When it comes to growing your savings — or paying off debt — the type of interest you're dealing with makes an enormous difference. Compound interest and simple interest follow different rules, and understanding those rules can save you thousands of dollars over time.
The short version: Simple interest is calculated only on your original amount. Compound interest is calculated on your original amount plus all the interest you've already earned. Over time, this difference becomes massive.
Simple interest is exactly what it sounds like — straightforward and predictable. You earn a fixed percentage of your original deposit every single period, and that's it. The interest never earns interest of its own.
Simple interest is common in short-term personal loans, car loans, and some bonds. It's easy to calculate and easy to understand — but it grows slowly compared to compound interest.
With compound interest, the interest you earn each period gets added to your balance — and then the next period's interest is calculated on that larger balance. Your money effectively earns money on top of money.
This creates exponential growth. Slowly at first, then faster and faster as the balance grows. The longer the time period, the more dramatic the difference becomes.
After 5 years the difference is $244. Not dramatic yet — but extend that to 30 years and the gap becomes tens of thousands of dollars.
Calculated on original principal only. Growth is linear. Common in car loans, short-term personal loans. Easy to predict.
Calculated on principal plus accumulated interest. Growth is exponential. Common in savings accounts, investments, mortgages, credit cards.
Compound interest isn't always your friend. When you carry a credit card balance, the interest compounds against you — usually daily. A $5,000 balance at 20% APR, with only minimum payments, can take over 15 years to pay off and cost more than $7,000 in interest alone.
The same force that builds wealth for investors actively destroys it for borrowers who don't pay attention.
As a saver or investor, always look for compound interest — and the highest compounding frequency available. As a borrower, simple interest loans are generally more favorable since your debt won't snowball the same way.
See exactly how compound interest grows your money with our free calculator.
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