75% | 7% | $1,262 | $378,600 | | $600,000 | 3.25% | 7% | $1,382 | $414,600 |

These are principal-and-interest savings only, not accounting for the equity gap. But they show the scale of what we're talking about.

Try It Yourself

I built an interactive calculator where you can plug in any home price, assumable rate, market rate, and loan term to see your exact savings. It updates in real time as you move the sliders.

Or, better yet, browse real Colorado listings where every property card shows the calculated monthly savings and total savings for that specific property. The numbers are already done for you.

The math doesn't lie. Assumable mortgages save buyers an extraordinary amount of money in this rate environment. The only question is whether the equity gap and timeline work for your situation.

Ready to Find an Assumable Mortgage in Colorado?

Browse available listings or schedule a free call with Ryan Thomson, Colorado's leading assumable mortgage specialist.

Browse Homes | Schedule a Call | (719) 624-3472

Frequently Asked Questions

How do I calculate assumable mortgage savings?

Compare monthly P&I at the assumed rate vs. today's rate on the same balance. Example: $350,000 at 3% = $1,476/mo. $350,000 at 7% = $2,329/mo. Monthly savings = $853. Annual savings = $10,236.

What's the break-even on the equity gap vs. payment savings?

Divide the equity gap by the monthly payment savings. If the gap is $100,000 and you save $800/month, break-even is 125 months (about 10 years). If you plan to hold the home that long, it's a strong financial case.

Does the equity gap wipe out the savings?

Usually not. Even covering a $100,000 equity gap with a second mortgage, the blended payment is often $400-$800/month less than a conventional purchase. The mortgage calculator can model this for your specific scenario.

How does a blended rate work with an assumable mortgage?

A blended rate combines your first mortgage (assumed at 3%) and a second mortgage (at 9%) on a weighted average basis. On $300,000 at 3% and $100,000 at 9%, your effective blended rate is about 4.5%, still well below market.

What's the true cost comparison over 30 years?

On $400,000 at 7% for 30 years, you pay $558,036 in total interest. At 3%, you pay $207,110. The difference is $350,926. Even if you only hold 10 years, the savings are significant.

Should I include closing costs in my savings calculation?

Yes. Add assumption closing costs ($3,000-$6,000) and equity gap costs to your total investment. Divide by monthly savings to find the true break-even period. In most cases, you break even within 1-3 years.