Should you pay off the mortgage early or invest the difference?

Your mortgage rate is the return

Every dollar of principal you remove early is a dollar that stops accruing interest for the rest of the term. On a 6.5% loan, prepayment pays 6.5%. That is not a rough analogy, it is the same arithmetic running backwards, and it comes with something almost no investment offers: the return is certain. The loan does not have a bad decade.

So the comparison is not "debt versus investing" in the abstract. It is a guaranteed 6.5% against whatever you actually expect to earn, after tax, net of fees, on money you might need to leave alone through a downturn.

The comparison almost everyone gets wrong

Here are the two numbers people reach for. Put $200 a month extra against this site's default mortgage, a $320,000 loan at 6.5% over 30 years, and the loan calculator says you save $105,429 in interest and finish 6 years 7 months early. Put that same $200 a month into the investment calculator at 7% for 30 years and it grows to $243,994.

$243,994 against $105,429 looks decisive. It is not a comparison at all. The first number is interest avoided over 23 years, the second is a balance after 30 years of contributions, and the payoff path has a second act the headline ignores: once the mortgage is gone you have the entire payment free, and 79 months left to invest it. Comparing those two figures is the most common error in this whole debate.

The fair version, and what it shows

A fair comparison holds two things constant: the money you spend each month, and the date you measure. So give both paths the same $2,222.62 a month, which is the $2,022.62 of principal and interest plus the spare $200, and measure both at month 360, when each one owns the house outright.

Path A, pay it down. Everything goes at the mortgage until it clears in month 280, then the whole $2,222.62 goes into the market for the remaining 80 months. Path B, invest alongside. Pay the normal $2,022.62 for all 360 months and invest $200 a month the entire time. Same outlay, same finish line. Here is where they land:

Look at the middle row. At an investment return of exactly 6.5%, the same number as the mortgage rate, the two paths finish $3 apart on a balance of $221,000. That is not a coincidence and it is not tuning. It is the proof that prepaying a 6.5% loan and earning 6.5% are the same act. The break-even is your mortgage rate, full stop.

How this was checked: the invest column comes straight from this site's investment calculator. The pay-down column was computed separately from the amortization formula, and at 6.5% it reproduces the calculator's figure to within $3, with the residual explained by the mortgage's partial final payment. Two independent routes, one answer. The method is described on how it's checked.

1. An employer match beats both

If your employer matches 401(k) contributions, that is an immediate 50% or 100% return on the money before it has earned anything. No mortgage rate competes with that. Capture the full match before this question applies to a single dollar.

2. Higher-rate debt beats both

The same logic that makes mortgage prepayment a 6.5% return makes credit card repayment a 22% one. Clear anything costing more than the mortgage first. The debt payoff calculator orders that for you, and the gap is usually so large that nothing else is close.

3. Prepaid principal is money you cannot reach

This is the cost nobody puts in the spreadsheet. Money in a brokerage account can be sold in a day. Money sent to your mortgage is gone until you sell or refinance, and prepaying does not lower next month's payment, it just shortens the term. If you lose your income, a brokerage balance pays the mortgage and a smaller principal balance does not.

So the emergency fund comes first, and then the honest framing is that the pay-down path buys you a guaranteed return and charges you liquidity for it. The net worth tracker shows that tradeoff as the liquid share of your assets.

4. Taxes move the bar in both directions

If you itemize, mortgage interest is deductible, so your effective rate is roughly the rate times one minus your marginal rate. A 6.5% mortgage at a 22% marginal rate behaves more like 5.1%, which lowers the bar investing has to clear. Most households now take the standard deduction and get none of this, which is why this site's calculators leave it out by default.

Pushing the other way, investing inside a 401(k) or IRA defers or avoids tax on the growth, which raises the investment side. Both adjustments are real and they partly cancel. Neither changes the shape of the table, only where the crossover sits.

An average return is not the return you get

Path B's 7% column assumes 7% every year. Real markets deliver 20% and then negative 15%, and the order matters once you are drawing on the money. The retirement simulator exists to show that effect, and it is the honest companion to any smooth projection, including the one above.

This is the strongest argument for the pay-down path and it does not appear anywhere in the arithmetic: 6.5% guaranteed and 7% expected are not 0.5% apart in any meaningful sense. One of them is a promise and the other is an average of outcomes you will only live through once.

The plan you follow beats the plan you optimize

A paid-off house is a thing people feel, and feelings drive whether a plan survives ten years. If watching a brokerage balance fall 30% would make you sell, your realized return is not 7% and the table above does not describe you. If a shrinking mortgage balance is what keeps you saving at all, that is worth more than the spread.

Both answers here are defensible. What is not defensible is picking one and claiming the numbers forced your hand. They do not. They tell you the break-even is your mortgage rate, and then they hand the decision back to you.