Compound Interest Explained (With Examples)

Published October 11, 2026

Free tool: Compound Interest Calculator→

Compound interest is interest earned on interest. Each period, the interest you've earned is added to your balance, and the next period's interest is calculated on that larger amount. Over a few years the effect is small; over decades it does most of the work. This guide explains how it works, with worked examples you can reproduce in the free Compound Interest Calculator.

Simple interest vs compound interest

With simple interest, you earn interest only on the money you put in. $10,000 at 5% earns $500 every year, so after 20 years you have $20,000.

With compound interest, each year's interest is added to the balance and earns interest itself. Here are the first three years for $10,000 at 5%, compounded once a year:

The extra $25 in year 2 is interest on the first year's interest. That difference grows every year: after 20 years, the balance is $26,532.98, about $6,500 more than simple interest would give.

The compound interest formula

For a single deposit, the balance after t years is:

A = P × (1 + r ÷ n)^(n × t)

where:

• A is the final amount
• P is the starting amount (the principal)
• r is the annual interest rate as a decimal (5% = 0.05)
• n is how many times a year the interest is compounded (12 for monthly)
• t is the number of years

For $10,000 at 5% compounded monthly for 20 years: A = 10,000 × (1 + 0.05 ÷ 12)^240 = $27,126.40.

Does the compounding frequency matter?

The more often interest is compounded, the faster the balance grows, but the effect is smaller than most people expect. Here is $10,000 at 5% after 20 years:

Going from simple to compound interest adds over $6,500, but going from monthly to daily compounding adds only about $55. The interest rate and the time matter far more. That's why banks quote savings accounts by their APY (annual percentage yield), which already includes compounding, so you can compare accounts directly; the APY Calculator converts between rates and APY.

Adding regular contributions

Most people don't invest a single lump sum; they add money every month. Each deposit then starts compounding from the day it goes in. For monthly deposits of PMT, the extra amount at the end is:

PMT × ((1 + i)^N − 1) ÷ i

where i is the monthly rate (annual rate ÷ 12) and N is the number of monthly deposits.

Starting with $10,000 and adding $500 a month at 5% for 20 years:

• You put in $10,000 + $500 × 240 = $130,000
• The account grows to $232,643
• So $102,643 of the final balance is interest

Time is the biggest factor

Because growth builds on itself, the early years count the most. Compare two savers who each invest until age 65 and earn 7% a year:

Starting ten years earlier more than doubles the result. Even saving 50% more each month doesn't let the later starter catch up.

A handy shortcut is the Rule of 72: divide 72 by the annual rate to estimate how many years it takes to double your money. At 6%, that's about 12 years. The Rule of 72 Calculator shows how close the shortcut is to the exact answer.

How to use the Compound Interest Calculator

1. Open the Compound Interest Calculator.
2. Enter your starting amount in Initial Investment (it can be 0).
3. Enter what you'll add each month in Monthly Contribution.
4. Enter the annual interest rate and the number of years.
5. Choose the compounding frequency, from annually to daily. It applies to your starting amount; monthly contributions are added at the end of each month and earn the monthly rate.
6. Read the results: the Future Value, the Total Contributed and the Total Returns (the interest earned). The charts and the yearly breakdown show how the growth speeds up over time.

Try changing one input at a time, such as the number of years, to see which one makes the biggest difference for you.

When compound interest works against you

Compounding works the same way on debt. Credit card interest is typically charged monthly on a balance that includes past interest, so an unpaid balance can grow quickly. At 24% a year, a balance you don't pay down roughly doubles in three years. The Credit Card Payoff Calculator shows how long a balance takes to clear and how much interest it costs.

Tips to make compounding work for you

• Start as early as you can, even with small amounts.
• Contribute regularly. Automatic monthly transfers make it effortless.
• Watch the fees. Fees compound too: $10,000 growing at 7% a year for 30 years becomes $76,123, but at 6% (7% minus a 1% annual fee) it becomes $57,435, about a quarter less.
• Leave the money alone. Withdrawals stop the growth on that money for good.
• Think in real terms. Inflation reduces what the final amount can buy; the Inflation Calculator shows by how much.

Frequently asked questions

What's the difference between simple and compound interest?

Simple interest is paid only on the original amount; compound interest is also paid on past interest. Over long periods, compound interest grows much faster. The Simple Interest Calculator shows the simple version.

How often is interest compounded?

It depends on the account. Savings accounts often compound daily or monthly, some bonds pay interest twice a year, and many loans charge interest monthly. Compare accounts by their APY, which includes the compounding.

What interest rate should I use?

For savings accounts and CDs, use the rate the bank quotes. Investment returns vary from year to year, so for investments, use a cautious long-run assumption and treat the result as an estimate, not a promise.