Compound Interest Calculator
Calculate how your money grows with compound interest. See future value with monthly contributions, different compounding frequencies, and year-by-year growth.
How This Calculator Works
Calculation methodology and assumptions
Compound interest is calculated using the formula FV = P(1+r/n)^(nt) + PMT×[((1+r/n)^(nt)-1)/(r/n)], where P is principal, r is annual rate, n is compounding periods per year, t is time, and PMT is periodic contribution. The calculator defaults to monthly compounding (n=12), which is how most investment accounts compound.
How to Use This Investment Calculator
- 1
Enter your initial investment
Input the lump sum you plan to invest today. This is your starting principal that will begin compounding immediately.
- 2
Set your monthly contribution
Enter the amount you plan to add each month. Consistent contributions accelerate growth through dollar-cost averaging.
- 3
Input expected return and time horizon
Set your expected annual return (7–10% for stocks historically, 4–6% for bonds) and investment period. Longer time horizons amplify compounding effects dramatically.
- 4
Review the growth projection
The results show your total invested amount, earnings from compound growth, and a year-by-year projection table showing how your money grows over time.
Example Calculation
How does compound interest build wealth over time?
Starting with $10,000 and adding $500/month at an 8% average annual return for 30 years: Your total contributions would be $190,000 ($10K initial + $180K in monthly deposits). But with compound growth, your portfolio would grow to approximately $745,000.
Result: Compound interest generated $555,000 in earnings on top of your $190,000 in contributions — nearly 75% of the final value came from returns, not deposits. Starting 5 years later would reduce the final amount by roughly $230,000. Time in the market is the most powerful factor in wealth building.
What Affects Your Results
Rate of Return
Even small differences compound massively over time. 7% vs. 8% over 30 years on $100K means a difference of $200K+. Asset allocation drives your expected return.
Time Horizon
Compounding accelerates exponentially. Most of your wealth is generated in the final years — a 30-year investment earns more in its last 5 years than its first 15.
Contribution Consistency
Regular monthly investments (dollar-cost averaging) smooth out market volatility and ensure you're always buying — including during dips when prices are low.
Fees & Expenses
A 1% annual fee vs. 0.1% fee on a $500K portfolio costs you $4,500/year extra. Over 30 years, high fees can consume 25–30% of potential returns. Use low-cost index funds.
Tips for Compound Interest Residents
- Start early. Thanks to compounding, $200/month invested from age 25 to 65 at 8% returns grows to ~$700K. Waiting until 35 cuts that to ~$300K — a $400K penalty for the 10-year delay.
- Don't try to time the market. Research consistently shows that time in the market beats timing the market. Missing the 10 best trading days over 20 years can halve your returns.
- Consider tax-advantaged accounts first: 401(k) (especially with employer match), IRA, HSA. These reduce your tax drag — a 25% tax bracket investor keeps more in a tax-deferred account.
- Rebalance annually. If stocks outperform and grow from 80% to 90% of your portfolio, rebalancing back to 80% locks in gains and manages risk.
- Factor in Compound Interest's tax treatment of investment income. Some states exempt certain investment income or have lower rates on capital gains.
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StateCalc Team
Editorial Team
The StateCalc team builds free financial calculators using data from official government sources including the IRS, U.S. Census Bureau, BLS, and state revenue departments. All formulas are validated by an automated test suite and cross-referenced against published data.
Our editorial standardsFrequently Asked Questions
What is compound interest?
Compound interest is interest earned on both the initial principal and previously accumulated interest. Unlike simple interest (earned only on principal), compounding creates exponential growth — often called "interest on interest."
What is the Rule of 72?
The Rule of 72 estimates how long it takes money to double: divide 72 by your annual return rate. At 7% returns, money doubles in approximately 72÷7 = 10.3 years.
What is a good annual return rate assumption?
The S&P 500 has historically returned about 10% annually (7% after inflation). For conservative planning, 6-7% after inflation is a reasonable assumption for a diversified stock portfolio.
How much should I invest monthly?
A common guideline is to invest 15-20% of your gross income for retirement. The exact amount depends on your goals, timeline, and current savings.