Mortgage Reinvestment Calculator

It depends on the math and on you: pay down the mortgage when your rate is high relative to what you could safely earn elsewhere and you value a guaranteed, risk-free return; invest when your expected market return comfortably clears your rate and you can stomach the risk.

There is no single right answer, and anyone who gives you one without knowing your rate, your timeline, and your risk tolerance is guessing. The honest framing is a trade-off between two real things:

• Paying extra principal earns you a guaranteed return equal to your interest rate — every dollar of principal you kill is a dollar of interest the bank can never charge you again.
• Investing the difference earns a hoped-for return that is usually higher on average, but is not guaranteed and can be negative in any given stretch.

This calculator does not pick a side for you — it shows both outcomes in real dollars at the moment you sell, so you can weigh the certain payoff against the bigger-but-riskier one.

It runs both strategies month by month with the same extra dollars and compares the wealth each one has built at the moment you sell — the guaranteed equity and interest you saved by paying down, versus the portfolio value of investing at your expected market return.

The two sides are scored on a level playing field — the extra wealth each strategy creates versus doing nothing:

Whichever number is larger at your sale date wins, and the calculator shows the dollar margin between them. If your extra payments retire the loan before you sell, the payment you used to make is freed up and invested for the remaining months — that money doesn’t vanish, so the payoff strategy gets full credit for it.

Your guaranteed, risk-free return is exactly your mortgage interest rate — if your loan is at 6.5%, every extra dollar of principal earns you a certain 6.5% by erasing future interest at that rate.

This is the single most useful idea in the whole debate. Paying down a 6.5% mortgage is financially identical to earning 6.5% on an investment that cannot lose money — there is no market, no volatility, no bad year. The "return" is the interest you would have owed and now never will.

That is a genuinely strong, safe return. The catch is that it is capped at your rate: a 6.5% loan can never pay you back more than 6.5%, while the market might (or might not) do better. So the comparison is really "a certain X% versus a probable-but-uncertain Y%."

Investing tends to win when your expected market return is meaningfully higher than your mortgage rate and your time horizon is long enough for compounding to work — but it only wins on paper if the market actually delivers, and it carries risk, taxes, and a longer payback that paying down does not.

Four things tip the scales:

With the calculator’s default 7% expected return against, say, a 6.5% mortgage, investing often edges ahead over a long horizon — but the margin is thin and depends entirely on the market hitting that 7%. Flip the loan to 7.5% or drop the expected return to 5% and paying down usually wins outright. Liquidity is the quiet tiebreaker: money in a brokerage account can be reached in an emergency, while a dollar of home equity cannot — you would have to sell or take out a loan to touch it.

No — on a normal fixed-rate mortgage, extra principal shortens the loan and slashes total interest, but your required monthly payment stays exactly the same until the loan is paid off (unless you formally recast it).

Here is the part that surprises people: you can pay an extra $300 every month for years and your minimum payment will not budge by a cent. What changes is the finish line — the balance hits zero sooner and you skip years of interest. On a $300,000 loan at 6.5% over 30 years, an extra $300/month pays the loan off years early and saves tens of thousands in interest, yet the bill in your mailbox reads the same number every month right up until it’s gone.

The one way to actually lower the required payment after a lump-sum principal reduction is a recast — see the next question.

A recast is when your lender re-amortizes your loan over its remaining term after you make a large lump-sum principal payment, producing a lower required monthly payment without refinancing.

Recasting keeps your existing rate and term — it just spreads a now-smaller balance across the months you have left, so the minimum payment drops. It typically costs a small flat fee (often a few hundred dollars) and requires a minimum lump sum.

Not every loan is eligible (many government loans are not), so confirm with your servicer first. If you want a lower payment and a lower rate, a refinance is the lever — run the numbers in our Refinance Calculator.

Peace of mind is a real, legitimate reason to pay down your mortgage — a paid-off home is a guaranteed outcome with no market risk, and that certainty has genuine value the spreadsheet doesn’t fully capture.

The investing case is built on averages. Over decades the market has trended up, but it does not deliver a smooth 7% every year — it delivers great years, flat years, and frightening years, in an order nobody can predict. Paying down your mortgage sidesteps all of that. The return is locked in, it never has a bad year, and owning your home free and clear lowers the fixed cost of simply living somewhere.

So even when the math nudges slightly toward investing, choosing the guaranteed path is a defensible, grown-up decision — especially as you near retirement, when a small chance of a large loss matters far more than a small chance of a large gain.

For most people the smart order is: capture any employer 401k match first, then split remaining cash between extra principal and investing — the match is free money no mortgage payoff can compete with.

A typical employer match is an instant 50–100% return on the dollars you contribute. No mortgage rate and no stock-market average comes close to that, so it almost always belongs at the front of the line. A sensible priority stack looks like this:

• 1. Capture the full 401k match — an immediate, guaranteed return that beats both strategies on this page.
• 2. Pay off high-interest debt — credit cards at 20%+ dwarf any mortgage decision.
• 3. Keep an emergency fund — cash you can reach, because home equity is locked until you sell.
• 4. Then choose — extra principal, more investing, or a blend of both.

Doing both is a perfectly good answer: many borrowers add a little to the mortgage and keep investing, buying a mix of certainty and growth instead of betting everything on one.

No — the calculator compares pre-tax outcomes, so it does not subtract capital-gains tax from the investing side or factor in any mortgage-interest deduction on the payoff side.

That keeps the core comparison clean, but it means real-world results can shift in either direction:

• Taxes can trim the investing edge — gains in a regular brokerage account are taxed when you sell, so the after-tax portfolio is smaller than the figure shown.
• Tax-advantaged accounts push the other way — money invested inside a 401k or IRA grows tax-deferred (or tax-free for a Roth), which strengthens the investing case.
• The mortgage side — if you itemize and deduct interest, paying down reduces that deduction slightly, nudging its real return a touch lower.

Enter your loan, the extra amount you could put toward either strategy each month, your expected investment return, and how long until you sell — the verdict updates instantly.

• Type in your loan amount, interest rate, and loan term from your statement.
• Set the extra per month — the same dollars are applied to both strategies for a fair fight.
• Enter your expected investment return. The S&P 500’s long-run average is roughly 10% before inflation; 7% is a common conservative planning number. It is not guaranteed, unlike your mortgage rate.
• Pick a compounding frequency and drag the "how long until you sell" slider — the single biggest factor in the result.

The verdict card names the winner and the dollar margin at your sale date, with side-by-side breakdowns of equity built, interest saved, and portfolio value. Still deciding what you can take on? Start with our What Can I Afford calculator.