Should you pay off your mortgage or invest your extra money? Start with your remaining principal, fixed mortgage rate, remaining term, monthly surplus and time horizon. The investment return you enter is only an assumption, so the larger projected number is not a promise.
Compare the same total monthly budget in every path, not extra principal against investments alone. Keep emergency savings outside the comparison. This guide models ongoing monthly surplus, rather than an immediate lump-sum payoff, and separates investments from the debt still owed.
Mortgage or investing: equal monthly budget
Compare three hypothetical paths. Keep emergency cash outside this model. Investment growth is an assumption, not a prediction.
Fixed-rate loan; principal and interest calculated from the balance and remaining term. Taxes, insurance, PMI, deductions, penalties and investment account restrictions are not modeled. Contributions occur at month end. After payoff, the entire principal and interest plus surplus budget goes to investing.
This tool runs in your browser. Your entries are not sent or saved. All amounts are in US dollars (USD).
Load example fills hypothetical figures. They are not current rates or recommendations.
Loading the calculator.
What money can you actually commit each month?
Write down the extra amount you can sustain after ordinary bills and cash reserves. If that amount depends on overtime or a temporary bonus, test a smaller recurring contribution first. A plan that requires withdrawing investments to cover next month's bills needs a different budget.
The calculator derives the scheduled principal-and-interest payment from the current balance, fixed rate and remaining months. Check that result against your mortgage statement. An unusual payment schedule, modification or inaccurate remaining term can make the derived payment differ from your actual obligation.
Do not enter the entire escrow-inclusive payment as principal and interest. The CFPB explains mortgage amortization: principal reduces the balance and builds equity, while interest does not. Many monthly payments also include taxes and insurance, which this calculator leaves out.
For a household cash-reserve check, use the emergency fund worksheet separately. Do not add that protected reserve to your investment contributions merely because it appears available in your bank account.
Why must the monthly budgets stay equal?
Suppose one path uses $300 of surplus for principal and another invests $300. Both still owe the scheduled mortgage payment initially. After the first path pays off the loan, ignoring its newly available scheduled payment would make the comparison unequal.
Here, each path commits the scheduled principal-and-interest payment plus the same surplus every month. While debt remains, the invest path pays the scheduled amount and invests the surplus. The prepay path sends both amounts to the mortgage until the payoff payment is reached.
The split path directs your chosen percentage of surplus to principal and invests the rest. A 50% setting means half the surplus, not half the entire mortgage budget. The scheduled loan payment remains due while the loan exists.
In any path's payoff month, the actual loan payment cannot exceed the remaining principal plus that month's interest. Unused budget is invested at month end. Thereafter, the full scheduled payment plus surplus is invested, including in the split path.
This is a disciplined reinvestment assumption, not a forecast of behavior. If you would spend the freed payment instead, the displayed investment balance would not describe your plan. Decide whether you can automate that transfer before relying on the comparison.
What does the hypothetical example show?
Load the hypothetical example: $120,000 remaining principal, a 6% fixed nominal mortgage rate, 120 remaining months, $300 monthly surplus, a 5% assumed net annual investment return, a 120-month horizon and a 50% principal allocation for the split path.
The calculated scheduled payment is $1,332.25, and the equal monthly budget is $1,632.25. The model retains unrounded amounts internally and displays cents, so lender rounding can differ slightly. The 5% return is illustrative, not a current market rate or expected outcome.
| Surplus allocation | Portfolio | Mortgage debt | Portfolio minus debt | Payoff month |
|---|---|---|---|---|
| Invest all | $46,308.95 | $0.00 | $46,308.95 | 120 |
| Prepay all | $48,539.59 | $0.00 | $48,539.59 | 92 |
| Split surplus 50/50 | $47,565.17 | $0.00 | $47,565.17 | 105 |
Under these inputs, prepaying produces a modeled net advantage of $2,230.64 over investing all surplus. The split advantage is $1,256.22. Neither figure is mortgage interest saved alone: both reflect the entire investment-and-debt path under the assumed return and reinvestment rule.
Mortgage interest paid is $39,869.52 in the invest path, $29,959.65 in the prepay path and $34,194.86 in the split path. Earlier principal reduction lowers modeled interest, but portfolio growth also changes because contributions arrive at different times.
How are the payments and investment balances calculated?
Let B be starting principal, r the mortgage annual rate divided by 12, and n the remaining months. The scheduled payment is B × r ÷ (1 − (1 + r) raised to −n). At a zero loan rate, use B ÷ n.
Each month, interest equals the opening debt balance times r. The actual payment is the smaller of debt plus interest and the planned loan payment. Remaining debt equals debt plus interest minus that actual payment.
For an assumed annual effective net investment return R, monthly growth is (1 + R) raised to 1/12, minus 1. The opening portfolio grows first; the equal monthly budget minus the actual loan payment is then contributed at month end.
Net for this comparison is portfolio minus remaining mortgage debt. The same home value cancels across all three paths and is omitted. These calculations are scenario arithmetic, not a total household balance sheet or a tax calculation.
What if the loan is not paid off by your horizon?
Shorten the horizon and read both the portfolio and debt columns. A large portfolio beside a large mortgage is different from a smaller portfolio beside less debt. Comparing account balances alone gives the investing path credit for money while ignoring its remaining obligation.
The net column makes that trade-off visible, but does not make those assets equally accessible. Home equity may require a sale or new borrowing to release. Investments may fall in value before you need them, and account rules can restrict withdrawals.
The SEC's Investor.gov risk-and-return guide distinguishes access, growth and safety. Stock prices can fall, and mutual funds cannot guarantee returns. A constant growth assumption does not reproduce losses concentrated just before your comparison date.
Run several return assumptions, including a lower or negative one, rather than choosing the number that makes your preferred path win. A narrow modeled difference is especially sensitive to costs, taxes and actual investment performance omitted from this comparison.
Which contract and tax details should you check first?
Ask your servicer how to designate extra money as principal and verify the following statement. Do not assume an extra payment automatically lowers next month's required payment. The model keeps the scheduled principal-and-interest budget unchanged until payoff.
Check your loan documents for a prepayment penalty. The CFPB says some mortgages have one; full payoff or a large principal payment can trigger it. Small extra principal payments normally do not, but the CFPB still recommends checking with the lender.
The tool does not calculate a mortgage-interest deduction, investment tax liability, account withdrawal taxes or fees. Its investment-return input should reflect your assumed net growth after relevant costs. Your actual tax treatment may make a simple nominal-rate comparison misleading.
If retirement contributions are involved, check employer-plan terms, any match, vesting and withdrawal restrictions before moving contributions elsewhere. Treat those details as a separate decision, not as benefits already included in these taxable-style portfolio scenarios.
Review expensive nonmortgage debt separately using the debt payoff comparison. The mortgage model does not decide which of several debts deserves your next dollar, and it does not include minimum payments on those other accounts.
When does this comparison not fit?
Do not use these outputs as a personalized recommendation for an adjustable-rate, interest-only or balloon loan. The payment formula assumes a fixed-rate amortizing balance. A lender recast, refinance or changing rate would need its own payment schedule and costs.
This model also does not fit a lump-sum inheritance decision, a near-term home sale, or a plan to stop investing after payoff. It assumes a constant monthly budget, the same home, and continued reinvestment through the chosen horizon.
If you are behind on payments or cannot preserve essential cash, focus on payment stability first rather than optimizing a distant modeled balance. The CFPB's amortization page points borrowers having payment difficulties toward HUD-approved housing counseling assistance.
Finally, avoid treating the winning net column as a reason to accept risk you cannot tolerate. Earlier debt freedom and access to usable cash are different goals. Choose a path you can maintain through changes in income and investment values.
Quick decision checklist
- Confirm the current principal, fixed rate, remaining months and principal-only payment instructions with your mortgage statement or servicer.
- Protect emergency cash and choose a surplus that leaves other required payments covered.
- Compare the same monthly budget, including full reinvestment of freed principal and interest after payoff.
- Read portfolio, remaining debt and net separately; rerun lower-return and shorter-horizon scenarios.
- Check penalties, tax treatment, investment account restrictions and fees outside the model before acting.
Sources
- CFPB, How does paying down a mortgage work? Checked October 9, 2026.
- CFPB, What is a prepayment penalty? Checked October 9, 2026.
- Investor.gov, Risk and return. Checked October 9, 2026.
Prepared with AI-assisted research, drafting, and calculation checks. Not independently reviewed by a human financial professional.
