Compound Interest Calculator
Compound interest is interest earned on your interest. the balance grows, then the growth itself grows. Enter your numbers below; results, chart, and table update as you type.
Future balance
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- Total you contributed
- $0
- Interest earned
- $0
- Growth multiple
- -
- Money doubles roughly every
- -
Constant rate, month-end deposits. A learning estimate, not a guarantee.
Year-by-year growth
| Year | Contributed (total) | Interest (total) | Balance |
|---|
What you put in against what it earned
The split shows how much of the total is money you put in and how much is interest it earned.
Move it and watch
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at {v} for starting amount
And what you can change
Why your own figure may differ
How to use the compound interest calculator
- Enter your starting amount - whatever is already saved or invested. $0 works fine.
- Add the monthly contribution you can sustain. Consistency beats size; the tool assumes month-end deposits.
- Set the annual rate: your account's APY for savings, or a long-run estimate like 7% for index investing.
- Pick the timeframe and compounding frequency, then read the chart - the gap between the two lines is money you never deposited.
The formula behind the numbers
Your starting amount grows by the classic compound interest formula, and each monthly deposit compounds for exactly the time it stays invested:
A = P(1 + r/n)nt + PMT × [((1 + i)m − 1) ÷ i]
- P. starting principal · r. annual rate (decimal) · n. compounds per year · t. years
- PMT. monthly contribution · i. effective monthly rate · m. total months
You can choose how often interest is added. If you pick anything other than monthly, the tool converts it to the matching monthly rate. That way your monthly deposits are handled exactly, not roughly.
Worked example: $10,000 + $200/month at 7% for 20 years
Direct answer: you end with about $144,573. Of that, $58,000 is your own deposits and roughly $86,573 is pure interest. the interest ends up larger than everything you contributed. That crossover, usually somewhere in the second decade, is why starting early beats starting big.
| Scenario (7%, monthly compounding, 20 yrs) | You contribute | Ending balance |
|---|---|---|
| $10,000 lump sum only | $10,000 | $40,387 |
| $200/month only | $48,000 | $104,185 |
| Both combined | $58,000 | $144,573 |
What rate should you enter?
| Where the money lives | Typical rate to model | Notes |
|---|---|---|
| High-yield savings account | 3.5–5% | Use your bank's exact APY; it changes with Fed policy |
| Certificates of deposit (CDs) | 3–5% | Locked rate for the term - model the quoted APY |
| Broad stock index funds | 6–8% | Conservative long-run planning band (history: ~10% nominal) |
| Bonds / bond funds | 3–5% | Varies with duration and credit quality |
Two honest warnings. Market returns do not arrive evenly - some years fall 20%, some rise 30%. And rising prices quietly eat away at what your money buys. US prices rose 2.51% a year on average over the 29 years to 2025, per the Bureau of Labor Statistics. A steady-rate projection is a planning tool, not a prophecy.
Compound interest examples: three savers compared
Three savers, three starting points - every figure below is this calculator's own output. Click any card to load it and change the numbers to yours.
- New graduate, 24
Starting from zero at $300/month
No savings yet - just $300 a month into a broad index fund averaging 7%, starting with the first paycheck.
After 30 years: $365,991 - $108,000 deposited, $257,991 created by compounding.
Load this scenario in the calculator → - Windfall recipient
A $15,000 inheritance, left alone
One deposit, no monthly additions - 25 years of 7% growth, untouched.
Grows to $85,881 - 5.7× the original, with zero further effort.
Load this scenario in the calculator → - Late starter, 50
Catching up with $800/month
$10,000 saved, 15 working years left, saving hard into a balanced 6% portfolio.
Reaches $257,196 by 65 - $154,000 saved, $103,196 earned. Late is not lost.
Load this scenario in the calculator →
Common questions about compound interest
What is compound interest?
Compound interest means you earn interest on your interest. Not just on the money you put in. Each period, interest joins your balance, and the next period's interest is calculated on that larger balance - so growth speeds up the longer you leave it alone.
What is the compound interest formula?
A = P(1 + r/n)nt, where A is the final amount, P is the starting principal, r is the annual rate as a decimal, n is compounding periods per year, and t is years. Each monthly payment also grows, for as long as it has been in. The tool adds all of that up.
How is compound interest different from simple interest?
Simple interest pays only on the original principal, so growth is a straight line. Compound interest pays on principal plus built up interest, so growth curves upward. $10,000 at 7% simple interest earns a flat $700 every year; compounded annually, year 10 alone earns about $1,287.
Does compounding frequency matter much?
Less than most people expect. $10,000 at 5% for 10 years grows to $16,289 with annual compounding and $16,486 with daily - a difference of under $200. The rate and the time horizon matter far more than the frequency.
What annual return should I assume?
Nobody can tell you, and be wary of anyone who says they can. Past stock market averages get quoted a lot, but they are not a promise about your years. The rate here is your assumption, so try a few and see how much the answer moves. For savings accounts and CDs you do not have to guess - use the exact APY your bank quotes - see our APR vs. APY guide.
What is the Rule of 72?
A doubling-time shortcut: divide 72 by your annual return. At 8%, money doubles about every 9 years (72 ÷ 8 = 9). This calculator shows your exact doubling time in the results card; the rule is most accurate between roughly 4% and 12%.
Does this account for taxes or inflation?
No, results are pre-tax, nominal dollars. In taxable accounts, interest and dividends are usually taxed yearly; in a 401(k) or IRA, growth compounds tax-deferred or tax-free. To think in today's buying power, take the rate of price rises off your assumed return. US prices rose 2.51% a year on average over the 29 years to 2025, and 3.16% a year across the whole record since 1913, per the Bureau of Labor Statistics.
The guides behind this calculator
- APR vs APY: Why Banks Quote One and Not the OtherAPR is what borrowing costs. APY is what saving earns. APY counts compounding and APR does not - which is why each product is sold with the flattering number.
- How Long to Double Your Money? Rule of 72, TestedDoubling times at every realistic rate, with the Rule of 72 tested against exact math - and why the last double is always the biggest.
- How Compound Interest Works, With Real Numbers$200 a month for 30 years puts in $72,000 and can end near $234,000. Why starting early beats saving more, and how the same maths works against you in debt.
- What to Do With $10,000: the Right OrderClear high-rate debt, fill the cushion, take the employer match, then invest. Killing a 24% card returns $2,400 a year, guaranteed.