Simple vs Compound Interest: A Plain-English Guide

Interest is the price of using money. Whether you are saving or borrowing, the way it is calculated changes the outcome far more than most people expect.

Simple interest

Simple interest is calculated only on the original amount. If you put 1,000 in an account paying 5 percent simple interest, you earn 50 every year, no matter how long you wait. After 10 years you have 1,500.

Compound interest

With compound interest, you earn interest on the interest. The same 1,000 at 5 percent compounded yearly becomes about 1,629 after 10 years. The difference looks small early on and becomes large over decades, which is why starting early matters so much.

What affects growth

• Rate: a higher rate grows faster, but also usually carries more risk.
• Time: the longer the money stays invested, the more compounding helps.
• Compounding frequency: monthly compounding grows slightly faster than yearly at the same rate.
• Regular contributions: adding a fixed amount every month often matters more than the starting amount.

Try your own numbers

Use the Simple & Compound Interest calculator to compare both methods. For longer-term planning, the Investment Return Calculator shows growth with regular deposits, and the CAGR Calculator turns a start and end value into an average yearly growth rate so you can compare investments fairly.

The same maths works against borrowers

Credit-card balances and many loans compound too. A debt that is only paid at the minimum can grow for years. The Savings Goal Calculator works the other way round: it tells you how much to save each month to reach a target by a date.

Inflation matters

A return of 5 percent when prices rise 4 percent is a real gain of only about 1 percent. Use the Inflation Calculator to see what a future amount is worth in today's money.

Calculators give estimates based on the numbers you enter. For decisions about loans, taxes or investments, confirm the figures with your lender or a qualified adviser.

Frequently asked questions

How can I estimate how long money takes to double?

The rule of 72 gives a quick estimate: divide 72 by the annual percentage rate. At 6 percent, money roughly doubles in 12 years.

Does more frequent compounding matter much?

It helps a little. Going from yearly to monthly compounding at the same rate adds a small amount; the rate and the time you leave the money invested matter much more.

Is a higher advertised return always better?

Not if it comes with higher risk, fees or lock-in periods. Compare after fees and consider whether you could lose capital.

Do these calculators include tax?

Generally not. Taxes on interest or gains vary by country and account type, so reduce your expected return accordingly.

Key takeaways

• Compounding rewards time more than anything else.
• Regular contributions are powerful.
• Compare returns after inflation, fees and tax.