Simple vs Compound Interest on Debt: Why Costs Snowball

Most people learn “compound interest” as a wealth-building concept. But the same mechanism can work against you when borrowing. The difference between simple and compound interest is one reason revolving debt becomes expensive quickly.

Simple interest in borrowing

Simple interest is calculated only on the original principal. Many basic loans approximate simple interest when payments reduce principal steadily and interest is charged on the remaining balance each period.

Compound interest in revolving debt

With revolving products (like credit cards), unpaid interest can effectively compound because balances roll forward and interest is computed frequently (often daily). If you pay only the minimum, the balance can decline very slowly.

Why minimum payments keep you stuck

• Interest consumes much of the payment early on.
• The remaining principal declines slowly, so interest stays high.
• Fees can add additional drag.

Model interest costs

To understand how fast interest snowballs, model the balance growth with the Daily Compound Calculator using the APR as a starting point and daily frequency. Then compare against a simple-interest growth assumption in the Simple Interest Calculator to see why compounding changes outcomes.

Takeaway

Compound interest is powerful—positively for investing and negatively for debt. The faster the compounding and the longer the balance persists, the more you pay. Pay more than the minimum whenever possible.