How to Calculate Monthly Compound Interest (Formula + Examples)
Learn the monthly compound interest formula, check the r/12 convention and compare deposits with a transparent calculator example [2026].
Use the monthly rate and the number of monthly periods: A = P x (1 + r/12)^(12t) + PMT x ((1 + r/12)^(12t) - 1) / (r/12). With $10,000 initially, $500 at each month-end, a nominal 8% rate and 30 years, the calculator returns $854,537 before fees, taxes or inflation.
A projection convention in which a nominal annual rate is converted to an equivalent monthly growth factor and applied each month.
Example: At 8% nominal compounded monthly, the monthly factor is about 0.6667% and the effective annual rate is about 8.30%.
The future value of equal deposits made at regular intervals. The formula below assumes deposits at the end of each month.
Example: At $500 per month, 8% nominal and 30 years, the deposits alone grow to about $745,180 under this convention.
The annual rate supplied to the formula before the selected compounding frequency is applied. It is different from an account APY.
Example: If a bank publishes an APY, do not also divide that APY by 12; enter a compatible nominal rate or use the account terms directly.
Monthly compound interest has two parts when you make deposits: the starting balance and the future value of each monthly contribution. Use a nominal annual rate r, monthly periods n = 12 and years t. The formula is A = P x (1 + r/12)^(12t) + PMT x ((1 + r/12)^(12t) - 1) / (r/12). This page uses end-of-month deposits, the same convention as the calculator.
Worked example: $10,000 plus $500 each month
Assumptions: $10,000 starting balance, $500 at each month-end, 8% nominal annual rate, monthly compounding, 30 years, no taxes, fees or inflation adjustment. The starting balance grows to $109,357; the deposits total $180,000 and grow to $745,180; the projected balance is $854,537. These are arithmetic scenarios, not a promised investment return.
The three steps
- Convert the annual nominal rate to a monthly rate: 8% / 12 = 0.6667% per month.
- Count the periods: 30 years x 12 = 360 monthly periods.
- Calculate the starting balance and the contribution stream separately, then add them and compare the result with total contributions.
Why the result depends on the inputs
The example is sensitive to the rate, deposit timing, frequency and time horizon. At 7% nominal, $500 at each month-end for 30 years is about $609,985; at 10% it is about $1,130,244. The rate is an assumption supplied by the user, not a forecast. Investor.gov explains that investments do not have a set rate of return and involve market risk.
Monthly, daily and annual frequency
For a $10,000 lump sum at 7% nominal over 20 years, this calculator convention gives about $38,697 with annual compounding, $40,387 with monthly compounding and $40,547 with daily compounding. The difference is a property of the selected rate and frequency; for a real account, use the institution's stated APY or crediting rules.
When monthly is a useful model
- Use monthly periods when deposits arrive monthly and the product terms support that approximation.
- For a bank account or CD, prefer the published APY, interest-crediting schedule and withdrawal terms over a generic frequency assumption.
- For stocks and funds, returns are uneven and not literally credited monthly. Treat a constant-rate projection as a scenario and include a range of outcomes.
- Separate nominal and inflation-adjusted results. A nominal balance does not show what the money will buy in the future.
Common mistakes
- Dividing an APY by 12 and then compounding it again. APY already reflects the annual effect of compounding.
- Mixing a real return with nominal deposits without stating the inflation assumption.
- Using an investment return as if it were guaranteed. Market investments can lose value and do not have a fixed return.
- Ignoring fees, taxes, contribution timing or changing deposits when comparing the projection with a real account.
How to use our calculator
Open the <a href="/monthly-compound-interest-calculator">monthly compound interest calculator</a>. Enter the starting amount, monthly contribution, nominal annual rate, years and frequency. Check the assumptions beside the output, then run a lower and higher rate to see how sensitive the result is.
$500 at each month-end: nominal-rate scenarios over 30 years
Projected balance with no starting amount, constant $500 monthly deposits and monthly compounding. These are arithmetic scenarios; rates are not forecasts.
| Dimension | Nominal rate | 10 years | 20 years | 30 years |
|---|---|---|---|---|
| 4% | $73,625 | $183,387 | $347,025 | |
| 6% | $81,940 | $231,020 | $502,258 | |
| 7% | $86,542 | $260,463 | $609,985 | |
| 8% | $91,473 | $294,510 | $745,180 | |
| 10% | $102,422 | $379,684 | $1,130,244 |
Frequently asked questions
What is the formula for monthly compound interest?
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How much does $500 a month grow over 30 years at 8%?
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Should I use monthly or daily compounding?
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Does the S&P 500 compound monthly?
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How does inflation change the result?
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What if I increase contributions over time?
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What is the effective annual rate for 8% nominal compounded monthly?
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Sources & further reading
Plug in your own amounts with our free calculators.