Compound Interest Calculator

What is compound interest?

Compound interest is the interest you earn on both your original deposit and the interest that has already been added to your balance. It's essentially "interest on interest," and it's the single most powerful concept in personal finance.

Unlike simple interest, which is calculated only on your initial principal, compound interest grows your money exponentially over time. The longer your money stays invested, the more dramatic the effect becomes. Albert Einstein reportedly called compound interest the "eighth wonder of the world."

How to calculate your interest

To calculate compound interest, you'll need these variables:

1. Principal (P): Your starting amount or initial investment.
2. Rate (r): The annual interest rate expressed as a decimal (e.g., 5% = 0.05).
3. Compounding frequency (n): How often interest is added to your balance — annually, monthly, or daily.
4. Time (t): The number of years you plan to leave your money invested.

Compound interest formula

The formula for compound interest is:

Where:

• A = the final amount (principal + interest)
• P = the initial principal balance
• r = the annual interest rate (as a decimal)
• n = the number of times interest is compounded per year
• t = the number of years the money is invested

What is the compound interest formula with an example?

Let's say you invest $10,000 at a 5% annual interest rate, compounded monthly, for 10 years with no additional contributions. Using the formula:

• P = $10,000
• r = 0.05
• n = 12 (monthly)
• t = 10

A = $10,000 × (1 + 0.05/12)^12×10 = $10,000 × (1.004167)¹²⁰ = $16,470.09

That means you'd earn $6,470.09 in interest on a $10,000 investment over 10 years — without adding a single extra dollar.

Interest calculator example

Let's say you want to put $10,000 into a high-yield savings account with a 4.5% annual rate, compounded daily. You plan to also contribute $100 per month for 20 years. Using this calculator:

• Initial investment: $10,000
• Monthly contribution: $100
• Annual interest rate: 4.5%
• Compounding frequency: Daily
• Years to grow: 20

After 20 years, your $34,000 in total contributions ($10,000 initial + $24,000 in monthly deposits) would grow to approximately $63,000+, with roughly $29,000+ in interest earned. That's nearly doubling the money you put in, all thanks to compound interest.

Compounding with additional contributions

The calculator above factors in regular contributions, which is how most people actually save. Making consistent deposits — even small ones — can dramatically accelerate your growth because each contribution begins earning compound interest immediately.

Here's what makes regular contributions so powerful:

• Dollar-cost averaging: Regular contributions smooth out market volatility over time.
• Habit building: Setting up automatic monthly contributions creates a "pay yourself first" mindset.
• Exponential growth: Each new dollar starts compounding the moment it lands, so earlier contributions have the longest runway to grow.

For example, investing $500/month for 30 years at a 7% return would give you about $567,000 in total value — but you only contributed $180,000 out of pocket. The remaining $387,000 is pure compound interest.

Compounding investment returns

While this calculator uses a fixed interest rate, real-world investment returns vary year to year. The stock market has historically returned about 10% annually before inflation (roughly 7% after inflation) over long periods.

A few key things to keep in mind when projecting investment returns:

• Past performance doesn't guarantee future results. Use conservative estimates (5–7%) for long-term planning.
• Fees matter. A 1% annual management fee on a $100,000 portfolio costs $1,000/year and compounds against you. Low-cost index funds typically charge 0.03–0.20%.
• Tax-advantaged accounts amplify compounding. Accounts like Roth IRAs and 401(k)s let your interest compound without being reduced by annual taxes.
• Inflation erodes purchasing power. While your nominal balance grows, consider using a "real return" rate (nominal rate minus inflation) for more realistic projections.

How compound interest can help you save

Compound interest rewards patience. The earlier you start saving — even in small amounts — the more time your money has to grow. Here are a few practical ways to put compound interest to work:

• High-yield savings accounts: Currently offering 4–5% APY, these are great for emergency funds and short-term savings goals.
• Index fund investing: Low-cost S&P 500 index funds provide diversified exposure to the stock market with minimal fees.
• Retirement accounts: 401(k)s and IRAs offer tax advantages that supercharge compound growth over decades.
• I Bonds & Treasury securities: Government-backed options that protect against inflation while earning compound interest.

The power of starting early

Consider two investors, both targeting retirement at age 65:

• Investor A starts at 25, invests $200/month for 40 years at 7% → ~$525,000
• Investor B starts at 35, invests $200/month for 30 years at 7% → ~$243,000

Investor A ends up with more than twice as much, despite only contributing $24,000 more. Those extra 10 years of compounding made a $282,000 difference. The takeaway: the best time to start investing was yesterday. The second best time is today.

Frequently asked questions

What's the difference between simple and compound interest?

Simple interest is calculated only on the original principal. If you deposit $1,000 at 5% simple interest, you earn $50 every year regardless of your balance. Compound interest, on the other hand, is calculated on the principal plus any accumulated interest, so your earnings accelerate over time.

How often should interest be compounded?

More frequent compounding means slightly more growth. Daily compounding earns a bit more than monthly, which earns more than annual. However, the difference is small for most savings accounts. What matters far more is your interest rate, how much you contribute, and how long you leave the money invested.

What's a realistic interest rate to use?

For savings accounts, use the current APY (around 4–5% in early 2026). For long-term stock market investments, 7% (adjusted for inflation) is a commonly used conservative estimate. For bonds, 3–5% is reasonable. Always err on the conservative side when planning.

Does compound interest work against me with debt?

Yes. Credit card debt typically compounds daily at rates of 18–28% APR, which means unpaid balances grow rapidly. This is why paying off high-interest debt should generally be your first financial priority before investing.