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Compound Interest Calculator

See how your money grows over time with the power of compound interest. Enter your starting amount, optional monthly contributions, interest rate, compounding frequency, and time period to see your future balance and total interest earned.

Enter Your Investment Details

Future Balance
$0

Where Your Balance Comes From

Initial Principal Your Contributions Interest Earned
$0
Initial Principal
$0
Total Contributions
$0
Total Interest Earned

Year-by-Year Growth

Calculate on the Calculator tab first to see your yearly growth breakdown.

YearContributions This YearInterest This YearBalance
Calculate to see your yearly growth

How Compound Interest is Calculated

Basic Compound Interest (Lump Sum)

A = P × (1 + r/n)^(nt)

A = Final amount
P = Principal (initial investment)
r = Annual interest rate (decimal)
n = Compounding periods per year
t = Number of years

Worked Example

$10,000 invested at 7% annual interest, compounded monthly, for 20 years:

A = 10,000 × (1 + 0.07/12)^(12×20)
A = 10,000 × (1.005833)^240
A ≈ $40,387

With Monthly Contributions

Each monthly contribution compounds from the time it's added. The future value of a series of regular contributions is:

FV of contributions = PMT × [((1 + r/n)^(nt) - 1) ÷ (r/n)]

Total Balance = A (lump sum growth) + FV of contributions

Total Interest Earned

Total Interest = Final Balance - Initial Principal - Total Contributions

Try These Examples

Click any example to instantly calculate.

$10,000 + $200/mo
7% for 20 years, compounded monthly
$5,000 + $500/mo
8% for 30 years, compounded monthly
$50,000 lump sum
5% for 10 years, compounded quarterly
$1,000 + $100/mo
6% for 40 years (starting young), compounded annually

Frequently Asked Questions

Compound interest is interest calculated on both your original principal and on the interest that has already accumulated. This means your money earns 'interest on interest', causing growth to accelerate over time rather than staying flat like simple interest. The more frequently interest compounds - daily versus monthly versus annually - the faster your balance grows, though the difference between compounding frequencies is usually small compared to the effect of rate and time.
The 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. If you add regular contributions, each contribution also earns compound interest from the time it is added, which is why consistent contributions matter more than trying to time a single lump sum.
Because compound interest grows exponentially, not linearly - the earlier money is invested, the more compounding cycles it goes through. For example, investing $10,000 at age 25 at 7% annual growth becomes roughly $149,000 by age 65 (40 years), while the same $10,000 invested at age 35 (30 years) grows to only about $76,000. That 10-year head start roughly doubles the outcome, even though the amount invested and rate were identical.
It makes a difference, but usually a smaller one than people expect compared to the effect of interest rate or time invested. For example, $10,000 at 7% for 20 years grows to about $38,697 compounded annually versus about $40,387 compounded monthly - a difference of roughly $1,690, much smaller than the impact of, say, an extra 1% annual return or five more years invested.
Historically, the S&P 500 has returned around 9-10% annually before inflation over long periods (roughly 6-7% after adjusting for inflation), though any individual year can vary widely and past performance never guarantees future results. High-yield savings accounts and CDs typically offer lower, more predictable rates (often 3-5% depending on conditions). This calculator lets you enter any assumption - it's worth trying a few different, conservative rates to see a realistic range of outcomes rather than relying on a single optimistic number.
Monthly contributions can dramatically increase your final balance because each contribution starts compounding from the moment it's added. Consistent contributions - even modest ones - often matter more than the size of your initial lump sum, especially over long time horizons. This calculator includes an optional monthly contribution field so you can see the combined effect of a starting balance plus ongoing deposits.

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Compound Interest Calculator - See the Power of Compounding Over Time

Albert Einstein reportedly called compound interest "the eighth wonder of the world" - whether or not he actually said it, the math backs up the sentiment. Unlike simple interest, which only grows your original principal at a flat rate, compound interest earns returns on your returns, creating exponential rather than linear growth. This calculator shows you exactly how a starting amount, combined with optional monthly contributions, grows over time at a given interest rate.

The formula: A = P × (1 + r/n)^(nt)
Quick example: $10,000 at 7% for 20 years, compounded monthly ~$40,387 (more than 4x your starting amount)

Why Time Matters More Than Almost Anything Else

The single biggest lever in compound interest is time, not the interest rate. Because growth is exponential, the last few years of a long investment period often add more in dollar terms than the first decade combined. This is why financial advisors consistently emphasize starting early over waiting to invest a larger amount later - a smaller sum invested for 30 years frequently outperforms a much larger sum invested for only 15 years, at the same rate of return.

Compounding Frequency: Does It Really Matter?

  • Annually: Interest is calculated and added once per year - the simplest form of compounding.
  • Quarterly: Interest compounds 4 times a year, common for some bonds and CDs.
  • Monthly: Interest compounds 12 times a year - the most common structure for savings accounts and many investment calculations.
  • Daily: Interest compounds 365 times a year, used by some high-yield savings accounts.

More frequent compounding does produce a slightly higher final balance, but the effect is modest compared to the impact of the interest rate itself or the length of time invested. Don't let compounding frequency distract from the two factors that matter far more: starting as early as possible, and contributing consistently.

Tips for Maximizing Compound Growth

What Helps

  • Starting as early as possible, even with a small amount
  • Setting up automatic, consistent monthly contributions
  • Reinvesting dividends and interest rather than withdrawing them
  • Choosing tax-advantaged accounts (401k, IRA, etc.) where available

️ What Hurts

  • Waiting for the "perfect time" to start investing
  • Withdrawing early and interrupting the compounding cycle
  • High fees that quietly erode returns over decades
  • Chasing short-term high returns instead of consistent long-term growth

How this calculator works, and where the numbers come from

The Compound Interest Calculator applies the standard formula for this calculation to the values you enter and updates the result as you type. The calculation itself happens in your browser, and the page explains the method so you can check any result by hand.

Please note: Projections assume the inputs you enter stay constant; real returns vary. Results are estimates, not financial advice.

Sources and further reading

Learn more

Read our guide: Compound Interest and the Rule of 72, Checked Against the Math

Last reviewed: by the CalcQube Editorial Team. See our editorial policy for how we build and check calculators, or report an error.