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Compound Interest Calculator With Monthly Deposits
Run the numbers with monthly deposits and five compounding frequencies, and find out why the same inputs give different answers on different sites.
$10,000 to start plus $500 a month, for 10 years at a 6% annual return with monthly compounding, ends at $100,133.64: $70,000 you put in and $30,133.64 of interest, which is 30.1% of the final balance. That is the same number the SEC calculator at investor.gov returns for the same inputs, to the cent. This tool uses the same fields, the same five compounding frequencies and the same contribution rule, verified by comparing thousands of values against their server.
Change one thing, the compounding frequency, and the answer changes. That is the single most common reason two compound interest calculators disagree on identical numbers, and almost nobody explains it. The second table on this page shows exactly how much it moves.
* DENOTES A REQUIRED FIELD
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A worked example: $500 a month for 10 years
Same inputs as above: $10,000 to start, $500 every month, 6% a year, compounded monthly. Here is where the balance sits at the end of years 1, 3, 5, 7 and 10. Read the middle column as the part that is not your money.
| Year | You contributed | Interest earned | Balance |
|---|---|---|---|
| 1 | 16,000.00 | 784.56 | 16,784.56 |
| 2 | 22,000.00 | 1,987.58 | 23,987.58 |
| 3 | 28,000.00 | 3,634.86 | 31,634.86 |
| 4 | 34,000.00 | 5,753.81 | 39,753.81 |
| 5 | 40,000.00 | 8,373.52 | 48,373.52 |
| 6 | 46,000.00 | 11,524.87 | 57,524.87 |
| 7 | 52,000.00 | 15,240.66 | 67,240.66 |
| 8 | 58,000.00 | 19,555.70 | 77,555.70 |
| 9 | 64,000.00 | 24,506.94 | 88,506.94 |
| 10 | 70,000.00 | 30,133.64 | 100,133.64 |
The shape of that table is the whole point of compounding, and it is not linear. In year 1 the interest is $784.56, roughly 4.7% of the balance. By year 10 the interest is $30,133.64, or 30.1% of the balance. The contributions grow in a straight line ($6,000 a year, every year), the interest does not.
Notice also that most of the balance is still just your own deposits. Ten years of a 6% return turns $70,000 of deposits into $100,133.64. It is not a small difference, but it is not a transformation either, and any calculator that makes it look like one is either using a bigger rate or a longer horizon.
Why two compound interest calculators give different results
You type the same five numbers into two sites and get two different answers. Usually nothing is broken. The difference is in what each tool does with your monthly contribution when the compounding is not monthly, and most calculators never say.
The rule this calculator follows, the same one investor.gov uses: the monthly contribution is annualized (multiplied by 12) and then split evenly across however many periods there are in the year, and each slice is credited at the end of its period. With annual compounding, that means your twelve monthly deposits become a single $1,200 deposit landing on the last day of the year, so those deposits earn nothing at all in their first year.
Here is the same scenario run through all five frequencies. Nothing to start, $100 a month, one year, 12% a year. Only the frequency changes.
| Compounding | Periods per year | Deposit credited each period | Balance after 12 months |
|---|---|---|---|
| Annually | 1 | $1,200.00 | $1,200.00 |
| Semiannually | 2 | $600.00 | $1,236.00 |
| Quarterly | 4 | $300.00 | $1,255.09 |
| Monthly | 12 | $100.00 | $1,268.25 |
| Daily | 365 | $3.29 | $1,274.75 |
Same money in, same rate, same year, and the spread between the best and worst case is $74.75. The annual row is the one that surprises people: the balance is exactly what you deposited, $1,200.00, and the interest is zero. That is correct. Every deposit arrives at the end of the only period there is, so there is no time left for any of it to earn anything.
So when a calculator disagrees with this one, check three things in order:
- Compounding frequency. Some tools quietly force monthly compounding when you enter a monthly deposit, so their annual option behaves differently from the one here.
- Deposit timing. A calculator that credits the deposit at the start of each period gives a bigger number for identical inputs, because every deposit gets one extra period of growth.
- Contribution frequency. If the other tool lets you deposit quarterly or annually, it is answering a different question than “I save this much every month”.
The formula, and the three assumptions behind it
No black box. This is the entire calculation, and you can reproduce every figure on this page with it in a spreadsheet.
i = r / n N = n × t pmt = A × 12 / n FV = P (1 + i)^N + pmt × [ (1 + i)^N - 1 ] / i
- P, what you start with. A, your monthly contribution. r, the annual return as a decimal (6% is 0.06). t, years.
- n, periods per year: 1 annually, 2 semiannually, 4 quarterly, 12 monthly, 365 daily.
- i, the rate per period. N, the total number of periods. pmt, the deposit credited in each period. FV, the future value.
Three assumptions are baked in, and they are the ones worth knowing:
- Daily compounding uses a fixed 365 days. No leap years, no 360 day banking convention. Your monthly deposit becomes 1,200 divided by 365 per day, which is $3.29 in the example above.
- The deposit is credited at the end of every period, never at the start. This is the ordinary annuity convention, and it is the conservative one.
- Every point comes straight from the closed formula. Year 5 is not built by stepping through year 4’s rounded balance, so no rounding error is carried forward and the table cannot drift by a cent or two over long horizons.
What this calculator does not tell you
The arithmetic here is exact. What it is arithmetic about is a guess, and the honest version of this page has to say so.
- The return is an assumption you typed in, not a forecast. Real returns do not arrive as the same percentage every single period, and a bad first few years hurts far more than the same bad years at the end. That is what the variance field is for: run your number 2 points lower and treat that as the plausible case, not the pessimistic one.
- It does not discount inflation. $100,133.64 in ten years will not buy what $100,133.64 buys today. If you want a rough real figure, enter your return minus expected inflation instead of the nominal return.
- It does not subtract taxes or fees. Fund fees, account fees and spreads all come out of the return before you see it. On the tax side, rules differ by where you file, and the timing matters as much as the rate: in the United States, interest is generally taxed as ordinary income in the year it is credited to you, not when you finally withdraw, so a taxable account grows slower than any untaxed projection. Other markets have their own allowances and tax free wrappers that change the answer completely. Check the rules where you actually pay tax.
One more thing, plainly: AI Money does not invest anyone’s money and does not recommend where to put it. We do not hold funds, we are not a broker and we have no product to steer you toward. This is a calculator, and AI Money is an app for keeping track of your own accounts. Where to invest is a decision for you, or for a licensed advisor in your country.
The projection is the easy part
A calculator tells you what $500 a month becomes. The hard part is actually putting the $500 aside, month after month, and knowing whether you did.
AI Money is a personal finance app for iOS and Android with savings pockets, so a goal is a real balance you can watch move instead of a line in a projection. The free plan is free permanently and includes 3 pockets, and you are never required to connect a bank account. You can log a transaction by speaking it, by photographing the receipt, or by sending it to the app on WhatsApp, which takes about as long as ignoring it would.
Almost always because of how each one handles a monthly contribution when the compounding is not monthly, and because of whether the deposit is credited at the start or the end of each period. This calculator annualizes the monthly contribution, splits it evenly across the periods in the year and credits each slice at the end of its period. A tool that credits deposits at the start of the period will always return a higher balance for identical inputs.
Yes, to the cent. It uses the same input fields, the same five compounding frequencies and the same contribution rule, and the match was verified by comparing thousands of values against their server rather than by eyeballing one example. AI Money is not affiliated with or endorsed by the SEC. If you ever see a difference, the SEC calculator is the reference and we want to hear about it.
It gets annualized into one deposit credited on the last day of each year. Twelve deposits of $100 become a single $1,200 deposit at the end of year 1, which is why that first year earns exactly zero interest and the balance is $1,200.00. The same $100 a month at 12% compounded monthly ends the year at $1,268.25, and compounded daily at $1,274.75.
At the end, always. This is the ordinary annuity convention and it is what investor.gov uses. In practice it means each deposit earns interest starting from the period after it lands. If your real savings plan deposits on the first of the month, your actual balance will run slightly ahead of this projection.
A fixed 365, with no adjustment for leap years and no 360 day convention. Your monthly contribution is divided as 1,200 per year over 365 days, so $100 a month becomes $3.29 a day (the calculation keeps the unrounded value). Note that daily compounding buys less than people expect: at 12% for a year it beats monthly by about $6.50 on a $1,200 deposit stream.
No, none of the three. The figure is a nominal projection before tax and before costs, so treat it as a ceiling rather than an expectation. To approximate a real, after inflation number, enter your return minus expected inflation. For tax, check the rules where you file: the timing of when interest becomes taxable varies by country and can change the outcome considerably over ten years.
Keep going
- All AI Money calculators and tools
- Credit card minimum payment calculator: work backwards from the number you need
- The AI Money blog: saving, budgeting and getting the numbers right
- The SEC compound interest calculator on investor.gov, the reference this tool is built to match
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Updated August 14, 2026
