Compound Interest Calculator
See how your money grows with the power of compounding. Add monthly contributions and watch interest build on interest.
Your Investment
Results
Future Balance
$144,626
Total Invested
$58,000
Interest Earned
$86,626
Return on Investment
149.4%
📐 Rule of 72: At 7%, your money doubles every 10.3 years.
For educational purposes only. Past returns do not guarantee future results.
⏰ Power of Time
See what 5 more (or fewer) years of compounding means with your current inputs.
Start 5 years earlier (25 years total)
$219,330
+$74,704 more than your current plan
Start 5 years later (15 years total)
$91,870
-$52,756 less than your current plan
Growth Over Time
Year-by-Year Breakdown
| Year | Year Contrib. | Year Interest | Total Invested | Balance |
|---|
How to Use This Compound Interest Calculator
Enter your initial investment — the lump sum you're starting with. If you're starting from zero, enter $0. Then set your monthly contribution, the amount you'll add every month going forward. Enter the annual interest rate you expect to earn, and the time period in years.
Choose your compounding frequency: daily, monthly, quarterly, or annually. Most savings accounts and money market funds compound daily. CDs typically compound daily or monthly. For stock market investments, monthly is a reasonable approximation.
Hit Calculate and you'll instantly see your projected balance, total contributions, and interest earned. The Power of Time cards show what happens if you start 5 years earlier or later — a powerful reminder that time is your most valuable financial asset.
How Compound Interest Is Calculated
The compound interest formula for a lump sum is: A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate, n is the compounding periods per year, and t is years. But most real-world investing involves regular contributions, so the full formula adds a future value of annuity component.
For monthly contributions, each $200 you add is also compounded from the moment it enters your account. The calculation simulates each month: apply interest to the current balance, then add your contribution. This means earlier contributions grow the most — your very first $200 earns 20 years of compounding, while your last $200 earns one month.
The effective monthly rate depends on compounding frequency:
- Daily: (1 + r/365)^(365/12) − 1
- Monthly: r/12
- Quarterly: (1 + r/4)^(1/3) − 1
- Annually: (1 + r)^(1/12) − 1
On $10,000 at 7% for 20 years with no contributions: daily compounding yields $40,088; monthly yields $40,025; annually yields $38,697. The frequency difference is real but modest — the rate and time period matter far more.
Understanding Your Results
Future Balance is the total projected value of your investment at the end of the period — your contributions plus all interest earned. Total Invested is the sum of your initial deposit plus all monthly contributions. Interest Earned is the difference — money created by compounding rather than saved.
The Return on Investment (ROI) shows interest earned as a percentage of total contributions. An ROI of 149% means for every dollar you invested, compounding generated an additional $1.49 — you more than doubled your money in pure earnings.
The Rule of 72 gives you a quick sanity check: divide 72 by your annual rate. At 7%, money doubles every 10.3 years. At 10%, every 7.2 years. At 4%, every 18 years. Use it to quickly estimate whether a rate seems reasonable for your goal.
The year-by-year table shows how your balance builds. Notice that early years show more growth from contributions, while later years show interest earning more than contributions — that's the compounding effect accelerating.
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the principal and previously earned interest. Unlike simple interest — which only applies to your original deposit — compound interest grows exponentially because each period's gains become the base for the next period's calculation. It's often called "interest on interest," and it's the foundation of long-term wealth building.
How much will $10,000 grow in 20 years at 7%?
With $10,000 initial investment, $200/month contributions, and 7% annual return compounded monthly, you'd have approximately $144,626 after 20 years — with $58,000 contributed and $86,626 earned in interest. Without contributions (just the initial $10k), it grows to about $40,025.
What is the Rule of 72?
The Rule of 72 estimates how long money takes to double: divide 72 by the annual interest rate. At 7%, money doubles approximately every 10.3 years (72 ÷ 7 = 10.3). At 10%, every 7.2 years. It's a quick mental math shortcut — not mathematically exact but very accurate for rates between 6% and 10%.
Does compounding frequency really matter?
Yes, but less than most people think at typical rates. Daily compounding yields slightly more than monthly, which yields more than annual. On $10,000 at 7% over 20 years: daily = $40,088 vs monthly = $40,025 vs annually = $38,697. The difference is less than $400. The rate and time period matter far more than compounding frequency.
What is the best compounding frequency?
Daily compounding gives the highest return, but the difference is small at typical rates. Most high-yield savings accounts and money market funds compound daily. CDs may compound daily, monthly, or quarterly — the APY (Annual Percentage Yield) already accounts for this. When comparing accounts, use APY rather than APR for a true comparison.
How do monthly contributions affect compound growth?
Monthly contributions dramatically accelerate growth. Adding $200/month to a $10,000 investment at 7% over 20 years triples the final balance (from ~$40,000 to ~$144,600). Consistency matters more than the amount — $100/month started at 25 beats $300/month started at 35, because you gain 10 extra years of compounding on every early contribution.
What rate of return should I use for long-term investing?
The S&P 500 has historically returned about 10% annually before inflation, or roughly 7% after inflation. For conservative planning, financial planners often use 6–7% for diversified equity portfolios and 3–4% for bonds. High-yield savings accounts currently offer 4–5%. Always account for inflation to understand actual purchasing power growth.