Compound Interest Calculator

How to use the Compound Interest Calculator

1. Enter the principal amount, annual interest rate and years.
2. Add a monthly contribution if you invest regularly.
3. Read the future value, total contributions and total interest.

Compound Interest Calculator example

Invest 100,000 at 10% per year for 10 years and it grows to about 259,374 with annual compounding — 159,374 of it is interest earned on interest.

Who uses the Compound Interest Calculator?

Savers, investors and students learning how savings grow over time.

About Compound Interest Calculator

See how your savings grow with compound interest. Add monthly contributions and get future value and total interest instantly. Free online, no sign-up needed. Everything runs in your browser, so your data stays private and results appear instantly. No signup and no payment — free to use.

What Is Compound Interest?

Compound interest is interest earned on interest. In the first year, you earn interest only on your starting amount. In the second year, you earn interest on the starting amount plus the first year's interest. In the third year, the base is bigger again. Year after year, the growth compounds — it builds on itself. That is why time matters more than the rate: money left to compound for 20 years grows far more than the same money left for 10 years, even at the same rate.

The formula is: future value = P × (1 + r/n)^(n×t), where P is the principal (your starting amount), r is the annual interest rate as a decimal, n is how many times interest is compounded per year, and t is the number of years. You do not need to solve this by hand — the calculator does it — but knowing the formula helps you understand why the result grows the way it does. The key point is the exponent: time sits in the exponent, so every extra year multiplies the result rather than just adding to it.

How to Use the Calculator (Step by Step)

Step 1: Enter the principal amount. This is the money you start with — the lump sum you invest or save today.

Step 2: Enter the annual interest rate. Type the rate as a percentage, for example 10 for 10% per year. If you are comparing two options, run the calculation once for each rate.

Step 3: Enter the number of years. Longer periods show the compounding effect much more clearly — try 5, 10, and 20 years with the same amount and rate to see the difference.

Step 4: Add a monthly contribution if you invest regularly. This is the amount you add every month, on top of the starting amount. Leave it at zero if you are only investing once.

Step 5: Read the future value, total contributions, and total interest. Future value is what you end up with. Total contributions are what you put in. Total interest is the difference — the money your money earned.

Compound Interest vs Simple Interest

Simple interest is calculated only on the original amount, every year, forever. If you invest 100,000 at 10% simple interest for 10 years, you earn 10,000 a year, every year — 100,000 of interest in total, and a final value of 200,000. Compound interest at the same rate gives you about 259,374 after 10 years, because each year's interest also earns interest in the years after. The gap between the two grows every year: after 20 years the difference is far larger than after 10. That gap is the whole reason compound interest is called the most powerful idea in saving — the longer you leave money alone, the harder it works.

Who Uses This Tool?

Savers use it to plan for a goal — a house deposit, a child's education, or retirement — and to see how much a monthly contribution adds up to. Investors use it to compare two rates or two time periods before choosing where to put money. Students use it to learn the formula and to see compounding in action with real numbers. Teachers use it in finance and maths lessons, because the worked example makes the idea click faster than the formula alone. Anyone who has ever wondered "what if I save a little every month?" will get a clear answer here.

Benefits of This Calculator

The first benefit is the monthly-contribution option, which matches how real people save — a little at a time, not once. The second is the breakdown: you see the future value, what you contributed, and what the interest contributed, so you know how much of the final amount is your money and how much is growth. The third is speed: changing the rate or the years updates the answer instantly, so comparing options takes seconds. And it is free, with no sign-up, so you can test as many scenarios as you like. One honest warning: the result is an estimate, not financial advice. Real investments have fees, taxes, and rates that change, so check those before making a decision.

Worked Example

Invest 100,000 at 10% per year for 10 years, with annual compounding and no monthly contributions. The future value is 100,000 × (1.10)^10 = about 259,374. Of that, 100,000 is your original money and about 159,374 is interest — more than you put in. Now add a monthly contribution of 5,000: over 10 years you contribute another 600,000, and the future value rises well above one million, because every monthly payment also earns compound interest for the years that remain. Try it in the calculator: change the years from 10 to 20 and watch the future value more than double, even though the time only doubled. That is compounding at work.

Related guide

Learn more in our guide: Compound Interest Explained: How Your Money Grows — Compound interest is the reason small, regular savings can grow into a large sum. Learn the formula, see a worked example, and what makes the biggest difference.