Compound Interest Guide
What Is Compound Interest?
Learn what compound interest means, why it matters, and how it can affect long-term savings and investing.
In this guide
Compound interest in plain English Why compounding feels slow at first Where people use itCompound interest in plain English
Compound interest means you earn returns not only on your original money, but also on the interest or gains that have already been added. If simple interest is a straight line, compound interest is closer to a curve: the longer the money stays invested or saved, the more previous growth can help create new growth.
A simple example makes the idea easier to see. If you deposit $1,000 and earn 5% for one year, you have $1,050. In the next year, 5% is calculated on $1,050 rather than only on the original $1,000. That extra $50 is now part of the base that can grow.
Why compounding feels slow at first
In the early years, compound interest can look unimpressive because the balance is still small. The effect becomes easier to see after time has passed, especially when interest is reinvested and contributions continue.
This is why many long-term savings examples focus on ten, twenty, or thirty years. Time gives compounding more opportunities to repeat.
Where people use it
Compound interest is relevant to savings accounts, retirement accounts, education funds, investment portfolios, certificates of deposit, and even some types of debt. For savers and investors, it can help money grow. For borrowers, it can make debt more expensive if unpaid interest is added to the balance.
The calculator on this site focuses on the positive planning side: estimating how savings or investments may grow under a set of assumptions.