compound interest


Definition of compound interest 

: interest computed on the sum of an original principal and accrued interest

Examples of compound interest in a Sentence

Recent Examples on the Web

This is the one time when compounding interest can actually work against you. Nathan Bachrach,, "P&G’s new 401(k) offering a great option for employees," 20 June 2018 Consider the power of compound interest via the 1626 purchase of Manhattan Island by Peter Minuit, director-general of New Netherland from the Man-a-hat tribe for an apparently ridiculously low sum of $24 in trinkets. WSJ, "Another Look at Foreign Investments in U.S.," 16 May 2018 That’s the power of time. More specifically, that’s the power of time and compound interest. Peter Dunn, USA TODAY, "College graduates: Here's how to set yourself up for a life of financial success on Day 1," 20 Apr. 2018 Source: Social Security Administration WSJ: What about the power of compound interest? Lisa Ward, WSJ, "Saving More for Retirement vs. Work Longer," 25 Mar. 2018 Joseph Hargadon, the department head for accounting and information management at Widener, will teach about how to make financial decisions, including reducing debt and the benefits of compound interest. Erin Arvedlund,, "Want to bet against U.S. Treasurys? These ETFs make the trade easy for investors," 22 Jan. 2018 Over time, the balance of the loan increases as a result of compounding interest and MIP, and fees, the CFPB wrote. Robert Powell, USA TODAY, "Should you use a reverse mortgage to delay taking Social Security?," 22 Sep. 2017 It has been estimated that the total cost to taxpayers will be $2.4 billion, due to compounding interest on bonds to build the facility. Usa Today Sports, USA TODAY, "MLB owners approve sale of Marlins to Derek Jeter-led group," 27 Sep. 2017 The issue with this approach is that the investor loses out on much of the benefit of nearly two decades of compounding interest. Ryan O’donnell, WSJ, "Why You May Not Want to Save for College," 10 Sep. 2017

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'compound interest.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

See More

First Known Use of compound interest

1660, in the meaning defined above

Keep scrolling for more

Learn More about compound interest

Share compound interest

Statistics for compound interest

Look-up Popularity

Time Traveler for compound interest

The first known use of compound interest was in 1660

See more words from the same year

Keep scrolling for more

More Definitions for compound interest

compound interest


Financial Definition of compound interest

What It Is

Compound interest is interest earned on principal plus interest that was earned earlier.

How It Works

Let's assume you have $100 to open a savings account at XYZ Bank on January 1. The annual interest rate is 5%. How much will you have in five years?

Well, if the bank simply gave you 5% of your $100 at the end of the year, you would have $105 on December 31. If you left the $105 in the account to earn another 5% next year, at the end of that second year, you would have: $105 x 1.05 = $110.25. Not only did you earn interest on your original $100 in year two, you earned interest on year one's interest. That is, it compounded. If you carried this out another eight years, here's what your account might look like:

At the end of 10 years, you would have $162.89 in the account just for doing nothing. But it gets better. The example above assumes the bank pays interest on the balance at the end of the year (that is, the interest compounded annually). In the real world, a bank would usually pay you interest on your account balance at the end of every month (that is, the interest compounds monthly). The bank simply divides the annual interest rate (5% in our case) by 12 months, and applies that rate to your balance at the end of each month. So in year one, let's see what happens:

Notice that when the bank compounded the balance annually, you only had $105 at the end of year one. But if the bank compounds monthly, you have $105.12 at the end of year one. It may not sound like much, but consider the effect on a $500,000 beginning balance: At the end of 10 years, the investor has $814,447.13 if the interest compounds annually, but she has $823,504.75 -- a full $9,057.62 more -- if the interest compounds monthly.

It is important to note that compounding doesn't just affect how much interest investors earn; it affects how much interest investors pay. For example, if that $1,000 savings account had really been a $1,000 loan to you from XYZ Bank, the amount of interest you pay would be influenced by how often the bank compounded the rate. The important lesson here is that the more frequently compounding occurs, the more interest is earned (or paid) on a balance. Some credit cards even compound interest daily, which greatly affects the borrower's balance owed.

Why It Matters

The financial world often refers to compound interest as "magic" because it is one of the most fundamental ways to build wealth yet takes the least amount of effort. But because of the variety of interest calculation methods out there, borrowers should compare lender offers, and investors should compare investment offers by carefully reading the disclosure accompanying those offers. Investors are well served to understand how an institution’s choice of interest-calculation methods affect the amount of interest applied to the investor’s account. Depending on the anticipated activity, the borrower might save money and the investor might make more money by preferring one calculation method to another.

This InvestingAnswers story features a real-life example of how compounding can work for you: How One Normal Lady Turned $200 into $7 Million.

Source: Investing Answers

compound interest


English Language Learners Definition of compound interest

finance : interest paid both on the original amount of money and on the interest it has already earned

compound interest

Legal Definition of compound interest 

see interest sense 5

More from Merriam-Webster on compound interest

See words that rhyme with compound interest

Comments on compound interest

What made you want to look up compound interest? Please tell us where you read or heard it (including the quote, if possible).


something that serves to warn or remind

Get Word of the Day daily email!

Test Your Vocabulary

Autumn Words of the Day 2018

  • a-top-down-image-of-road-through-an-autumn-forest
  • Which is a synonym of fugacious?
Name That Thing

Test your visual vocabulary with our 10-question challenge!

Bee Cubed

Test Your Knowledge - and learn some interesting things along the way.


Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!