Compound Interest Explained (Step by Step, with Examples)
Albert Einstein supposedly called compound interest the eighth wonder of the world. Whether or not he did, the maths is genuinely worth understanding.
By ToolJolt Team · May 28, 2026
Simple vs compound
Simple interest is earned only on your original amount. Compound interest is earned on your original amount plus all the interest already added — so you earn interest on your interest. Over short periods the difference is small; over decades it is enormous.
The formula
A = P × (1 + r/n)^(n×t), where P is the principal, r is the annual rate (as a decimal), n is how many times per year it compounds, and t is the number of years. The amount of interest is A − P.
Frequency matters (a little)
More frequent compounding — monthly vs yearly — increases your return, but less than people expect. Going from annual to monthly on a typical rate adds a fraction of a percent. The headline rate and the time invested matter far more than the frequency.
Time is the real lever
Because growth compounds, the early years you leave money invested are worth the most. Investing a fixed amount ten years earlier can easily double the final result versus starting later, even with identical contributions. Starting early beats trying to catch up.
See it for your numbers
ToolJolt's compound interest calculator lets you plug in a rate, term and compounding frequency and watch the total grow. It is the fastest way to feel why time in the market matters.
Free tools mentioned in this guide
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original amount; compound interest is calculated on the original amount plus accumulated interest, so it grows faster over time.
Does daily compounding make a big difference?
Slightly more than monthly or yearly, but the effect is small compared with the interest rate and the length of time invested.
How can I make compounding work for me?
Start early, leave the money invested, and reinvest the returns. Time is the most powerful factor in the formula.