Compound Interest Calculator
See the power of compounding. Enter a starting amount, an interest rate, a time period and an optional monthly contribution to find out exactly how much your money will grow.
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Save ⟶How to use the Compound Interest Calculator
- 1Enter initial amount in the form on the left.
- 2Fill in the remaining fields — the result updates automatically as you type.
- 3Review the highlighted result and the supporting breakdown on the right.
- 4Use Copy, Share or Print to save or send your result.
How compound interest works
Compound interest means you earn interest on your interest. The formula for a lump sum is A = P(1 + r/n)^(nt), where P is the principal, r the annual rate, n the number of compounding periods per year and t the time in years. The more often interest compounds, the faster the balance grows.
Regular contributions supercharge the effect: each deposit starts its own compounding clock. Starting ten years earlier often beats contributing twice as much later — time in the market is the most powerful variable in the formula.
The rule of 72
A quick mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes money to double. At 8% a sum doubles roughly every 9 years; at 6%, every 12 years.
Use the calculator to test realistic scenarios — for long-term stock market investing many planners model 6–8% annual returns after inflation, while savings accounts typically earn far less.
Frequently Asked Questions
▸What is the compound interest formula?
A = P(1 + r/n)^(nt) for a lump sum, where P is principal, r is the annual rate as a decimal, n is compounds per year and t is years. Contributions are added with the future-value-of-annuity formula.
▸Does compounding frequency matter much?
It helps, but with diminishing returns. Moving from yearly to monthly compounding makes a noticeable difference; moving from monthly to daily adds very little.
▸Is compound interest good or bad?
Both — it grows investments and savings in your favor, but it also makes debt like credit cards grow against you. The same math that builds wealth can deepen debt.