The Interest Rate That Tells You Whether to Pay Off Debt or Invest

The debt vs. investing question can feel complicated, but a simple comparison settles most cases. It comes down to one number: the interest rate on the debt, compared fairly with what investing has historically earned.

When a debt costs more than investments are likely to earn, paying it off tends to come out ahead. When it costs less, investing has tended to come out ahead over long periods, with market risk along the way. The comparison only works if both numbers are measured the same way.

The certain-outcome framing

Paying off a debt that charges 20% interest removes a 20% cost for certain. The U.S. stock market has returned about 10% a year on average before inflation (about 7% after inflation), and that average includes years of steep losses. Against a 20% debt, the certain saving is about twice the market's long-run average.

This framing makes the extremes clear. Credit card debt at 20 to 25% APR costs about twice the market's historical average, which is why paying it off is usually the first step after a small emergency cushion. A mortgage at 3.5% costs far less than that average, which is why many people keep it and invest the difference. Student loans at 4 to 5% sit on the same side as the mortgage. The hard cases are in the middle.

The gray zone: roughly 6 to 10% debt

This is where most people get stuck, and where the answer is less mechanical. At these rates the historical gap between investing and paying down the debt is about 0 to 4 points, close to the extra return investors have historically earned for accepting stock market swings. The certain saving and the expected return are nearly a wash. A few factors tip the decision.

Age and timeline matter. Someone who is 25 with 40 years until retirement has a long runway for investments to compound and to recover from bad years. That tends to tip the gray zone toward investing for younger people with a long horizon.

Risk tolerance matters. Market returns are not guaranteed. The long-run average includes years like 2008, when the S&P 500 fell about 37%. The savings from paying down debt are certain. For someone whose behavior changes when markets drop, that certainty has real value.

Tax treatment matters. Mortgage interest may be deductible for people who itemize, which lowers the effective rate. Student loan interest is deductible up to $2,500 a year, subject to income limits. A 6% debt that produces a deduction may effectively cost 4 to 5%, which moves it below the gray zone.

Why the 401(k) match usually comes first

Most frameworks put an employer 401(k) match ahead of extra payments on any debt other than credit cards. A 50% match on the first 6% of salary adds 50 cents to each dollar contributed, up to that limit: an immediate 50% return before any market gain. Some plans make the match vest over a few years, so it only counts once it's yours. Few debts cost anywhere near 50%, which is why the match usually comes ahead of extra debt payments.

The debt types I avoided personally

I have never carried a credit card balance. I financed my first car but paid it off early when I had the cash. I had a mortgage in the mid-4% range, refinanced to a meaningfully lower rate a few years later, and invested the difference instead of rushing to pay it down. I eventually sold that house for a profit. At that rate, the math of keeping the mortgage instead of prepaying it was clear.

The debts I would never carry are credit cards and high-interest personal loans. They compound against you exactly the way an investment compounds for you. At 24% APR, charged monthly, a $5,000 balance grows to about $6,340 in a year if nothing is paid. That is an effective cost of about 27% a year, close to three times the stock market's long-run average.

Frequently Asked Questions

Should I pay off debt or invest first?

Compare the debt's interest rate with the stock market's long-run average measured the same way: about 10% a year before inflation from 1928 through 2025, or about 7% after inflation. Debt rates are quoted before inflation, so the 10% figure is the fair comparison. A debt above about 10% costs more than that average, and paying it off removes that cost for certain. Below about 6%, investing has historically come out ahead over long periods, with market risk. An employer 401(k) match is usually considered first, since it adds to every dollar contributed regardless of the debt's rate.

What counts as high-interest debt?

Generally anything above about 10%, which is more than the stock market's long-run average before inflation. Credit card debt commonly runs 20% or higher, about twice that average. Many mortgages and older student loans carry lower rates and fall on the investing side of the comparison or in the gray zone.

What if my debt's interest rate is between 6 and 10%?

That's the gray zone. The historical gap is about 0 to 4 points, close to what investors have earned for accepting market risk, so the math doesn't clearly favor either choice. Risk tolerance and peace of mind matter as much as the math here. Some people prefer the certainty of a paid-off balance, and others prefer the higher expected return of investing. Both are defensible in that range.