Compound vs. Simple Interest: What's the Difference?

Interest is the price of money — what you earn when you save and what you pay when you borrow. But there are two very different ways to calculate it, and the difference compounds (literally) into thousands over time.

Simple interest

Simple interest is charged only on the original principal. The formula is:

Interest = Principal × Rate × Time

Put $1,000 in at 5% for 3 years and you earn a flat $50 a year — $150 total. The balance never affects the calculation. You can check any figure with the simple interest calculator.

Simple interest shows up in some short-term loans, car financing, and certain bonds.

Compound interest

Compound interest is charged on the principal plus all the interest already added. Each period the balance is bigger, so the next interest payment is bigger too. Albert Einstein supposedly called it the eighth wonder of the world, and the math is why.

That same $1,000 at 5%, compounded annually for 3 years, grows to about $1,157.63 — and the gap widens every year after that. Try longer horizons in the compound interest calculator and watch the curve bend upward.

Why the difference matters

• When you save or invest, you want compounding on your side. Reinvested

dividends and interest are what turn small monthly contributions into a large balance over decades.

• When you borrow, compounding works against you. Credit-card debt compounds

monthly — paying only the minimum lets the balance snowball.

• Time is the multiplier. Over one year the two methods barely differ. Over

20 or 30 years, compounding pulls far ahead.

A quick rule of thumb

The Rule of 72: divide 72 by the annual rate to estimate the years it takes compound interest to double your money. At 6%, that's about 12 years. Simple interest would need over 16.

Put it to work

1. Estimate the interest on a short-term loan with the

simple interest calculator.

1. Project long-term growth — with monthly contributions — in the

compound interest calculator.

Understanding which one applies to a given account or loan is one of the highest-leverage things you can know about your money.