Simulate Your Financial Future
Adjust the sliders below to see your wealth compound in real-time. Fast, private, and 100% free.
Compound Interest
Simulate savings growth over time with monthly additions.
Compound Interest Calculator Guide
Use this guide to understand how compounding works, how to read the results, and how to plan realistic long-term savings goals.
How to use this calculator
Set your starting balance with Initial Deposit, then choose how much you can add each month with Monthly Contribution. Adjust the Annual Interest Rate to match a savings account, brokerage portfolio, or conservative investment assumption. Use Years to Grow to model short goals (3–5 years) or long-term wealth building (20–40 years). Finally, pick a compounding interval—monthly compounding usually grows faster than annual compounding at the same stated rate.
Worked example
Suppose you start with $10,000, contribute $500 each month, earn 8% annually, and compound monthly for 20 years. Your total contributions are $10,000 + ($500 × 240) = $130,000. Because interest is earned on both principal and prior interest, the projected ending balance is much higher than $130,000—often more than $350,000 depending on exact compounding assumptions. The chart shows when interest begins to overtake new contributions, which is the “snowball” phase of compounding.
How the math works
Future value with regular contributions can be modeled as: future principal growth plus the future value of an annuity. The common formula is A = P(1 + r/n)^(nt) for the starting balance, plus a contribution formula for recurring deposits. Here P is principal, r is annual rate, n is compounds per year, and t is years. Small changes in rate or time can create large differences over decades, which is why long horizons matter more than perfect short-term timing.
Practical planning tips
Use conservative rates if your money is in cash or bonds, and higher historical averages only if you accept market volatility. Increase monthly contributions before stretching for aggressive return assumptions. Re-run the simulation after raises, windfalls, or expense cuts so your plan stays current. Remember that taxes, fees, and inflation can reduce real purchasing power even when the nominal balance looks strong.
What is compound interest?
Compound interest is interest earned on both your original principal and previously earned interest. Over time this creates exponential growth compared with simple interest, which only applies to the starting amount.
Does compounding frequency matter?
Yes. Monthly compounding credits growth more often than annual compounding, so the same nominal rate usually produces a higher effective yield when compounds more frequently.
Should I prioritize rate or contributions?
Early on, higher contributions often matter more because they increase the base that can compound. Later, rate differences become more visible as the balance grows large.
Is this financial advice?
No. FinanceSliders provides educational simulations only. Actual investment returns vary, and you should consult a qualified advisor before making decisions.
Why does time matter so much?
Each extra year gives interest more periods to grow on itself. Starting earlier with smaller amounts can outperform starting later with larger deposits.
What should I export the PDF for?
PDF reports are useful for personal planning, comparing scenarios, or discussing assumptions with a partner or advisor without sharing account logins.
Disclaimer: Results are estimates based on constant rates and uninterrupted contributions. Markets, fees, taxes, and inflation can change outcomes. This tool does not recommend specific products or strategies.