Compound Interest Calculator
See how your money grows over time with the power of compound interest.
Compound interest is the most powerful force in personal finance. This calculator shows how your money grows over time when interest earns interest. Enter your starting amount, monthly contribution, and expected return rate to see your future balance with a detailed year-by-year breakdown.
Whether you are investing in a 401(k), building a savings account, or planning for retirement, understanding compound interest helps you set realistic goals. Even small monthly contributions grow dramatically over decades thanks to compounding.
What Is a Compound Interest Calculator?
Compound interest means you earn interest on both your original deposit (principal) AND on previously earned interest. This creates exponential growth rather than linear growth. Albert Einstein reportedly called it "the eighth wonder of the world." The longer your money compounds, the faster it grows — which is why starting early matters more than investing large amounts later. Even a 5-year head start can mean tens of thousands more at retirement.
How It Works
After year 1, your $10,000 earns $700 interest. In year 2, you earn interest on $10,700 — not just $10,000. By year 30, a single $10,000 investment at 7% grows to $76,123 without adding another dollar. Add $200/month and it becomes $303,219. Use our investment calculator to model different contribution scenarios.
Formula Used
The compound interest formula is: A = P(1 + r/n)^(nt), where A = future value, P = principal, r = annual interest rate, n = compounding frequency per year, t = time in years. With monthly compounding (n=12): $10,000 at 7% for 20 years = $10,000 × (1 + 0.07/12)^(12×20) = $40,387.
Worked Example: $500/Month for 30 Years
Starting with $0 and investing $500/month at 7% annual return (S&P 500 historical average):
- After 10 years: $86,542 (you contributed $60,000)
- After 20 years: $260,464 (you contributed $120,000)
- After 30 years: $606,438 (you contributed $180,000 — interest earned $426,438)
That is the power of compound interest: your money earned more than double what you put in. Start with our savings calculator to plan your monthly contributions.
Compound Interest Growth Table ($10,000 Initial, 7% Return)
| Year | Balance | Interest Earned |
|---|---|---|
| 5 | $14,026 | $4,026 |
| 10 | $19,672 | $9,672 |
| 20 | $38,697 | $28,697 |
| 30 | $76,123 | $66,123 |
How to Maximize Compound Interest
Start early: A 25-year-old investing $300/month at 7% has $703,765 at 65. A 35-year-old investing the same has only $340,025 — starting 10 years earlier more than doubles the result.
Increase contributions over time: Add an extra $50/month each year. Reinvest dividends: Never withdraw earnings. Minimize fees: A 1% fee reduces your 30-year balance by 25%. Use tax-advantaged accounts: 401(k) and IRA accounts let your money compound without annual tax drag.
Related Guides
Frequently Asked Questions
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. It causes wealth to grow exponentially rather than linearly, which is why starting to invest early is so important.
More frequent compounding produces slightly higher returns. Most savings accounts compound daily, while investments typically compound annually. The difference is modest for most situations, but over long periods it can add up.
The S&P 500 has historically returned about 10% annually (7% after inflation). For conservative planning, many financial advisors recommend using 6-7% for stock investments and 3-4% for bonds or savings accounts.