You have a spare $300 a month. Send it at the mortgage, or invest it? This is one of personal finance's longest-running arguments, and most takes on it are secretly advice in a trench coat. Here is just the math of each side — the decision is yours.
What paying extra earns — exactly
Every dollar of extra principal stops accruing interest at your loan's rate. Paying down a 6.5% mortgage is therefore a guaranteed, risk-free 6.5% return — no market, no volatility, no luck. On a $400,000 loan at 6.5%, $300/month extra saves roughly six figures of interest and shortens the loan by years; run your own numbers in the extra payment calculator.
What investing might earn
Broad equity markets have historically returned more than 6.5% annually over long periods — but as an average across decades that included gut-wrenching drawdowns. The investing side offers a higher expected return in exchange for uncertainty: sequence risk, behavior risk (selling in a crash), and no guarantee your particular decades match the average.
The comparison, stated fairly
- Rate matters most. Against a 3% loan, the guaranteed return is small and investing has a wide edge in expectation. Against an 8% loan or 24% credit card, the guaranteed return is excellent and hard to beat reliably — run the card math in the credit card calculator.
- Taxes cut both ways. Mortgage interest may be deductible (lowering the effective loan rate); investment gains are usually taxed (lowering the effective market return). Tax-advantaged accounts, especially any employer match, change the picture sharply.
- Liquidity is asymmetric. Money invested can be sold in an emergency; money paid into a mortgage is locked in the walls until you sell or borrow it back out.
- The psychological return is real. A paid-off home produces no spreadsheet yield, but the behavior change it enables — risk tolerance, career flexibility — is not nothing.
How to use the calculators
Get the guaranteed side precisely: enter your loan in the extra payment calculator and note the interest saved. That figure is the bar the uncertain alternative has to clear, after taxes, with risk. Where that bar sits — and how you feel about uncertainty — is the whole decision.
A worked comparison
Take $400,000 at 6.5% with $300/month spare for the full 30 years. Sent at the loan, the engine says it saves on the order of $145,000 of interest and ends the mortgage about six years early — a guaranteed outcome. Invested instead at a hypothetical steady 8%, the same stream compounds to roughly $440,000 — but the loan also ran its full course, paying its full $510,180 of interest, and "steady 8%" smooths over the years it was −20%. The expected-value gap favors investing; the certainty gap favors the loan. Both statements are true at once, which is why this argument never ends.
The middle paths most people actually take
This isn't binary. Common splits: capture any employer match first (an instant 50–100% return no loan can match), then divide the remainder; or pay extra until PMI drops (a dated, guaranteed win) and invest after; or pay the loan down to a planned recast, cutting required payment and redirecting the freed cash to investing. Each path has a precise guaranteed side — compute it first, then weigh the uncertain side against your own tolerance. The error isn't choosing either answer; it's choosing without running the numbers.
The honest summary
Pay down the loan and you earn its rate, guaranteed, with zero liquidity. Invest and you chase a higher expected return, unguaranteed, with full liquidity. High-rate debt makes the first answer nearly automatic; cheap fixed-rate debt makes the second mathematically attractive; everything between is a personal risk decision. Compute the guaranteed side first — it takes thirty seconds — and let the uncertain side argue against a real number instead of a feeling.