Simulate future investment wealth with monthly compounding deposits, custom compounding frequencies, and see how time accelerates compound interest over 1 to 40 years.
Compound interest combines exponential growth on your starting balance with an annuity future-value stream for recurring monthly deposits:
Where P is initial deposit, PMT is periodic monthly contribution, r is nominal annual interest rate, n is compounding periods per year (12 for monthly), and t is duration in years.
• Worked Example: Starting with $10,000 and adding $500 per month at an 8.0% annual return for 20 years:
Total out-of-pocket deposits equal $10,000 + ($500 × 240 months) = $130,000.
Through compound returns, the initial $10,000 grows to $49,268, while the recurring $500 monthly contributions accumulate to $276,219.
Total ending portfolio equals $325,487, generating $195,487 in pure compound interest (150% more than your deposits).
The Rule of 72 is an easy mental formula to determine how many years it will take to double your investment at a fixed interest rate. Divide 72 by your annual interest rate: at an 8% return, money doubles in approximately 9 years (72 / 8 = 9).
The more frequently interest compounds (daily vs. monthly vs. annually), the faster your wealth multiplies because accrued interest begins earning interest sooner. However, the difference between monthly and daily compounding is relatively modest compared to the impact of your savings rate and time horizon.
Compound interest follows an exponential hockey-stick curve. The dollars you invest in your 20s have 30 to 40 years to double multiple times, generating vastly more wealth than larger sums contributed later in life.
A 1.0% annual expense ratio or advisory fee can reduce your final portfolio value by 20% to 30% over a 30-year horizon because fees compound negatively against your returns. Favoring low-cost index funds with expense ratios below 0.05% protects your compounding growth.
Add this interactive compound growth tool to your personal finance site or blog.