How Compound Interest Grows Debt

Compound interest is either your best friend or your worst enemy. In savings and investments, it builds wealth. In credit card debt, it's the mechanism that can double or triple the cost of every purchase you make.

Simple vs. Compound Interest

Simple interest is calculated only on the principal. If you borrowed $10,000 at 24% simple interest for one year, you'd owe $2,400 in interest. Compound interest is calculated on the principal plus accumulated interest. Credit cards compound daily, which means interest accrues on yesterday's interest. Over a year, daily compounding at 24% results in an effective annual rate of about 27%.

The Rule of 72

The Rule of 72 is a quick way to estimate how long it takes for debt to double at a given interest rate: divide 72 by the rate. At 24% APR, your debt would double in just 3 years if you made no payments. At a HELOC rate of 8%, doubling takes 9 years. This simple math illustrates why high-interest debt is so dangerous.

Real Dollar Impact

On a $20,000 credit card balance at 24% APR, making only minimum payments, you'll pay approximately $39,000 in interest over the life of the debt — nearly double the original balance. The same $20,000 on a HELOC at 8.5% with the same monthly payment would cost about $5,800 in interest. The compound interest difference: $33,200.

Breaking the Compound Interest Cycle

Every extra dollar you pay above the minimum directly reduces principal, which reduces the base on which interest compounds. Even $50 extra per month on a $10,000 balance at 24% saves $5,300 in interest and cuts the payoff time from 27 years to about 6 years. Consolidation amplifies this effect by reducing the rate at which interest compounds.