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
Compound interest is often called 'the eighth wonder of the world' because it makes your money grow exponentially over time. The earlier you start saving and investing, the more dramatically compound interest builds wealth.
How does compound interest actually build wealth?
Compound interest means you earn returns on your previous returns. If you invest $10,000 at 8% annual return, after year 1 you have $10,800. In year 2, you earn 8% on $10,800 (not just $10,000), so you grow to $11,664. Over 30 years, that initial $10,000 becomes $100,627 — without adding another cent.
How much will $100/month grow into over 30 years?
At an 8% average annual return (typical S&P 500), $100/month becomes $149,036 in 30 years. Of that, you only contributed $36,000 — the other $113,036 is pure compound growth. At $500/month, you'd have $745,180 from $180,000 contributed.
Daily, monthly, or annual compounding — does it really matter?
Less than you'd think. On $10,000 at 8% over 30 years: annual compounding = $100,627, monthly compounding = $109,357, daily compounding = $110,176. Compounding frequency matters more for short timeframes than long ones.
When should I start investing to maximize compound interest?
As early as possible. Starting at 25 with $300/month at 8% gives you $1.05M by 65. Starting at 35 (10 years later) requires $700/month to reach the same goal. Starting at 45 requires $1,700/month. Time matters far more than amount.
What's the rule of 72 and how do I use it?
The rule of 72 is a quick mental shortcut: divide 72 by your annual return rate to find how long until your money doubles. At 8% return, money doubles every 9 years. At 6%, every 12 years. At 12%, every 6 years. It's not perfectly precise but very close.