Compound Interest Calculator
See exactly when you'll hit your wealth milestones — with real-time growth tracking, historical S&P 500 returns, and cost-of-waiting analysis. Free, no signup.
Last reviewed: March 2026
Future Value
Your money earns
$6.95/hour
while you sleep
+$0.0000
Retirement Readiness (4% Rule)
$2,537/month
$30,443/year passive income
Rule of 72
Your money doubles every 9.0 years
Investment Growth Chart
Wealth Milestones
$100K
Age 36
April 2037
$250K
Age 44
April 2045
$500K
Age 51
April 2052
Your Money vs The Market's Money
Crossover at year 16: interest surpasses your contributions
| Delay | You Get | You Lose |
|---|---|---|
| 1 year | $697K | -$64K (8%) |
| 3 years | $583K | -$178K (23%) |
| 5 years | $486K | -$275K (36%) |
| 10 years | $301K | -$460K (60%) |
Good start — room to optimize
Score: 69/100
$500/month contribution. Consider increasing your monthly investment.
30 years gives compound interest 3 doubling cycles.
S&P 500 historical CAGR: ~10% nominal, ~7% real. Your 8% is realistic.
Starting with $1,000 gives you a head start.
Enable inflation toggle to see real purchasing power.
You'll hit 3 wealth milestones. Next: $100K by age 36.
What Is Compound Interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Often attributed to Albert Einstein as the "eighth wonder of the world," compound interest is the single most powerful force in wealth building.
Unlike simple interest — which only earns returns on your original investment — compound interest creates a snowball effect. Each year, you earn returns on your returns. Over decades, this exponential growth turns modest monthly contributions into substantial wealth.
The Compound Interest Formula Explained
Compound Interest Formula
A = P(1 + r/n)^(nt) + PMT × ((1 + r/n)^(nt) − 1) / (r/n)
Where:
- P = Principal (initial investment)
- r = Annual interest rate (decimal)
- n = Compounding frequency per year
- t = Time in years
- PMT = Monthly contribution
Example: $1,000 initial + $500/month at 8% for 30 years = $745,832. Of that total, you contributed $181,000 — the remaining $564,832 (76%) was earned by compound interest alone.
The Rule of 72 — How Quickly Does Your Money Double?
The Rule of 72 provides a quick mental estimate: divide 72 by your annual return rate to find how many years it takes your money to double.
At 8% returns, $10,000 becomes $20,000 in 9 years, $40,000 in 18 years, and $80,000 in 27 years. The Rule of 72 becomes less accurate at extreme rates but is remarkably precise for typical investment returns (4–15%).
The True Cost of Waiting to Invest
Every year you delay, compound interest has one less doubling period to work with. For $500/month at 8%:
The best time to start investing was yesterday. The second-best time is today. Use the cost-of-waiting table above to see exactly what delay costs you.
Compound Interest with Monthly Contributions
Regular monthly contributions are the engine of wealth building for most people. While a large lump sum has more time to compound, consistent monthly investing (dollar-cost averaging) reduces timing risk and builds discipline.
Comparison: $50,000 lump sum + $0/month at 8% for 20 years = $233,048. Versus $0 lump sum + $500/month at 8% for 20 years = $294,510. Regular contributions win — and most people can sustain monthly investing far more easily than saving a lump sum.
How Much Should You Invest Each Month?
A common guideline is to invest 20% of your gross income. The FIRE community aims for 50% or higher. But even modest amounts compound dramatically over time:
At 8% annual return, monthly compounding
A powerful strategy: increase your contribution by 1% of income each year. You barely notice the change, but over a decade it dramatically accelerates your wealth timeline.
Historical Stock Market Returns (S&P 500)
The S&P 500 has delivered approximately 10.5% compound annual growth rate (CAGR) since 1970, including dividends. Notable years: best was 1995 (+37.6%), worst was 2008 (-37.0%). The Time Machine feature in this calculator lets you test your scenario against actual historical returns.
Long-term averages smooth out volatility. Even investors who started at the worst possible times (2000, 2008) saw strong recovery within 5-7 years. Time in the market consistently beats timing the market. Data sourced from the Shiller dataset and NYU Stern research.
The 4% Rule — Turning Investments into Passive Income
The 4% Rule, originating from the 1998 Trinity Study, suggests you can withdraw 4% of your portfolio annually in retirement with a very low risk of running out of money over 30 years.
Example: A $1,000,000 portfolio supports $40,000/year ($3,333/month) in passive income. Some modern advisors recommend a more conservative 3.5% rate. This calculator shows your projected passive income based on the 4% rule — adjust your inputs to find your target retirement number.
Methodology & Data Sources
This calculator uses standard compound interest mathematics with period-by-period compounding. Historical S&P 500 total return data (including dividends) is sourced from NYU Stern's Damodaran dataset and the Shiller CAPE database. Inflation adjustment uses a user-configurable rate (default 3%, the long-term US average). The 4% withdrawal rate is based on the Trinity Study methodology. All calculations run entirely in your browser — no data is sent to any server.
Frequently Asked Questions
What is compound interest?
Compound interest is interest earned on both your original principal and on previously accumulated interest. This creates exponential growth over time — the longer your money compounds, the faster it grows.
How does compound interest work?
Each compounding period, interest is calculated on your total balance (principal + all previous interest). At 8% annual return, $10,000 becomes $10,800 after year 1, then $11,664 after year 2 (8% of $10,800, not $10,000). This snowball effect is what makes long-term investing so powerful.
What is the Rule of 72?
Divide 72 by your annual return rate to estimate how many years it takes your money to double. At 8% returns: 72 / 8 = 9 years to double. At 10%: 72 / 10 = 7.2 years.
What is the average stock market return?
The S&P 500 has returned approximately 10% per year on average since 1970 (nominal, including dividends). Adjusted for inflation, the real return is about 7%.
What is the 4% rule for retirement?
The 4% rule suggests you can withdraw 4% of your portfolio annually in retirement without running out of money over 30 years. A $1M portfolio = $40,000/year passive income.