Compound Interest Calculator
Calculate how your money grows with compound interest over time. See the power of regular contributions.
Your investment will grow to $300,851 over 20 years, earning $170,851 in compound interest.
Why This Calculation Matters
The Compound Interest Calculator helps you make better investments decisions by putting the math directly in front of you. Instead of relying on averages or guesswork, plug in your own numbers and see how the key inputs, rate, term, amount, and timing, interact. Small changes to any one of them can have outsized effects over years or decades.
How to Use This Calculator
- Enter your values in the input fields, each one has a label and help text explaining what to type.
- Results appear instantly as you type; there's no "calculate" button to press.
- Change any input to compare scenarios side by side.
All math happens in your browser. Nothing you type is sent to a server, saved, or shared.
Key Inputs to Get Right
The most important numbers are usually the interest rate and the time horizon. Over years or decades, small rate differences compound into large dollar differences, so it's worth sanity-checking the rate against current market data before acting on any result.
The Power of Compound Interest
Compound interest earns interest on your interest, creating exponential growth. Albert Einstein reportedly called it the "eighth wonder of the world."
The Formula
A = P(1 + r/n)^(nt) + PMT × [(1 + r/n)^(nt) - 1] / (r/n)
Where:
- A = final amount
- P = principal
- r = annual rate
- n = compounding frequency
- t = years
- PMT = periodic contribution
Key Factors
- Time: The earlier you start, the more compounding works for you
- Rate: Even small rate differences compound significantly
- Contributions: Regular additions dramatically accelerate growth
Formula
Compound growth follows:
A = P(1 + r/n)^(nt) + PMT × ((1 + r/n)^(nt) − 1) / (r/n)
Where P is the starting balance, r the annual rate, n the compounding periods per year, t the years, and PMT any recurring contribution. The second term captures the future value of regular deposits.
Worked Example
$10,000 invested at 7.0% for 20 years with no further contributions.
- Principal
- $10,000
- Annual rate
- 7.0%
- Years
- 20
- Result
- $38,697
The investment earns about $28,697 in compound interest.
When to Use This Calculator
- Model scenarios before making a major financial decision involving investments.
- Compare different inputs side by side to see how rate, term, or amount changes your outcome.
- Sanity-check numbers a lender, advisor, or spreadsheet has given you.
- Build a realistic financial plan grounded in your actual numbers, not averages.
Limitations & Common Mistakes
- Results are estimates, actual terms depend on credit, lender policy, taxes, and fees not captured here.
- Rates and prices change daily; recompute with current numbers before signing documents.
- Does not constitute financial advice. For major decisions, consult a licensed advisor.
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (which only applies to the principal), compound interest grows exponentially because you earn interest on your interest.
How often should interest compound?
More frequent compounding produces slightly higher returns. Daily compounding earns marginally more than monthly, which earns more than quarterly or annually. However, the difference between daily and monthly compounding is usually minimal.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by your annual interest rate. At 8% return, your money doubles roughly every 9 years (72/8 = 9).
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Key terms
- Compound InterestInterest calculated on both the principal and the accumulated interest from previous periods. The formula A = P(1 + r/n)^(nt) shows how principal P grows at ann…
- APR vs. APYAPR (Annual Percentage Rate) is the simple annualized interest rate on a loan, excluding the effect of compounding. APY (Annual Percentage Yield) is the effecti…
- FIRE NumberThe investment balance required to retire and live on passive withdrawals, following the Financial Independence, Retire Early movement. The standard formula is …
- Half-LifeThe time required for half of a radioactive substance to decay. The remaining fraction follows N(t) = N₀ × (1/2)^(t/t½). Carbon-14's 5,730-year half-life powers…
- Inflation RateThe annual percentage change in a price index, most commonly the Consumer Price Index (CPI-U) published monthly by the Bureau of Labor Statistics. The Federal R…
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