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Compound Interest Calculator

See how your money grows exponentially over time. Model different compounding frequencies, monthly contributions, and time horizons to understand the most powerful force in investing.

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Compound Interest Calculator Results update as you move the sliders
Starting amount $5,000
The amount you are starting with today
Monthly contribution $200
Amount you add every month. Even $50 makes a real difference
Annual interest rate 8%
S&P 500 historical average is around 10%. Savings accounts currently offer 3 to 5%
Time period 20 years
How long you plan to keep your money invested
Compounding frequency
Final balance after
$0
20 years at 8% compounded monthly
You put in
$0
Interest earned
$0
Effective rate
0%
Money multiplied
0x
Rule of 72
At 8%, your money doubles approximately every 9 years.
Total balance
Amount contributed
YearBalanceYou ContributedInterest EarnedMultiple
Understanding compound interest

The most powerful force in building wealth

Compound interest is often called the eighth wonder of the world, and for good reason. It is the process by which your money earns returns, and then those returns earn returns of their own. Over time this creates an exponential growth curve that simple interest can never match.

With simple interest, you earn a fixed percentage on your original amount only. With compound interest, you earn on your growing balance. Each year, your returns are added to your principal, and the following year those returns also earn returns. The longer this goes on, the more dramatic the difference becomes.

Compounding frequency
How often interest is calculated and added to your balance. Daily compounding produces slightly more than monthly, which produces more than annual. For most investments, monthly compounding is standard. The rate and time horizon matter far more than frequency.
Regular contributions
Adding money consistently each month dramatically accelerates compound growth. Each contribution immediately begins compounding. This is called dollar-cost averaging and it is one of the most effective wealth-building habits you can build. The amount matters less than the consistency.
Time is the real variable
In compound interest, time does most of the heavy lifting. The final years of a long investment produce more growth than all the early years combined. Starting now, with whatever you have, beats waiting for the perfect moment every single time.
Simple vs compound interest

See the difference for yourself

This table shows what happens to a $10,000 investment at 8% over time with no additional contributions, comparing simple interest against compound interest.

YearSimple Interest (8%)Compound Interest (8%)Advantage of Compounding
Year 1$10,800$10,830+$30
Year 5$14,000$14,898+$898
Year 10$18,000$22,196+$4,196
Year 20$26,000$49,268+$23,268
Year 30$34,000$109,357+$75,357
Frequently asked questions

What people ask about compound interest

What is the formula for compound interest?+
The standard compound interest formula is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is the time in years. When you add regular monthly contributions, the formula adds the future value of an annuity on top. This calculator handles all of it automatically.
What interest rate should I use for stocks?+
The S&P 500 has returned approximately 10% per year on average over the past fifty years including all major crashes. A more conservative estimate after inflation is around 7%. For a mixed portfolio of stocks and bonds, 6 to 8% is a reasonable assumption. CD accounts currently offer around 4 to 5%. Use a rate that reflects what you are actually invested in, not the most optimistic scenario.
What is the Rule of 72?+
The Rule of 72 is a quick mental math shortcut to estimate how long it takes to double your money. Divide 72 by your annual interest rate. At 8%, your money doubles in approximately 9 years. At 10%, about 7.2 years. At 6%, about 12 years. It is not perfectly precise but gives you a fast, intuitive sense of what different rates mean in practice.
Does compounding frequency make a big difference?+
More frequent compounding produces slightly more growth, but the difference between daily and monthly compounding is smaller than most people expect. On a $10,000 investment at 8% over 20 years, daily compounding produces about $49,640 compared to $49,268 with monthly compounding. The rate and time horizon matter far more.
How much should I start with?+
There is no minimum. You can start with $50, $500, or $50,000 and the mathematics of compounding works the same way. What matters more than the initial amount is consistency. Someone who invests $100 a month for 30 years will often outperform someone who waits to save $10,000 and then invests a lump sum.
Does this calculator account for taxes and fees?+
No. This calculator shows pre-tax, pre-fee projections. In reality, taxes on investment gains and annual fund expenses will reduce your actual returns. A 1% annual management fee can reduce your final balance by 20 to 25% over 30 years. Tax-advantaged accounts like IRAs help you keep more of your compounded growth.
Why this matters

Markets that were once out of reach are now within yours

For too long, the mechanics of wealth creation were concepts that existed in boardrooms and business schools but rarely reached the people who needed them most. Understanding how money compounds over time is not complicated. It was simply never explained clearly enough.

Whether you are starting with $100 or $100,000, whether you are in your twenties or your forties, the principles on this page apply to you. Compound interest does not ask where you are from. It rewards anyone who starts and stays consistent.

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