Pay Off Debt or Invest? The Rule of Thumb, and When It Breaks

The whole “pay off debt or invest” question turns clean once you notice both sides are quoted in the same unit: a percentage. Clearing a card at 22.15% APR is a guaranteed, tax-free 22.15% return, because every dollar you send stops that interest with certainty. Investing that dollar instead earns an expected return of about 10% a year over the long run, and that number comes with real risk of a bad year. So the order almost always runs: capture the full employer 401(k) match, then clear any debt above roughly 8% APR, then invest the rest. You can model your own balances in the Debt Payoff Calculator, but here is why the math lands where it does.

That is the entire argument in one box. The 22.15% figure is the Federal Reserve’s average APR on accounts actually carrying a balance in the second quarter of 2026 (G.19, released July 8, 2026). The 10% is the long-run compound return of the S&P 500 with dividends reinvested, 1928 through 2025, from the NYU Stern historical-returns dataset. One is a rate you are being charged right now, guaranteed. The other is an average that any single year can miss by a lot.

The match comes first, before any debt

There is one dollar that beats even a 22% card, and it is the employer 401(k) match. The SEC’s own investor-education office puts it plainly: “If your employer contributes 50 cents for every dollar you save, that’s an immediate 50 percent return on your money. 50 percent! No other investment will likely give you that kind of guaranteed return” (Investor.gov, “Free Money!”). Their worked example: you earn $50,000, your employer matches 50 cents on the dollar up to 5% of pay, you put in at least $2,500, and your employer adds $1,250. That is an instant 50% return the day the money lands.

Nothing on your debt list clears 50% guaranteed. So the sequence starts there. Contribute at least enough to capture the entire match, because not doing so is “passing up free money for your retirement savings” in Investor.gov’s words. Only after the match is captured does the debt-versus-invest contest begin.

The head-to-head: $500 at the card, or in the market

Say the match is handled, you have $6,000 on a card at 22.15%, and $500 a month of spare cash. Send it to the card and the Debt Payoff Calculator engine clears the balance in 14 months for about $846 of interest. Invest the same $500 a month at the market’s long-run 10% instead, and over those 14 months it grows to about $7,392, a gain of roughly $392. But the untouched $6,000 keeps charging 22.15%, about $1,329 a year, or about $1,550 across the same 14 months.

Line the two up and investing loses by about $1,158 over 14 months, and the $392 it did earn was never guaranteed while the $1,550 card bill was. Pay the card first and you are ahead by that gap, with certainty, and the full $500 a month is free to invest the moment the balance hits zero. Taxes only widen it: credit-card interest is not deductible, so the 22.15% you avoid is a pure, untaxed return, while the market gains you skipped would eventually be taxed.

Why the line sits near 8%

The rule of thumb, “pay debt above about 8% APR, invest below it,” is just the point where a guaranteed number stops obviously beating a risked one. The market’s long-run 10% is nominal, before inflation, and it is a geometric average, meaning it already accounts for compounding. Strip out roughly 3% of long-run inflation and the real return is closer to 7%. Then remember it is an average that any year can miss badly. The Damodaran series shows single years running from about -43.84% at the worst to +52.56% at the best. A guaranteed 22% flattens all of that risk.

So above 8% or so, the guaranteed debt return wins for almost anyone, because you would need the market to reliably clear that hurdle after tax and after risk, and it does not. Below 8%, the two get close, and other factors can tip the scale toward investing: a long time horizon, tax-advantaged account space, and deductible interest all favor keeping a low-rate loan and investing the difference. The 8% figure is a fuzzy band, call it 6% to 9%, not a bright line. Use your own card’s real APR, because many run 25% to 30% and push the answer even harder toward payoff.

When the rule breaks

A few real situations flip the default, and it is worth naming them.

Low-rate, deductible debt. A 4% mortgage or a 6% student loan is a different animal from a 24% card. Mortgage interest and some student-loan interest can be deductible, which lowers the effective rate further, and both sit below the invest-instead line for most people with a long horizon. The getting out of debt guide walks through where each type of debt falls.

Tax-advantaged space you would lose. A 401(k) or IRA contribution limit is use-it-or-lose-it each year. If aggressively paying a 7% loan means skipping years of tax-sheltered growth you can never reclaim, the long-run case in compound interest, shown with real numbers can tip toward investing part of the money.

The emergency fund and the minimum payment. Most planners put a small starter cash buffer and a never-missed minimum payment ahead of both aggressive payoff and aggressive investing, so a surprise expense does not force you back onto the 22% card. And when you do attack debt, the avalanche method (highest APR first) is exactly what these return numbers endorse; the snowball is about motivation, not math.

A 0% balance-transfer window. If a high-rate balance is parked at 0% for 18 months, its guaranteed “return” from payoff is temporarily zero, so investing can win inside that window. Just watch the 3% to 5% transfer fee and the rate that snaps back when the promo ends.

The caveat

The 10% market figure is a long-run, nominal, geometric average from 1928 to 2025. It is an average, not a forecast, and any single year has ranged from about -43.84% to +52.56% in the Damodaran data. After inflation the real return is nearer 7%. The 22.15% card rate, by contrast, is a certainty, and it is the Federal Reserve’s average for accounts assessed interest, so your own card may be higher or lower. Rates move every quarter. The single fact that decides this whole question is your actual APR, so put your real balances and rates into the Debt Payoff Calculator, capture your full employer match first, and clear anything above the roughly 8% line before you invest a dollar in a taxable account. For the growth math on the investing side, see compound interest, shown, and for the wider set of these trade-offs, the Money Decisions and Comparisons hub lines them up side by side.