Should You Refinance Your Mortgage? A Number-Crunching Guide

Refinancing sounds like free money — until you do the math

I’ve refinanced my own mortgage twice. The first time, in 2007, I dropped from a 6.8% rate to 5.1% and saved over $340 a month. That one was a clear win. The second time, in 2014, I moved from 5.1% to 3.75% — a smaller gap, but still worthwhile because I planned to stay in the house for at least another decade. Both times, the decision came down to the same question: does the monthly savings justify the upfront cost, given how long I plan to keep this mortgage?

That’s the question most homeowners get wrong. They see a lower interest rate and assume refinancing is automatically the smart move. Sometimes it is. Sometimes it costs you money. The difference comes down to three numbers: how much your payment drops, what you pay in closing costs, and how many months you need to recoup those costs. Everything else is noise.

Start with your new monthly payment

The first step is straightforward. Figure out what your monthly payment would be under the new loan terms. You need three inputs: the remaining balance on your mortgage (not the original loan amount — what you owe today), the new interest rate, and the new loan term.

Here’s a common scenario. Say you bought your home five years ago with a $350,000 mortgage at 7.2% on a 30-year term. Your remaining balance is roughly $330,000. A lender is offering you 5.9% on a new 30-year mortgage. Your current payment is around $2,377 per month for principal and interest. What would the new payment be?

Let’s use the Mortgage Calculator to find out. Enter $330,000 as the loan amount, 5.9% as the rate, and 30 years as the term:

With those numbers, your new payment comes to roughly $1,955 per month — a savings of about $422. That looks compelling on its surface. But we’re not done yet, because you had to pay to get that lower rate.

Factor in closing costs and find your break-even point

Refinancing isn’t free. You’ll pay closing costs that typically range from 2% to 5% of the loan amount. On a $330,000 refinance, expect somewhere between $6,600 and $16,500. Let’s split the difference and say your costs come to $10,000.

This is where the break-even calculation matters. Divide your total closing costs by your monthly savings:

$10,000 / $422 = 23.7 months

That means you need to stay in the home and keep the new mortgage for at least 24 months before you’ve recovered the cost of refinancing. After that point, every month of savings is genuine money in your pocket. If you’re planning to sell in 18 months, refinancing would actually cost you money.

When I did my first refinance, my break-even was about 14 months. I knew we’d be in the house for years, so it was an easy call. My second refinance had a break-even closer to 30 months, which made me hesitate — but we had no plans to move, so the long-term savings won out. Over the remaining life of that loan, the refinance saved us roughly $48,000 in interest.

One thing I always told my clients: if your break-even is under 24 months and you’re confident you’re staying put, refinancing is almost always worth pursuing. If it’s over 48 months, proceed with caution. Between 24 and 48 months is where the decision gets personal.

Look at the full amortization picture

Monthly payment comparisons only tell part of the story. What matters just as much — sometimes more — is how your amortization schedule changes. When you refinance into a new 30-year mortgage, you’re resetting the clock. Even with a lower rate, you may end up paying more total interest over the life of the loan because you’ve extended your repayment timeline.

Take our example. You had 25 years left on your original mortgage. If you refinance into a new 30-year loan, you’ve added five years of payments. The monthly payment is lower, yes, but you’re making payments for 60 additional months. Depending on the rate difference, the total interest paid over the life of the new loan can actually exceed what you would have paid by sticking with the original.

This is why I always recommend comparing amortization schedules side by side. Let’s use the Amortization Calculator to see exactly how your principal and interest break down over time. Try modelling both your current loan (remaining balance, current rate, 25 years left) and the proposed refinance (same balance, new rate, 30 years):

Pay close attention to the total interest column. If the new loan’s total interest is higher than what remains on the old loan, you need to weigh that against the monthly cash flow improvement. Sometimes the lower monthly payment is worth the extra total interest — for instance, if it frees up money to pay down higher-rate debt. Other times, you’re genuinely better off staying put.

A smarter alternative for many people: refinance into a shorter term. If you can afford the payment, refinancing from a 30-year at 7.2% to a 20-year at 5.9% drops your total interest dramatically while keeping the monthly payment close to what you were already paying. Run both scenarios through the amortization calculator to see the difference.

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What to do with the savings

Here’s where most people leave money on the table. They refinance, enjoy the lower payment, and spend the difference. The financially optimal move is to invest that monthly savings — or at least a portion of it.

Going back to our scenario: if you save $422 per month and invest it at a modest 6% average annual return, after 20 years that money grows to approximately $195,000. That’s not pocket change. It’s a meaningful addition to your retirement savings, a college fund, or a future down payment on an investment property.

When I refinanced the second time, I committed to putting $200 of the monthly savings into an index fund. Over the nine years between that refinance and my retirement, that $200 per month grew to just over $28,000. Not life-changing, but a nice demonstration that small, consistent amounts add up when compound interest has time to work.

Let’s use the Compound Interest Calculator to project what your specific savings could grow into. Enter your monthly savings amount, an expected rate of return, and the number of years you plan to invest:

This step turns a refinance from a cost-reduction exercise into a wealth-building strategy. The refinance itself might save you $50,000 in interest. Investing the savings could add another $100,000 or more to your net worth over time. That’s the full picture most refinancing articles never show you.

The refinance decision checklist

After thirty years of walking clients through this decision, I’ve boiled it down to five questions:

1. Is the rate drop at least 0.75%? Smaller gaps rarely justify the closing costs unless your loan balance is very large.
2. Is your break-even under 36 months? And are you confident you won’t sell or refinance again before then?
3. Have you compared total interest, not just monthly payments? Resetting to a 30-year term can quietly cost you tens of thousands.
4. Are your closing costs reasonable? Get quotes from at least three lenders. Costs vary more than most people realize.
5. Do you have a plan for the savings? The monthly difference between your old payment and your new one is an opportunity. Don’t let it vanish into general spending.

Refinancing is not inherently good or bad. It’s a financial tool, and like any tool, its value depends entirely on whether you’re using it in the right situation. Run the numbers, compare the scenarios, and make the decision with your eyes on the full picture — not just next month’s payment.

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