Compound Interest Calculator

Your investment

Initial Amount ($)

Monthly Addition ($)

Annual Interest Rate 7%

Years 20 years

The magic of compounding

⏰Starting 10 years earlier can more than double your final balance.
📈The S&P 500 has averaged ~10% annually before inflation (7% after) over the long run.
💡Compound interest works against you with debt — the same math that builds wealth destroys it.

Compound interest: the most powerful force in personal finance

Albert Einstein is often (perhaps apocryphally) credited with calling compound interest the eighth wonder of the world. Accurate attribution aside, the sentiment is right. Compound interest is what makes patient, consistent investing so powerful — and what makes high-interest debt so dangerous. Understanding it deeply is one of the most valuable things you can do for your financial life.

Simple interest is calculated only on the principal. Compound interest is calculated on the principal plus all previously accumulated interest — meaning your interest earns interest. Over short periods, the difference is modest. Over decades, it is staggering. A $10,000 investment at 7% simple interest grows to $17,000 after 10 years. At 7% compounded annually, it grows to $19,672. Over 40 years: $38,000 simple vs. $149,745 compound — a difference of over $111,000 on the same original $10,000.

How compound interest is calculated

The 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 times interest compounds per year, and t is the number of years.

Most investment accounts compound daily or monthly in practice, though the difference between daily and monthly compounding is negligible over typical investment horizons. What matters far more is the interest rate and the time horizon.

Example: The effect of compounding frequency on $10,000 at 7% over 20 years

Annually: $38,697. Monthly: $40,075. Daily: $40,137. The difference between monthly and daily is only $62 after 20 years. The difference between 20 years and 30 years at monthly compounding is $54,435 — time dwarfs compounding frequency as a factor.

The rule of 72: a mental shortcut for compound growth

The Rule of 72 is a quick way to estimate how long it takes for money to double at a given interest rate. Simply divide 72 by the annual interest rate to get the approximate number of years to double.

At 6%, money doubles in about 12 years (72 ÷ 6). At 10%, it doubles in about 7.2 years. At 4%, it doubles in 18 years. This simple calculation explains why small differences in return rate matter so much over long time horizons — and why high-interest debt grows so frightening fast.

📈
At 7% (typical diversified portfolio)
Your money doubles roughly every 10 years. $50,000 becomes $100,000 in 10 years, $200,000 in 20 years, $400,000 in 30 years — without adding a single dollar.
💳
At 22% (typical credit card)
Your debt doubles every 3.3 years. A $5,000 balance becomes $10,000 in 3 years if you make no payments — which is why minimum payments feel like treading water.
🏦
At 4.5% (current HYSA rate)
Your emergency fund doubles in 16 years — not exciting, but the point of a HYSA is liquidity and safety, not maximum growth.

Frequently asked questions

APR (Annual Percentage Rate) does not account for compounding within the year. APY (Annual Percentage Yield) does. For savings accounts, APY is the more useful number because it shows what you actually earn. For loans, APR is the standard disclosure. Always compare savings accounts using APY and loans using APR.

Your 401(k) does not compound in a fixed-interest way — it invests in funds whose value fluctuates daily. The 'compounding' effect comes from reinvesting dividends and capital gains, and from your account growing larger so that even the same percentage gain represents more dollars each year.

Most high-yield savings accounts and money market accounts compound daily and credit interest monthly. The daily compounding on typical savings amounts produces only marginally more than monthly compounding — the rate matters far more than the compounding frequency.

Over long time horizons, broadly diversified, low-cost index funds (such as a total stock market or S&P 500 fund) have historically provided the best compound growth for most investors. The lower the fees, the more of the return compounds in your favor rather than the fund manager's.

All returns must be adjusted for inflation to understand real purchasing power. A 10% nominal return during 3% inflation represents only about 7% real growth. Always think in real (inflation-adjusted) terms when planning for long-term goals like retirement.