Compound Interest Calculator
Watch compound growth do the heavy lifting on your savings, 401(k) or brokerage account.
| Total you contributed | |
| Interest / growth earned |
Illustration only — not a guarantee of future returns. Investments can lose value.
What is this tool?
Compound interest is the reason starting early beats saving more later. Your money earns returns, and then those returns earn returns of their own, so growth accelerates the longer you leave it alone. This compound interest calculator lets you model a starting balance plus steady monthly contributions — exactly how most Americans build wealth through a 401(k), IRA or high-yield savings account.
Small differences matter enormously. An extra one percent of annual return, or an extra $50 a month, can mean tens of thousands of dollars after 20 or 30 years. Use the calculator to test contribution increases you could actually afford, and remember that these are illustrations, not predictions — real market returns vary year to year.
How to use it
- Enter your starting amount (current savings or investment balance).
- Add a monthly contribution if you plan to keep adding money.
- Enter the expected annual return and the number of years.
- Choose how often interest compounds, then click Calculate.
Formula & method
Worked example
Invest $10,000 at a 5 percent annual return for 10 years, compounded annually, and it grows to $16,288.95 — a gain of $6,288.95 without adding a cent. Add $200 a month for the same 10 years at the same return and the balance climbs to roughly $47,400 instead.
Frequently asked questions
What does compounding frequency change?
Interest can compound yearly, quarterly, monthly or daily. The more often it compounds, the slightly faster your money grows, because earned interest starts earning its own interest sooner.
Is this how a 401(k) or IRA grows?
Broadly, yes. Retirement accounts grow through market returns and your contributions rather than a guaranteed rate, but the compound growth model is the right way to estimate long-term results.
How much difference does starting early make?
$200 a month from age 25 to 65 at a 7 percent return grows to about $525,000. Starting at 35 instead — just 10 years later — reaches only about $244,000, less than half.