Why use this calculator
Compound interest is the most powerful force in personal finance. This calculator shows you exactly how your money can grow exponentially over time when interest earns interest. Understanding compound interest helps you make better decisions about savings, investments, and the true cost of debt.
How to use it
Enter your initial investment, expected annual return rate, time horizon, compounding frequency, and any regular monthly contributions. The calculator shows your projected future value and breaks down how much comes from contributions vs. earned interest.
The formula
A = P(1 + r/n)^(nt) + PMT × [(1 + r/n)^(nt) - 1] / (r/n), where A is the future value, P is principal, r is annual rate, n is compounds per year, t is years, and PMT is the monthly contribution.
Worked examples
$10,000 at 7% compounded monthly for 30 years with $200/month contributions
Future value of $317,884 — over $245,000 in interest earned
$5,000 at 10% compounded annually for 20 years, no contributions
Grows to $33,637 — your money triples through compound interest alone
When people use it
- Planning long-term investment growth
- Comparing savings account returns
- Understanding retirement account projections
- Teaching financial literacy concepts
Tips
- Start investing early — time is the biggest factor in compound interest
- Even small monthly contributions add up significantly over decades
- More frequent compounding (daily vs annually) yields slightly more
- The Rule of 72: divide 72 by your interest rate to estimate doubling time
Questions people ask
- What is the difference between simple and compound interest?
- Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest, creating exponential growth over time.
- How often should interest compound?
- More frequent compounding yields slightly higher returns. Monthly or daily compounding is common for savings accounts. The difference between monthly and daily is minimal, but the difference between annual and monthly can be meaningful.
- Is 7% a realistic return rate?
- The S&P 500 has historically returned about 10% annually before inflation, or roughly 7% after inflation. For savings accounts, rates vary widely. Always use conservative estimates for planning.