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The 8th Wonder of the World

Compound Interest Calculator

Unlock the power of exponential growth. Model your wealth accumulation with professional granularity—accounting for recurring deposits, taxes, and inflation.

Capital Configuration

Wealth Mechanics

The Physics of
Unstoppable Wealth

Albert Einstein reportedly called Compound Interest the eighth wonder of the world. It is the mathematical force that allows small, consistent actions to transform into massive financial results over time. By earning "interest on interest," your money begins to grow at an accelerating rate.

Compounding Frequency

The more frequently interest is added to your balance, the faster you grow. Monthly compounding earns more than annual; daily earns even more. Our tool supports everything up to "Continuous" compounding.

The Power of Time

Because compounding is exponential, the final years of an investment are the most productive. Waiting just 5 years to start can cost you hundreds of thousands of dollars in "Lost Growth" by the time you retire.

Recurring Synergy

Adding new capital while your existing capital earns interest creates a "Double Momentum" effect. This is the core strategy of successful 401(k) and IRA investors.

The Compound Interest Formula

The standard formula for compound interest is:

A = P(1 + r/n)^nt

Where A is the final amount, P is principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is time in years.

The Rule of 72

A quick way to estimate compounding power is the Rule of 72. Divide 72 by your expected annual return to find how many years it takes for your money to double.

  • At 6% return: ~12 years to double.
  • At 10% return: ~7.2 years to double.

Inflation: The Silent Eroder

While your balance grows, the "Purchasing Power" of each dollar decreases due to inflation. If your money grows at 5% but inflation is 3%, your Real Rate of Return is only about 2%. Our calculator shows you both the "Nominal" balance and the inflation-adjusted "Real Value."

Standard Compounding Rules:

  • • Savings Accounts: Usually compound daily and credit monthly.
  • • Credit Cards: Compound daily on your balance.
  • • Investments (Mutual Funds): Usually compound daily based on the NAV movement.

Start Today, Not Tomorrow

If Investor A starts with $1,000/mo at age 25 and stops at 35, and Investor B starts with $1,000/mo at age 35 and continues until 65—Investor A will still have more money at retirement. The early decade of compounding is impossible to catch up to.

Taxes and Growth

If your investment is in a taxable account, you must pay taxes on the interest earned each year. This reduces your "Effective Interest Rate" and slows down the compounding process. This is why Tax-Advantaged Accounts (like Roth IRAs or 401ks) are so powerful—they allow for "Gross Compounding" without the tax drag.

How the calculation works

Compound interest adds each period's interest to the balance, so later periods earn interest on earlier interest.

Balance grows by P × (1 + r/12)^(12×t), plus each monthly deposit. Deposits in this example are added at the end of the month. Tax is ignored.

Compound Interest Calculator also accepts other compounding schedules. This example uses monthly compounding so the steps stay visible.

Worked example: $20,000 at 5% for 5 years

  1. Starting balance = $20,000.00. Monthly deposit = $0.00.
  2. Monthly rate = 5% ÷ 12 = 0.4167% of the current balance.
  3. After 5 years the balance is $25,667.17.
  4. Of that, $20,000.00 was deposited and $5,667.17 is growth.

How the balance changes

CaseEnding balanceGrowth
$0.00 / month at 5%$25,667.17$5,667.17
No monthly deposit at 5%$25,667.17$5,667.17
$0.00 / month at 7%$28,352.51$8,352.51

Frequently asked questions

What is $20,000 worth after 5 years at 5% with $0.00 added each month?▼
About $25,667.17, of which $5,667.17 is growth. Deposits are added at the end of each month and compounding is monthly.
Does a higher rate change the deposit total?▼
No. The same deposits still total $20,000.00. At 7% the ending balance is about $28,352.51 instead of $25,667.17.
Where does the monthly rate come from?▼
Divide the annual rate by 12. At 5%, each month multiplies the balance by 1.004167 before the next deposit.