Mortgage Payoff vs. Invest Calculator Methodology

The calculator compares two paths over a selected time horizon: paying extra toward a mortgage and investing the same extra cash flow. It estimates mortgage balances, interest paid, investment balances, home equity, liquidity, and net position.

If you enter a current principal and interest payment, the calculator uses that amount. If the field is blank, it estimates the payment from the current balance, interest rate, and years remaining using a standard amortized mortgage formula.

payment = principal * (monthlyRate * (1 + monthlyRate)^payments) / ((1 + monthlyRate)^payments - 1)

The payoff path applies the normal mortgage payment, the extra monthly amount, and any lump sum to the mortgage. If the mortgage is paid off before the horizon and the redirect option is selected, the freed-up mortgage payment and extra amount are invested for the remaining months.

The investing path keeps the mortgage on its regular payment schedule and invests the extra monthly amount. The lump sum is invested in the selected month. If the scheduled mortgage is paid off before the horizon and the redirect option is selected, the regular mortgage payment is invested after payoff.

The model uses a steady monthly investment return based on the entered expected annual return minus the entered tax drag or fees. This is a simplified planning assumption, not a market forecast or guaranteed return.

Home value is estimated using the entered appreciation rate. Both paths own the same home, so appreciation affects both paths. Home equity equals estimated home value minus the remaining mortgage balance.

Net position is estimated as investment balance plus home equity. This highlights the tradeoff between lower mortgage balance, higher investment balance, and liquidity.

The calculator tests annual investment return assumptions from -5% to 15% using a binary search. The break-even return is the approximate return where the payoff path and investing path produce a similar net position.

The calculator shows the ending liquid investment balance in each path. Mortgage payoff may build home equity, while investing may leave more money in accessible accounts.

This calculator does not include guaranteed investment returns, market volatility modeling, exact tax deductions, exact itemized deduction treatment, mortgage prepayment rules unless entered, investment account eligibility, brokerage fees not entered, inflation unless reflected in your assumptions, or professional financial, investment, tax, legal, or mortgage advice.

These calculators are for educational purposes only and are not financial, tax, legal, insurance, investment, real estate, employment, medical, childcare, vehicle-buying, or professional advice.

This calculator uses user-entered assumptions and simplified steady-return modeling. Real investment returns vary, and tax treatment can materially affect the result. It is educational only and is not financial, investment, tax, legal, mortgage, or professional advice.