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

People make the debt vs. investing question more complicated than it is. There is a simple framework that resolves it in most cases, and it comes down to 1 number: the interest rate on your debt.

If your debt costs more than your investments are likely to earn, pay it off. If your debt costs less, invest. The math is the decision.

The guaranteed return framing

Paying off a debt that charges you 20% interest is equivalent to earning a guaranteed 20% return on your money. No investment in the world offers that reliably. The stock market returns roughly 7% on average, historically, after inflation. When you pay off a 20% debt, you are doing better than the market, risk-free, guaranteed.

This is the framing that makes the decision obvious at the extremes. Credit card debt at 20-25% APR: pay it off immediately. A mortgage at 3.5%: keep it, invest the difference. Student loans at 4%: same logic as the mortgage. The hard cases are in the middle.

The gray zone: 5 to 7% debt

This is where most people get stuck, and where the answer is less mechanical. At these rates, the expected returns from investing and the guaranteed return from paying off debt are close to equal. A few factors tip the decision.

Your age and timeline matter. If you are 25 with 40 years until retirement, the compounding effect of investing now is significant. Even if the expected returns are roughly equal in present value terms, the earlier you invest, the more time the returns have to compound. That tips toward investing for younger people with a long horizon.

Your risk tolerance matters. Market returns are not guaranteed. 7% is an average that includes years of -30% and years of +30%. The return on paying down debt is guaranteed. If market volatility genuinely affects your behavior, the psychological value of the guaranteed return is real.

Tax treatment matters. Mortgage interest may be deductible if you itemize, which lowers the effective rate. Student loan interest is partially deductible in some cases. A 6% debt that generates a tax deduction may effectively cost you 4-5%, which changes the math.

One rule that is never wrong: capture the 401(k) match first

Before you pay off any debt other than credit cards, make sure you are contributing enough to your 401(k) to capture your full employer match. A 50% match on 6% of your salary is a 50% guaranteed return. That beats paying off virtually any debt, including relatively high-interest debt. The match comes first, always.

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 at 4.5%, refinanced to 3.625%, and invested the difference. I've since sold that property, but if I still had it I would not even consider paying it off early. At that rate, the math of keeping the mortgage debt is clear.

The debts I would never carry are credit cards and high-interest personal loans. The interest rates are designed to be inescapable. They compound against you exactly as an investment compounds for you. At 24% APR, a $5,000 balance becomes $6,200 in a year if you pay nothing. Many people pay fall into the minimum payment trap, and the balance grows each month. No investment matches that rate of return on the downside.