Compound Interest Calculator

Watch your investment grow with compound interest

Compound interest is one of the most powerful forces in personal finance. Albert Einstein is often (though apocryphally) quoted as calling it the eighth wonder of the world: "He who understands it, earns it; he who doesn't, pays it." This calculator shows you exactly how any starting amount grows over time at a given interest rate, with the compounding frequency of your choice.

What is compound interest?

With simple interest, you earn a fixed amount of interest each period calculated only on the original principal. With compound interest, you earn interest on the principal plus on all previously accumulated interest. This causes balances to grow exponentially rather than linearly.

Example: Invest $1,000 at 10% annual interest.

• Simple interest after 10 years: $1,000 + ($100 × 10) = $2,000
• Compound interest (annual) after 10 years: $1,000 × 1.10^10 = $2,593.74

The difference, $593.74, is the extra growth from compounding. Over longer periods, this difference becomes enormous. At 10% for 30 years, the compound result is $17,449 vs simple interest's $4,000.

The compound interest formula

A = P × (1 + r/n)^(n×t)

Where:

• A = future value
• P = principal (starting amount)
• r = annual interest rate as a decimal (5% = 0.05)
• n = number of compounding periods per year
• t = time in years

The effect of compounding frequency

The more frequently interest compounds, the more you earn — but the difference between monthly and daily compounding is small. At 10% annual rate on $10,000 for 10 years:

• Annual compounding: $25,937
• Monthly compounding: $27,070
• Daily compounding: $27,179

The jump from annual to monthly compounding is significant; from monthly to daily is modest.

The Rule of 72

A quick way to estimate how long it takes money to double: divide 72 by the annual interest rate. At 6%, money doubles in approximately 72/6 = 12 years. At 9%, it takes 8 years. This approximation is surprisingly accurate for rates between 2% and 20%.

Real-world applications

Savings accounts and CDs: Most banks compound interest monthly or daily. A savings account at 4.5% APY with monthly compounding on $10,000 grows to $10,450 after one year.

Retirement accounts: The long time horizons of retirement investing amplify compounding dramatically. Starting at 25 versus 35 with the same contributions and returns can result in a retirement fund 2-3 times larger simply because of the extra decade of compounding.

Debt: Compound interest also works against you with debt. Credit cards typically charge 15-30% APR compounded daily. A $5,000 balance paid only the minimum can grow to $8,000-10,000 before it is paid off.

Mortgages: Mortgages use compound interest calculated monthly. The early payments in a 30-year mortgage are mostly interest because the principal is large; as principal decreases, more of each payment goes to principal.

How to use the calculator

Enter your starting amount, the annual interest rate, how often interest compounds, and the number of years you plan to leave the money invested. The future value appears immediately, along with the total interest earned, and every figure recalculates instantly as you adjust any input, making it easy to compare how a higher rate, a longer time horizon, or more frequent compounding would change the outcome.

Negative compounding: debt

The same exponential growth that makes investments grow also makes debts grow if left unpaid, which is the less pleasant side of compound interest. Credit card debt commonly carries an annual rate of 15 to 30 percent, compounded daily, and a balance on which only the minimum payment is made can take fifteen years or more to clear, ultimately costing far more in interest than the original purchase. Running a credit card balance through this same calculator, treating the rate as a growth rate on what you owe rather than what you save, is a sobering way to see exactly why paying down high-interest debt quickly is one of the highest-value financial moves available to most people.

Inflation and real returns

When evaluating investments, distinguish between nominal returns (the stated rate) and real returns (adjusted for inflation). If an investment earns 8% annually but inflation is 3%, the real return is approximately 5% (exactly: (1.08/1.03) − 1 = 4.85%). Using real returns makes the future value calculation directly meaningful in today's purchasing power, while using nominal returns requires discounting the result by cumulative inflation afterward to understand what it will actually buy.

The effect of fees on compounding

Investment fees dramatically reduce compound growth over time, more than their small annual percentage suggests. A 1% annual fee on a 7% gross return fund reduces the net annual return to approximately 6%. On a $100,000 investment over 30 years, growing at 7% reaches roughly $761,000, while growing at 6% reaches only about $574,000 — the seemingly modest 1% fee costs approximately $187,000 in lost compounding over three decades, because the fee is deducted every single year and therefore compounds against you in exactly the same way returns compound for you.

Tax-advantaged compounding

In many countries, tax-advantaged retirement accounts allow compounding to occur without annual tax drag on the gains, which significantly increases long-term wealth accumulation compared with an equivalent taxable account. Because taxes on investment gains would otherwise reduce the amount available to keep compounding each year, sheltering that growth from yearly taxation is one of the most reliable, low-effort ways to improve a long-term compounding outcome without taking on any additional investment risk.

Private and instant

All calculations run entirely in your browser using the standard compound interest formula, so results update instantly as you adjust any input and no financial figures you enter are ever sent to a server, logged or shared.