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Compound Interest Calculator

See how money grows when interest earns interest. Model principal, regular contributions, and any compounding frequency from annual to continuous.

Compound interest inputs

This calculator is for informational purposes only and does not constitute financial advice. Consult a licensed financial advisor before making financial decisions.

What is compound interest and why does it matter?

Compound interest is the phenomenon where the interest you earn on an investment is added back to the principal, so the next period's interest is calculated on a slightly larger balance. Over years and decades, that small addition each period snowballs into an enormous difference compared to "simple" interest, which only pays interest on the original principal. Understanding compound interest is the foundation of nearly every serious financial decision: retirement planning, mortgage payoff, saving for a home, choosing a bond fund, evaluating a savings account.

The reason compounding is so powerful is that it is exponential, not linear. At 7% a year, a balance doubles in about ten years (the rule of 72 says 72 / 7 ≈ 10.3). Another ten years and it quadruples. Thirty years and it is almost eight times the starting amount — all without adding a single extra dollar. This is why financial advisors endlessly repeat "start early". Every year you delay is a year of compounding you don't get back.

How this calculator works

The calculator uses the standard compound interest formula for periodic compounding:

FV = P × (1 + r/n)n × t

where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the number of years. For continuous compounding it uses the exponential form:

FV = P × er × t

If you add a monthly contribution, the calculator applies the future-value-of-annuity formula for each contribution, with the timing (start vs. end of month) factored in. For a detailed reference, see the US SEC's investor.gov compound interest calculator and any standard finance textbook such as Bodie, Kane & Marcus, Investments.

The calculator is currency-agnostic: you enter whatever symbol or code you want (dollars, euros, yen, pesos, rupees, pounds, or any other). It performs no tax, no jurisdiction-specific logic, and no inflation adjustment. Results are nominal.

Worked example

Suppose you start with 10,000 and add 200 per month for 20 years at 7% annual interest, compounded monthly. The monthly rate is 7% ÷ 12 = 0.5833%. Over 240 months, the principal of 10,000 grows to roughly 40,387 on its own. The 48,000 of contributions (200 × 240) grows to approximately 104,185 thanks to compounding. Final balance: about 144,572. Of that, about 86,185 is interest earned — almost as much as the total contributions themselves. The year-by-year table below shows exactly how the balance accelerates.

How to interpret the result

The final balance is a nominal number. That means it does not account for inflation: a dollar in 30 years will not buy what a dollar buys today. To get the real (inflation-adjusted) value, subtract expected inflation from the interest rate before running the calculation. For example, if you expect 7% returns and 3% inflation, use 4% as the rate to see what the result is worth in today's purchasing power.

The year-by-year breakdown is particularly useful for visualising how compounding accelerates. In the early years most of the growth comes from your contributions; in the later years, the majority comes from interest on existing balance. This is the reason the phrase "your money working for you" exists.

Common mistakes

  • Using an unrealistic rate. Picking 12% because a blog post promised it. Long-run global equity returns are closer to 7% nominal.
  • Forgetting inflation. 1 million in 40 years is not what 1 million is today. Plan in real terms.
  • Ignoring fees. A 1% annual fund fee can cut your final balance by 20–30% over 30 years because the fee compounds too.
  • Stopping contributions early. Compounding is strongest in the later years. Pulling money out in year 5 removes most of the future compound growth.
  • Assuming a constant rate. Real returns are volatile. Use this calculator as a long-run average estimate, not a year-by-year prediction.

When to consult a professional

This calculator is a teaching tool. Real financial planning involves taxes, inflation, risk, liquidity needs, insurance, estate planning, and your personal circumstances — none of which a formula can capture. If you are planning for retirement, buying a home, managing an inheritance, or dealing with debt, speak to a fiduciary financial advisor (someone legally required to act in your interest) or a certified financial planner. They can build a plan tailored to your situation rather than a generic projection.

This calculator is for educational purposes only and is not financial advice.

Frequently Asked Questions

What is compound interest?
Compound interest is interest earned on both your original principal and on the interest that has already been added to the account. Because the interest itself begins earning interest, the balance grows faster over time than simple interest would allow. Albert Einstein reportedly called it "the eighth wonder of the world" — and whether or not he actually said that, the underlying math is genuinely powerful over long horizons.
How often should interest compound?
More frequent compounding produces slightly higher final balances, but the difference between daily and continuous compounding is tiny once rates are below 10%. Monthly compounding is the most common real-world frequency for savings accounts, and it is the default here. Annual compounding is typical for some certificates of deposit and bonds.
What formula does this calculator use?
For periodic compounding: FV = P × (1 + r/n)^(n × t), where P is the principal, r is the annual rate as a decimal, n is compoundings per year, and t is years. For continuous compounding: FV = P × e^(r × t). Monthly contributions are handled with the standard future-value-of-annuity formula, with timing (start or end of month) applied appropriately.
Does this calculator include inflation or taxes?
No. Results are nominal, meaning they are not adjusted for inflation or taxes. To estimate real (inflation-adjusted) returns, subtract your expected inflation rate from the interest rate before entering it. For after-tax returns, multiply by (1 − your marginal tax rate) on the interest portion. For precise tax handling, consult an accountant in your jurisdiction.
Should I contribute at the start or end of the month?
Contributing at the start of each month (annuity-due) earns slightly more interest because your money has an extra month to compound each period. The difference is usually under 1% over a long horizon but grows at higher rates. Most automatic payroll deductions occur at the end of each pay period, so "end of month" is the more common real-world setting.
Is 7% a realistic return assumption?
Historically, diversified global equity markets have returned roughly 7–10% per year nominal over multi-decade periods, with significant year-to-year volatility. Bonds typically return 3–5%. Savings accounts in most developed countries return 0–5% depending on the rate environment. Your actual return depends on what you invest in and the market conditions you experience.
Why does the final balance grow so much in later years?
That is compound growth in action. Early on, most of your gains come from contributions. As the balance grows, interest on interest becomes the dominant driver. This is why starting early matters so much: a dollar invested at age 25 has 40 years to compound before retirement; the same dollar at 45 has only 20.
Is this financial advice?
No. This calculator is for educational and illustrative purposes only. It assumes a constant rate of return, which never happens in real life — markets go up and down, and fees, taxes, and inflation erode returns. Consult a licensed financial advisor before making investment decisions that affect your retirement, home purchase, or other major financial goals.