Mortgage payoff vs invest calculator

The core trade-off

Every extra dollar you can spare can either pay down your mortgage - a guaranteed return equal to your interest rate - or be invested for a higher but uncertain return. This calculator quantifies both sides so you can decide with numbers, not just feelings.

Worked example

A $300,000 mortgage at 6% with 30 years left, and $500 a month to spare, assuming a 7% investment return:

• Pay extra: the loan is paid off about 148 months early and you save roughly $160,295 in interest.
• Invest instead: that $500 a month grows to about $609,986 over the original 30 years.

Here the 7% expected return beats the 6% mortgage, so investing wins in expectation - but only if the market actually delivers 7%. The mortgage payoff is certain.

How to decide

• Compare the rates. If your expected after-tax return clearly exceeds your mortgage rate, the math favors investing. If they are close, the guaranteed payoff often wins on a risk-adjusted basis.
• Fund priorities first. Capture any 401(k) match, fill an HSA, and keep an emergency fund before doing either - those beat both options.
• Value the certainty. A paid-off house lowers your fixed costs and your required FIRE number, which has real psychological and practical value.

The liquidity catch

Money sent to the mortgage is locked in the house - you cannot easily get it back without a refinance or HELOC. Invested money stays liquid. If your emergency fund is thin, that flexibility argues for investing (or at least splitting the difference).

Common mistakes

• Prepaying the mortgage before capturing a full employer 401(k) match.
• Ignoring that investment returns are uncertain while the payoff is guaranteed.
• Draining liquidity into the house and then needing a high-rate loan later.

To see how the invested side compounds, use the compound interest calculator; to lower the payment instead with a lump sum, see the mortgage recast calculator.