Compound Interest Calculator — Investment Growth Over Time Free
See how your money grows with the power of compound interest. Enter your initial investment, monthly contributions, and interest rate to project your portfolio up to 50 years into the future. View year-by-year breakdown, portfolio composition, Rule of 72 insight, and download your projections as CSV.
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How to Use the Compound Interest Calculator
- 01
Set your investment
Enter your initial investment, monthly contribution amount, annual interest rate, and compounding frequency.
- 02
Adjust the time horizon
Use the years slider (1–50 years) to see how compound interest grows your portfolio over time.
- 03
Analyze and export
Review the portfolio composition bar, Rule of 72 insight, and year-by-year table. Download as CSV.
Frequently Asked Questions
What is compound interest?
Compound interest means earning interest on both your original principal and the interest already accumulated. Unlike simple interest (which only applies to the principal), compound interest grows exponentially. For example, $10,000 at 7% annual interest becomes $19,672 after 10 years with compound interest, versus $17,000 with simple interest.
How does compounding frequency affect returns?
The more frequently interest compounds, the more you earn. Daily compounding produces slightly more than monthly, which produces more than annual. For example, $10,000 at 10% for 1 year: annual compounding yields $11,000; monthly compounding yields $11,047; daily compounding yields $11,052. The difference grows significantly over longer periods.
What is the Rule of 72?
The Rule of 72 is a mental shortcut to estimate how long it takes to double your money at a fixed annual rate. Divide 72 by your annual interest rate. At 6%, your money doubles in roughly 12 years (72 ÷ 6). At 9%, it doubles in 8 years. At 12%, in 6 years. It's an approximation but remarkably accurate for rates between 4% and 20%.
How important are monthly contributions vs. the initial investment?
Over long periods, consistent monthly contributions often matter more than the initial lump sum. For example, $1,000 initial investment + $200/month at 7% for 30 years grows to about $242,000 — with only $72,600 ever deposited. The power comes from contributions being invested early and compounding for many years. Starting earlier and contributing consistently beats a larger one-time investment.
What interest rate should I use for projections?
Common benchmarks: savings accounts (0.5–5%), bonds (3–6%), diversified stock index funds (historically 7–10% average annual return before inflation). For conservative projections use 5–6%; for moderate projections use 7%; for aggressive scenarios use 9–10%. Remember these are averages — actual returns vary year to year.
Does this account for inflation?
This calculator shows nominal (not inflation-adjusted) returns. To get real returns, subtract the inflation rate from your interest rate. If you expect 7% returns and 3% inflation, use 4% in the calculator to see inflation-adjusted purchasing power growth. Alternatively, enter 7% to see nominal growth and understand that actual purchasing power will be somewhat lower.