Should You Pay Off Your Mortgage Early or Invest Instead

An extra $500 a month against a 3% mortgage builds $113,486 of home equity over fifteen years. The same $500 invested at 7% grows to $158,481. Against a 7.5% mortgage the order flips. The answer to "pay it off or invest" is mostly one number on your mortgage statement.

The full cost of owning, beyond the rate, is in rent vs buy and the assumptions every calculator hides. This piece answers the narrower question an owner faces every month: where should the spare money go once the house is bought?

The guaranteed return of a prepayment

Every extra dollar you send to principal stops being charged interest at your mortgage rate for the rest of the loan. That is the same as earning the mortgage rate on that dollar, with no market risk and no tax on the "earnings", because interest you avoid paying is not income.

So a prepayment on a 6.5% mortgage is a 6.5% risk-free return. Nothing else available to an ordinary saver pays that with certainty. A prepayment on a 3% mortgage is a 3% return, which a Treasury bill or a high yield savings account has beaten in recent years.

The general version of this comparison, for every kind of debt, is in invest or pay off debt first. The mortgage is the hardest case because the rates sit right where expected investment returns sit.

The after tax rate most owners actually pay

The mortgage interest deduction used to lower the real cost of a mortgage. For most owners it no longer does. The 2026 standard deduction is $32,200 for a married couple filing jointly, according to IRS Revenue Procedure 2025-32. Mortgage interest only helps if your itemized deductions, interest plus state and local taxes plus charity, beat that figure. On a $300,000 loan at 6.5% the first year's interest is about $19,400, short of the threshold on its own.

If you take the standard deduction, your after tax mortgage rate is your mortgage rate. If you itemize, the rules on how much interest is deductible are in IRS Publication 936, and your effective rate is roughly the rate times one minus your bracket.

The investment side does pay tax. Inside a 401k or IRA it grows sheltered. In a taxable account, dividends and gains are taxed along the way, which shaves the 7% down. That matters in the close cases below.

Liquidity and the emergency fund first

Home equity is the least liquid money you own. You cannot spend it without selling or borrowing against it, and lenders cut home equity lines exactly when times are bad. A family that prepaid $40,000 and then lost a job owns a house that is $40,000 more paid off and a checking account with nothing in it. The bank still wants the full monthly payment.

So prepaying comes after, not before:

1. an emergency fund sized to your household, worked out in how much emergency fund you need
2. the full employer 401k match
3. any debt at a higher rate than the mortgage, including car loans and every credit card

Only money left after those three is really deciding between the mortgage and the market.

Three households and three answers

Three owners each have $500 a month spare for fifteen years. Each has a large enough balance that the extra payments do not finish the loan inside fifteen years. The extra principal paid against a loan grows at the loan rate, so the equity it builds is the future value of $500 a month at that rate. The investment is $500 a month at an assumed 7% average annual return, compounded monthly, inside a tax-sheltered account. The worst case uses 4%, for a bad fifteen years.

The formula behind every cell is the future value of a monthly payment: 500 × ((1 + r/12)¹⁸⁰ − 1) ÷ (r/12), with r as the annual rate.

The 3% owner should invest. Investing beats prepaying by $44,995 at 7%, and still wins by $9,559 in the bad case. There is almost no version of the next fifteen years where prepaying a 3% loan comes out ahead.

The 7.5% owner should prepay. Prepaying beats the hoped-for 7% outcome by $7,075 and beats the bad case by $42,511, with no risk at all. If this owner's credit has improved, a refinance may beat both. That decision is in when refinancing your mortgage pays off.

The 6.5% owner is the hard case. Investing wins by $6,709 if returns average 7%, and loses by $28,727 if they average 4%. Invested in a taxable account instead of a 401k, the 7% case would shrink further. When the expected gain is that small and the downside that large, we think splitting the $500 is the honest answer: $250 to the mortgage, $250 to the market.

What the arithmetic leaves out

Behavior. Money invested "instead of prepaying" only beats the mortgage if it is actually invested, every month, and left alone through a crash. Money sent to the mortgage cannot be spent on a car. Some households know which of those they are.

Retirement timing. A mortgage that ends the year you stop working lowers the income you need from savings, and so the tax and the Medicare premium surcharges that come with larger withdrawals. Owners within ten years of retiring often value that more than a modestly higher expected return.

Paying off before retirement

Aim to have the mortgage gone by your planned retirement date if you can do it from cash flow. Do not do it with a lump sum from a 401k or traditional IRA. Every dollar withdrawn is taxed as income in that year, so pulling $80,000 to clear the loan could move a couple from the 12% bracket into the 22% bracket and cost thousands that steady payments never would.

If the payoff date and the retirement date do not line up, compare the remaining balance with the extra monthly amount needed to finish on time. That is a monthly number you can budget, the same way the 15 vs 30 year mortgage comparison frames the term choice.

How to prepay without wasting it

If the numbers say prepay, the mechanics decide whether the money does what you meant. Three things go wrong often enough to check.

The payment is not applied to principal. Some servicers treat an extra amount as an early version of next month's payment, holding it or applying it to interest that has not accrued yet. That saves you nothing. Use the servicer's "additional principal" field online, or write "apply to principal" on the check, and confirm on the next statement that the principal balance fell by the full extra amount.

A biweekly program charges for something free. Paying half the monthly payment every two weeks adds up to 13 full payments a year instead of 12, which is a real prepayment. But some third-party biweekly services charge a setup fee and hold your money between payments. You get the same result by adding one twelfth of a payment to each monthly payment, for nothing.

The loan has a prepayment penalty. Most mortgages made since 2014 cannot charge one after the first three years, and many never can, but check the note or the closing disclosure before sending a large lump sum.

Monthly extra or one lump sum

Prepaying works the same whether the money goes in monthly or as a lump sum from a bonus. Interest stops on each dollar the day it reduces the balance, so a $6,000 lump sum in January saves slightly more than $500 a month through the year. The lump sum is the right shape for irregular money, such as a bonus or a tax refund. The monthly amount is the right shape for a steady surplus, because it happens without a decision each time.

Recasting after a large prepayment

Prepaying does not lower your required payment; it shortens the loan. If you want the lower payment instead, for example before retiring on a smaller income, ask the servicer for a recast. The lender reamortizes the remaining balance over the remaining term for a fee, often $150 to $500, and your required payment drops. Not every loan type allows it, and government-backed loans usually do not, so ask before you send the lump sum.

When this does not apply

If you have an adjustable rate mortgage due to reset. The comparison above uses a fixed rate. If your rate could jump to 8% in two years, prepaying before the reset lowers the balance the new rate applies to, which changes the math in favor of prepaying.

If you expect to move within a few years. Equity you prepay comes back at sale, but tied up until then. If you will need that money for the next down payment, a savings account keeps it reachable.

If you are not getting the full employer match. A 50% match is an immediate 50% return. No mortgage rate competes with that.

Next steps

1. Find your mortgage rate on your latest statement and write it next to the 5% and 7% marks above.
2. Confirm the emergency fund, the match and every higher-rate debt are covered.
3. If your rate is under 5%, set up the monthly investment; if it is over 7%, set up the extra principal payment.
4. If you are in between, split the spare amount and review the split once a year.
5. Tell your servicer the extra payment is for principal, and check the next statement to confirm it was applied that way.