Pay Off Your Mortgage or Invest? How to Compare the Numbers

 

Comparing mortgage prepayments with investing requires more than comparing two percentages. The alternatives must use the same monthly budget and the same time horizon. You also need to account for any mortgage debt still outstanding and decide what happens to the payment money after a loan is fully repaid.

The Pay Off Mortgage or Invest Calculator models two consistent scenarios. Its results depend on your assumptions, especially the investment return. It does not recommend a strategy or guarantee that a projected return will occur.

Define the monthly budget

Start with the outstanding mortgage balance, fixed nominal interest rate and regular monthly principal-and-interest payment. Exclude property taxes, insurance and escrow from that payment. Those costs do not disappear when the mortgage principal is repaid and are outside this model.

Add the extra amount you could allocate each month. With a regular payment of $1000 and an additional budget of $200, both scenarios use $1200 monthly. In the prepay scenario, that budget goes toward the mortgage first. In the investing scenario, the regular payment goes to the mortgage and the extra $200 goes into investments.

Invest the freed payments after payoff

A comparison would be uneven if the prepay scenario stopped using its budget when the mortgage disappeared while the investing scenario continued contributing. This calculator invests the freed payment after payoff, keeping the same total budget through the selected horizon.

It also invests any unused budget in the payoff month. For example, if only $500 is needed to clear the loan and the monthly budget is $1200, the remaining $700 becomes an investment contribution that month. The full budget becomes available for investment in later months.

Compare investments minus remaining debt

An investment account balance alone does not reveal which scenario has the stronger modeled position. One scenario may hold more investments but also owe more mortgage principal. The relevant comparison here is projected investments minus remaining mortgage debt at the same date.

Home value is assumed identical in both scenarios and excluded from the calculation. This position is therefore not your complete net worth. Other assets, debts, cash reserves and household expenses are not included.

Work through a hypothetical example

Enter a $100000 mortgage balance, a 6% fixed nominal interest rate, a $1000 regular monthly principal-and-interest payment, an extra $200 monthly budget, a hypothetical 5% effective annual investment return and a ten-year horizon.

Under the model's monthly rounding and timing rules, prepaying clears the mortgage in month 109 and produces projected investments of $14640.80 at the horizon. Investing the extra amount produces $30872.63 of investments but leaves $18060.29 of mortgage debt, for a position of $12812.34.

The invest-minus-prepay difference is therefore approximately negative $1828.46. That is a result for these particular assumptions, not a universal advantage for prepaying. Changing the return, budget, horizon or mortgage terms can change the comparison.

Understand interest and contribution timing

Mortgage interest is calculated monthly from the outstanding balance using the nominal annual rate divided by twelve, rounded to cents each month. Payments occur at month-end. Extra payments shorten repayment without lowering the regular payment through a recast.

The investment assumption is an effective annual return, converted to an equivalent monthly growth rate. Growth occurs before each month-end contribution. The projection keeps investment growth unrounded until display and assumes a constant return rather than a changing market path.

Recognize what is outside the model

Taxes, investment fees, mortgage-interest deductions, prepayment penalties, changes in mortgage insurance, adjustable rates and inflation are not calculated. Liquidity, investment risk and emergency-fund needs also matter when making a real decision. Treat the result as a comparison of scenarios, not individualized financial advice.

For a payment estimate on a new loan, use the Mortgage Payment Calculator. For growth without a debt comparison, use the Compound Interest Calculator. Entries in this comparison are calculated locally without being saved or transmitted by the calculator.