Compound Interest Calculator
See what your money could grow to. Change any number and the answer updates.
Reviewed by Associate Professor Charles Lo, CPA, PhD · Updated October 13, 2026Free, no signup
Your investment details
Fill in the assumptions below. The calculator updates the projection, chart and year by year table.
Illustrative projection from your inputs, not market data or a forecast. Effective monthly interest and the selected contribution timing are modeled. Nominal values are future dollars; real values are today’s dollars. Taxes, fees and variable returns are excluded. Full method.
Formula reviewed by Associate Professor Charles Lo, CPA, PhD.
Educational projection only. Check the assumptions; results exclude taxes, fees and market volatility. Nominal results are future dollars; real results are today’s dollars. The inflation comparison applies only to nominal returns. Disclaimer
See this in the $1M simulator →View the detailed year by year breakdown
| Metric | Value | Notes |
|---|---|---|
| Starting Amount | $10,000.00 | Initial principal |
| Annual Rate | 7.00% | APR before compounding |
| Compounding | Yearly | Frequency |
| Years | 20 | Growth period |
| Additional Contributions | $48,000.00 | Excludes the starting amount |
| Total Deposits | $58,000.00 | Starting amount plus additional contributions |
| Dollar Basis | Future dollars | Selected nominal or real return basis |
| Total Interest | $82,204.12 | Growth from compounding |
| Final Value | $140,204.12 | Projected ending balance |
| Year | Start Balance | Additional Contributions | Interest Earned | End Balance |
|---|
What does my result mean?
You put in $58,000. Interest adds $82,204, which is 59% of your $140,204.
The share link includes your starting amount, return basis and rate, inflation assumption, years, contribution schedule and target. Save keeps those inputs in this browser only, with Preferences consent. The CSV contains your assumptions and annual projection.
What is compound interest?
Money makes money. Then that new money makes money too.
Put $100 in an account paying 10%. After a year you have $110. The next year you earn 10% of $110, not $100, so you get $11 and finish with $121. The extra dollar is interest on interest. It looks tiny. Over 20 or 30 years it becomes most of what you have.
Maria puts $10,000 in an index fund at 25 and never adds to it. At 7% a year she has about $76,000 at 55. Her friend Sam waits until 35 to invest the same $10,000: about $38,700 at 55. Same money, same fund; Maria’s ten extra years doubled the result.
- Shows what any amount grows to, for any rate and any number of years
- Splits the result into money you put in and money the interest made
- Tells you when your money doubles and when interest overtakes your deposits
- Lets you change one number at a time and see exactly what it changes
For anyone deciding whether to start investing now or later, how much to put away each month, or what a savings rate really means.
How much will my money grow with compound interest?
Three things decide it: how long it stays invested, the yearly rate, and what you add. $10,000 plus $200 a month at 7% grows to $140,204 in 20 years, and $82,204 of that is interest. Leave it 30 years and it reaches $310,013. Time is the input you control most.
What is the number telling me?
“Interest” in the result is money you never deposited. With the calculator’s starting numbers ($10,000 plus $200 a month at 7%) it is 59% of the total at 20 years and about 74% at 30. The box under the calculator recalculates this for your numbers.
72 divided by the rate is the years to double. At 7% that is about 10 years: $10,000, then $20,000, then $40,000, then $80,000.
One year the interest you have earned passes everything you put in. With the starting numbers that is year 16; the box under the calculator names the year for your own inputs.
How does compound interest work?
Each month the calculator adds that month’s interest to your balance, then adds your contribution if one is due. The new balance is next month’s starting point. That loop is the whole idea, and it is why money invested early is worth more than the same money invested late: $500 a month from age 30 reaches about $610,000 by 60 at 7% with interest added monthly; started at 40 it reaches about $260,000.
$10,000 at 7%, interest added yearly
| Year 1 | $10,000 × 1.07 | $10,700 |
| Year 2 | $10,700 × 1.07 | $11,449 |
| Year 3 | $11,449 × 1.07 | $12,250 |
| Year 20 | $10,000 × 1.0720 | $38,697 |
Year 3 earns $801, not $700, because it is 7% of $11,449. By year 20 the yearly interest is $2,531.
What changes the result most?
Your own numbers, changed one at a time.
| Change one thing | Result | Versus yours |
|---|
With the starting numbers, ten more years beats three more percentage points. The table above uses your inputs, so check whether it still holds for you.
What could make the real result different?
7% is an average. Real years swing from minus 20% to plus 30%. The return simulator shows a range.
A 1% yearly fund fee takes the default result from $140,204 to $122,759. Use the after tax calculator for a taxable account.
Choose Nominal for a return before inflation: the main result is in future dollars and the purchasing-power comparison divides it by (1 + inflation)years once. Choose Real only for a return already after inflation: the result and constant contributions are in today’s dollars, with no second adjustment. The default is a nominal 7% illustration with a separate editable 3% inflation assumption.
Compound interest FAQ
How does compound interest work?
Interest is added to your balance, then the next interest is worked out on the bigger balance. Early on most growth is your own deposits; later most of it is interest on interest. With the calculator’s starting numbers ($10,000 plus $200 a month at 7% for 20 years) you finish with $140,204, and $82,204 of that is interest.
What is the compound interest formula?
A = P × (1 + r/n)n×t. P is the starting amount, r the yearly rate as a decimal, n how many times a year interest is added, t the years. Regular contributions are added inside the same loop, month by month, which is what the calculator does.
What is the difference between simple and compound interest?
Simple interest is paid only on the original amount: $10,000 at 7% earns $700 a year, $24,000 after 20 years. Compound interest earns on the growing balance: $38,697 after 20 years. The $14,697 gap is interest on interest.
Does it matter if interest is added monthly or yearly?
A little. $10,000 at 7% for 20 years is $38,697 yearly, $40,387 monthly and $40,547 daily. With the default $200 a month added, switching from yearly to monthly moves $140,204 to $144,573. Rate and years matter far more.
Is 7% realistic?
7% is an illustrative assumption, not a forecast or guaranteed return. The default treats it as nominal (before inflation). If your chosen rate already accounts for inflation, select Real so it is not adjusted twice. Test several rates and time periods; actual returns vary.
Does this include tax and fees?
No. A 1% yearly fee takes the default result from $140,204 to $122,759. For a taxable account use the after tax calculator, or lower the rate by your fee.
How much a month to reach $1 million?
About $820 a month for 30 years at 7%, with interest added monthly and contributions at the end of each month. Use the “Road to $1 million” preset under the calculator to see the path.
Can I save or share my result?
Yes. Press “Copy link to this result” under the calculator. The link opens with every number you entered.
Where the numbers come from.
- Method: the calculator works month by month. Interest uses the effective monthly rate for the compounding you choose; contributions are added at the start or end of each contribution period.
- 7% example rate: an illustrative nominal assumption by default, not a market forecast. Select Real when entering an already inflation-adjusted return.
- 3% inflation: an editable illustration, not a forecast. A real-return result is not deflated again.
- Written by Felix La Spina, reviewed by Associate Professor Charles Lo, CPA, PhD. Last updated October 13, 2026. Editorial standards.
- Spotted an error? Tell us.
Give your projection a clear target.
Set an amount and a deadline, then calculate the contribution needed to reach them.
Sources and reference material
Primary sources and original reference material for the concepts discussed on this page.
- Investor.gov: Compound interest calculator and assumptions
- Investor.gov: Asset allocation, diversification and rebalancing
References added 14 October 2026.
