Compound Interest Calculator
See how money grows over time — with monthly contributions
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$
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Final value
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Invested: $0
Interest earned: $0
📊 Year-by-year breakdown
| Year | Contributed | Interest | Balance |
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📚 Learn more — how it works, FAQ & guide Click to expand
Learn more — how it works, FAQ & guide
Click to expand
Free compound interest calculator with monthly contributions
Toololis Compound Interest Calculator shows long-term investment growth. Factor in starting principal, monthly contributions, annual return rate, and time horizon. Perfect for retirement planning, savings goals, and understanding the power of time.
How to use this tool
- 1
Enter starting principal
How much you have today or plan to invest upfront.
- 2
Set rate & years
Annual interest rate (5-10% is typical for stocks long-term). Investment horizon in years.
- 3
Add monthly contribution
How much you add monthly. Often more impactful than the starting amount over long periods.
The magic of compounding — real examples
- $500/month at 7% for 30 years = $566,764 (you contributed $180,000)
- Starting at 25 vs 35 (same $500/month, 7%) — age 25 start = ~$1.2M at 65 vs ~$580K starting at 35
- Time beats amount — 20 years × $1000/month = less than 30 years × $500/month
Frequently Asked Questions
What is compound interest?
Interest earned on both your principal AND previously earned interest. Einstein allegedly called it the "eighth wonder of the world". The longer the time horizon, the bigger the effect.
Realistic rates of return?
US stock market historical average: 10%/year nominal, 7%/year real (after inflation). Bonds: 4-5%. Savings accounts: 0-5% depending on era. Crypto: highly variable. Use 6-8% for conservative planning.
Rule of 72?
Quick mental math: 72 ÷ rate = years to double. At 7%: money doubles every 10.3 years. At 10%: every 7.2 years.
Should I reinvest monthly?
Yes — the difference between monthly and annual compounding over 30 years is small but positive. What matters most is: start early, contribute consistently, don't withdraw.
Does this include inflation?
No — we show nominal returns. For "real" purchasing power, subtract 2-3% (inflation). $1M in 30 years has ~$400K in today's purchasing power.
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