How is compound interest calculated?

The standard formula is A = P × (1 + r/n)^(n·t), where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the term in years. This calculator uses exactly that formula for the principal and adds monthly contributions at the end of each month at the equivalent monthly rate.

What does compounding frequency mean?

It's how often earned interest is added to the balance so it starts earning interest itself. Daily compounding adds interest 365 times a year, monthly 12 times, yearly once. More frequent compounding gives a slightly higher final amount at the same nominal rate.

What's the difference between simple and compound interest?

Simple interest is paid only on the original principal, so growth is a straight line. Compound interest is also paid on previously earned interest, so growth curves upward — the longer the term, the bigger the gap between the two.

Are the monthly contributions included in the interest calculation?

Yes. Each contribution is added at the end of the month and starts compounding from that point on, at the monthly rate implied by your chosen compounding frequency. The Deposits column in the table shows what you paid in each year, and the Interest column shows what the money earned.

Can I use this for savings accounts, CDs, or investments?

Yes — any product with a fixed rate and regular compounding: savings accounts, certificates of deposit, bonds held to maturity. For stocks or funds, use your expected average annual return as the rate; the result is an estimate since real returns vary year to year.

Is my financial data private?

Completely. The calculator runs entirely in your browser with JavaScript. The amounts, rate, and term you enter are never sent to a server, logged, or stored anywhere.

Compound Interest Calculator — Growth & Final Amount

Starting amount

Interest rate

% / yr

Term

years

Compounding

Monthly contribution

Final amount

—

Total deposits

—

Total interest earned

—

Year-by-year growth

Enter a starting amount, interest rate, and term to see how your money compounds over time. Choose the compounding frequency, add an optional monthly contribution, and read the final amount, total interest earned, and a year-by-year growth table. Everything is calculated in your browser — nothing you type is sent anywhere.

How it works

1
Enter the starting amount and rate Type your initial deposit (principal) and the annual interest rate as a percentage. You can start from zero if you plan to save monthly.
2
Set the term and compounding frequency Enter the number of years and pick how often interest compounds — yearly, quarterly, monthly, or daily. Add a monthly contribution if you'll keep depositing.
3
Read the results and growth table The final amount, total deposits, and interest earned update as you type. Scroll the year-by-year table to watch the balance grow and see how interest accelerates over time.

Your data stays private

All processing happens entirely in your browser. No files, text, or data are ever sent to our servers. You can disconnect from the internet and this tool will still work.

Frequently asked questions

How is compound interest calculated?: The standard formula is A = P × (1 + r/n)^(n·t), where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the term in years. This calculator uses exactly that formula for the principal and adds monthly contributions at the end of each month at the equivalent monthly rate.
What does compounding frequency mean?: It's how often earned interest is added to the balance so it starts earning interest itself. Daily compounding adds interest 365 times a year, monthly 12 times, yearly once. More frequent compounding gives a slightly higher final amount at the same nominal rate.
What's the difference between simple and compound interest?: Simple interest is paid only on the original principal, so growth is a straight line. Compound interest is also paid on previously earned interest, so growth curves upward — the longer the term, the bigger the gap between the two.
Are the monthly contributions included in the interest calculation?: Yes. Each contribution is added at the end of the month and starts compounding from that point on, at the monthly rate implied by your chosen compounding frequency. The Deposits column in the table shows what you paid in each year, and the Interest column shows what the money earned.
Can I use this for savings accounts, CDs, or investments?: Yes — any product with a fixed rate and regular compounding: savings accounts, certificates of deposit, bonds held to maturity. For stocks or funds, use your expected average annual return as the rate; the result is an estimate since real returns vary year to year.
Is my financial data private?: Completely. The calculator runs entirely in your browser with JavaScript. The amounts, rate, and term you enter are never sent to a server, logged, or stored anywhere.

From the blog

How Compound Interest Works: Formula, Examples, and the Rule of 72 What compounding frequency really changes, why starting early beats saving more, and how to check the math yourself. Read the post →