Compound Interest Calculator
Calculate how your investments grow over time
About the Calculator
Compounding is slow at first and then surprising. This calculator shows how time, rate, and contributions interact so you can plan for goals like retirement or a future purchase. Enter a starting balance and a monthly amount, then test different rates and compounding schedules. You will see how much growth comes from contributions versus interest, which helps you stay consistent. Use it when deciding how much to invest, or when you want motivation to keep going. Small increases today can create a meaningful gap in ten or twenty years. It is also useful for comparing savings accounts, CDs, or long term investment assumptions.
Future Value
$37,405.09
Interest Earned: $15,405.09
Growth Over Time
The Formula
How to Calculate Manually
- 1
Identify your principal amount (P) - the initial investment.
- 2
Determine the annual interest rate (r) and convert it to decimal form (divide by 100).
- 3
Choose your compounding frequency (n) - monthly = 12, quarterly = 4, annually = 1.
- 4
Decide on the time period (t) in years.
- 5
If making regular contributions (PMT), add them to the formula.
- 6
Plug all values into the formula and calculate.
Examples
What will $10,000 grow to in 10 years at 7% interest, compounded monthly?
$10,000 × (1 + 0.07/12)^(12×10) = $20,096.61
How much do I need to invest monthly to have $100,000 in 20 years at 6%?
Using the future value of annuity formula, approximately $216/month.
💡 Tips
- •The more frequently interest compounds, the faster your money grows.
- •Even small differences in interest rates can have huge impacts over long periods.
- •Start investing early - time is your greatest ally with compound interest.
- •Reinvesting dividends accelerates compound growth.
🎉 Fun Facts
- •Einstein's "8th Wonder": Albert Einstein allegedly called compound interest "the eighth wonder of the world," saying "He who understands it, earns it; he who doesn't, pays it." Though this quote's authenticity is debated, the principle is mathematically sound.
- •The Penny Doubling Myth: If you doubled a penny every day for 30 days, you'd have $5,368,709.12 on day 30. This demonstrates exponential growth, though no investment actually compounds daily at 100%.
- •The Rule of 72: To estimate how long it takes to double your money, divide 72 by your interest rate. At 8% returns, your money doubles every 9 years; at 6%, every 12 years. This simple mental math has worked for centuries.
- •Time Beats Timing: Someone who invests $5,000/year from age 25-35 (just 10 years, $50K total) and then stops will have more at age 65 than someone who invests $5,000/year from age 35-65 (30 years, $150K total). Assuming 8% returns, the early starter ends with $787K vs. $566K.
- •Credit Card Reverse Compounding: Credit card debt compounds against you. A $5,000 balance at 18% APR, making only minimum payments, takes 13+ years to pay off and costs $4,800+ in interest, nearly doubling what you owe.
- •Daily vs Monthly Compounding: $10,000 at 5% compounded daily for 10 years yields $16,487, while monthly compounding yields $16,470. Daily is only $17 more, showing frequency matters less than rate and time.
- •The Redwood Tree Analogy: Compound interest grows like a redwood tree, slow and unnoticeable at first, then exponentially massive. A $10,000 investment at 8% grows just $800 in year 1, but $3,669 in year 30.
- •Historical Returns Reality: The S&P 500 has averaged approximately 10% annual returns since 1928, but real returns after inflation are closer to 7%. Always factor inflation into compound interest calculations.