A credit card charges 20% APR. Roughly how long to double a balance if only paying interest?

The Math Behind Doubling Your Credit Card Debt

At 20% APR, a credit card balance doubles in roughly 3.5 years when you're only covering the interest charges and not touching the principal. That's a sobering number, and understanding the mechanics behind it can genuinely change how you think about carrying a balance.

Where the 3.5 Years Comes From

The quickest way to estimate doubling time is the Rule of 72 — a handy shortcut used by financial professionals and savvy investors alike. You simply divide 72 by the annual interest rate. At 20% APR, that's 72 ÷ 20 = 3.6 years. Round it down slightly for the way credit card interest compounds monthly rather than annually, and you land right around 3.5 years.

The Rule of 72 isn't just a trick — it's grounded in logarithmic math. The precise formula uses the natural log of 2 (approximately 0.693), divided by the periodic interest rate. The Rule of 72 is simply a friendlier approximation of that calculation, accurate enough for real-world planning.

Why "Only Paying Interest" Is the Danger Zone

Credit card companies calculate a minimum payment that often covers little more than the monthly interest charge. On a $5,000 balance at 20% APR, the monthly interest alone is about $83. If your minimum payment is $100, only $17 is chipping away at the actual debt. At that pace, you're essentially parked — and the clock on doubling is ticking.

After 3.5 years of interest-only payments, that $5,000 balance becomes $10,000. After another 3.5 years, it's $20,000. This is the compounding trap that catches millions of cardholders off guard every year.

20% APR Is Increasingly Common — or Even Low

Twenty percent might sound steep, but by current standards it's closer to average. According to the Federal Reserve's consumer credit data, the average credit card interest rate in the United States surpassed 21% in 2023 and has remained elevated since the Federal Reserve's rate-hiking cycle began in 2022. Some store cards and subprime products charge 29.99% or higher — at that rate, the Rule of 72 gives a doubling time of just under 2.5 years.

The Practical Takeaway

Knowing this math reframes every minimum payment decision. Paying even a modest amount above the minimum — say, an extra $50 a month — dramatically shortens repayment time and slashes the total interest paid. Financial advisors consistently rank high-interest credit card debt as the first priority for payoff, precisely because no investment reliably returns 20% annually to offset it.

The Rule of 72 is one of those rare financial tools simple enough to use in your head, yet powerful enough to change behavior. Once you know your debt can double in 3.5 years, that balance feels a lot more urgent to pay down.