Abel's Calculators

You've probably seen the advice: "If you can lower your rate by at least 1%, refinancing is worth it." It's simple. It's memorable. And it's incomplete enough to cost you money.

The problem with this rule is that it reduces a complex financial decision to a single variable — the interest rate. It ignores closing costs, how long you plan to stay in the home, what happens to your equity, whether you'll trigger or eliminate PMI, and what you'd do with the monthly savings. All of those things affect whether a refinance actually makes you wealthier.

Here's how to think about it more clearly.

The "Monthly Savings" Trap

When a lender quotes your new monthly payment, that number feels concrete. You're paying $2,100 instead of $2,400 — that's $300 a month back in your pocket. Sounds like a win.

But several things are hidden inside that number:

• You reset the clock. If you're 5 years into a 30-year mortgage and refinance into a new 30-year term, you just added 5 years of payments. Your monthly payment dropped, but your total interest paid over the life of the loan might go up.
• Closing costs are real. A refinance typically costs 2%–5% of the loan amount. On a $350,000 loan, that's $7,000–$17,500. Your monthly savings have to overcome that upfront cost before you actually come out ahead.
• Your equity trajectory changes. A lower rate means more of each payment goes to principal, but a longer term means more payments go to interest early on. These effects work against each other.

Monthly savings tells you about cash flow. It doesn't tell you about wealth.

Think in Net Worth, Not Monthly Payments

A better question than "How much will I save per month?" is: "Will refinancing make me wealthier over the next 5, 10, or 15 years?"

To answer that, you need to account for everything that changes when you refinance:

1. Home equity — How does the new amortization schedule change the rate at which you build equity?
2. Investment opportunity cost — If your monthly payment drops by $300, and you invest that $300 at a reasonable market return, how much does that grow over time?
3. Tax implications — Mortgage interest is deductible (if you itemize). A lower rate means less deductible interest. Does that change your effective savings?
4. PMI — If your home has appreciated and a new appraisal gets you above 80% LTV, refinancing could eliminate private mortgage insurance — often $100–$400/month that builds zero equity.
5. Closing costs — Not just the dollar amount, but the opportunity cost. That $10,000 in closing costs could have been invested instead.

When you model all five of these together, you get a net worth impact curve over time. That curve is what actually tells you whether refinancing is worth it — and exactly when you break even.

The Break-Even Month Is More Useful Than You Think

Most people have heard of the "break-even point" — the month when your cumulative savings exceed your closing costs. But the standard calculation only looks at the payment difference divided by closing costs. That's a napkin-math version.

A real break-even analysis should factor in:

• The equity difference between the old and new loan at each month
• The investment growth of your monthly savings
• The investment growth you lost by spending cash on closing costs
• PMI payments saved (or added)

When you include all of this, the break-even month often shifts — sometimes by a lot. A refinance that looks like it breaks even in 18 months on a simple calculation might actually take 28 months when you account for opportunity cost. Or it might break even in 10 months if it eliminates PMI.

Three Common Refinance Scenarios

1. Rate Reduction — Usually Good, But Harder to Evaluate Than It Looks

The classic case. Your rate drops, your payment drops, and you save money over time. This is almost always worth modeling, but two things make it trickier than it sounds.

First, how long you plan to stay matters a lot. If you might move in 2–3 years, the closing costs might not be recoverable. That said, if you can get a lender credit to cover most or all of the closing costs, the calculus changes — you give up a slightly better rate, but you break even almost immediately. This is especially worth considering if there’s any chance you’ll move or refinance again within a few years, or if you’d rather invest that closing cost money instead of tying it up in the refi.

Second, and this is the one most people miss: refinancing resets your amortization schedule. If you’re 5 years into a 30-year mortgage, you’ve already front-loaded a lot of interest payments. Refinancing into a new 30-year term means you start that front-loading over again. Your monthly payment drops, but the split between principal and interest resets — so you may build equity more slowly than you’d expect, even at a lower rate. Understanding exactly how this plays out over time is genuinely confusing, which is why a basic monthly savings number doesn’t tell the full story.

Here’s a real example:

2. Cash-Out Refinance — It Depends on What You Do With the Money

You borrow more than you owe and pocket the difference. The conventional wisdom is that this is usually a bad idea, but the reality is more nuanced. Yes, you’re increasing your loan balance, paying more interest over time, and potentially re-triggering PMI. But you also have cash in hand — and if that cash earns more than the effective cost of borrowing it, you can come out ahead.

The key question is: what does the cash do? If you invest $40k in a diversified portfolio averaging 8–10% annually, and your mortgage rate is 6.25%, the spread works in your favor over time. If you’re paying off a 22% credit card, it’s almost certainly worth it. If the cash goes toward a depreciating asset or lifestyle spending, you’re likely worse off.

Here’s how the same cash-out looks under two different assumptions:

3. Principal Pay-Down — When It Makes Sense and When It Doesn’t

The opposite of cash-out: you bring money to closing to reduce your loan balance. The trade-off is straightforward — money locked in home equity earns your mortgage rate, not market returns. If your mortgage is at 5% and the market averages 8%, that pay-down has a real opportunity cost.

Where pay-down clearly wins is when it eliminates PMI. If you’re at 83% LTV and can bring enough to closing to cross below 80%, the PMI savings alone can dwarf the opportunity cost of the cash. Most basic calculators don’t model PMI at all, which is why they undervalue this scenario.

Where it’s less clear is when you’re already below 80% LTV. Paying down an extra $20k to reduce your balance sounds responsible, but that’s $20k that could be earning market returns instead of saving you 5–6% in mortgage interest. The math often favors investing.

What Most Calculators Get Wrong

Most refinance calculators online ask for your current rate, your new rate, and your loan balance. They spit out a monthly savings number and a simple break-even point. That's better than nothing, but it misses the picture.

Here's a side-by-side of what they show versus what actually matters:

Without these factors, you're making a five-figure decision with two-variable math.

A Quick Checklist Before You Refinance

Before calling a lender, run through these questions:

1. How long will you stay? If you might move in under 3 years, the math rarely works unless you're eliminating PMI.
2. What's your current LTV? If you're close to 80%, a new appraisal could eliminate PMI — that alone might justify the refi.
3. What will you do with the savings? Monthly savings that go to investing compound over time. Savings that get absorbed into lifestyle spending don't improve your net worth.
4. Have you shopped around? Rates and closing costs vary significantly between lenders. Get at least 3 quotes.
5. What's the real break-even? Not the napkin version — the one that accounts for equity, investments, and opportunity cost.

Refinancing can be one of the best financial moves a homeowner makes. But it's only a good move if the math says so — and only if the math includes everything that matters.