I started with $10,000 and added $200 at the end of every month, at 7%, compounded monthly, for 10 years. I put in $34,000. The rate added $20,713.57. The balance is $54,713.57.
The first month is the one I can check by hand. One twelfth of 7% of $10,000 is $58.33, after half-up rounding to the cent. The $200 lands after that interest. The balance is $10,258.33. The $200 has not earned anything yet.
The page is Compound Interest Calculator.
The first month is $58.33
$10,000 times 7% is $700. A month is a twelfth of that, $58.333, and the page rounds half up to the cent, so the interest cell is $58.33. Then the contribution of $200 is added. $10,000 plus $58.33 plus $200 is $10,258.33.
That order is the whole product. Interest posts on the balance at the start of the month. The deposit that arrives at the end of the month waits. Next month, interest is one twelfth of 7% of $10,258.33, rounded the same way, and then another $200 arrives.
Month two is $59.84
The balance entering month two is $10,258.33. One twelfth of 7% of that is $59.84 after the same half-up step. Then $200 is added. $10,518.17 is month two. The CSV writes that balance on row 2 as the text 10518.17.
I write the second month down because the first month is too clean. $58.33 is 7% of a round $10,000, split into twelve, and it looks like a textbook. $59.84 is the same rule on a balance that is no longer round. If a calculator shows $59.8407 there, it has not chosen a cent yet. This page chooses the cent on every row, then carries that cent into the next row. The last row is the sum of those choices, not a fresh formula that ignores them.
A closed-form annuity, the one textbooks print for an ordinary monthly deposit, lands within a dollar of this 120-row table. I keep both. The formula is the check. The rows are what you can export. When they disagree by a few cents, the cents win, because that is the rounding you can see.
I kept the rounding on every row instead of running a float formula once at year ten and hoping the cents agree. A closed-form annuity on this plan lands within a dollar of the table. The table is the one the CSV copies.
The first month's interest is $58.33. The $200 lands after it. The balance is $10,258.33.
$34,000 is the part you can count on your fingers
$10,000 at the start. $200 times 120 months is $24,000. Together that is $34,000. The page calls that column "put in." It does not include interest.
If the rate box is 0, the ending balance is $34,000 and the interest column is $0.00. Timing does not matter at a zero rate, because there is nothing for an early deposit to earn. I use that row as the check that the deposits were counted and the interest was not invented.
The growth column on the 7% sample is $20,713.57. Add it to the $34,000 and you get $54,713.57. If those three numbers ever fail to add, the page is wrong. They add.
The rate is the number in the box
7% is not a market quote. I typed it. Change it to 5% and the same deposits end at $47,526.51, with $13,526.51 from the rate. Change it to 9% and they end at $63,216.48, with $29,216.48 from the rate.
The deposits stay $34,000 in all three. The only thing that moved is the growth. That is why the three rates sit on one table. A single ending balance looks like an answer. Three of them look like what they are: the arithmetic of three guesses.
| Rate | Ending balance | Growth | Today's dollars at 3% |
|---|---|---|---|
| 5% | 47526.51 | 13526.51 | 35364.19 |
| 7% | 54713.57 | 20713.57 | 40712.03 |
| 9% | 63216.48 | 29216.48 | 47039.00 |
None of those rows is a forecast. A year of 7% followed by a year of 2% will not match the 7% column. The page does not know next year's return. It knows the rate you typed and the months you asked for.
End of the month, then the deposit
On the sample, interest posts and then the $200 is added. The deposit does not earn the interest of the month it arrived in. It earns interest in later months, once it is already sitting in the balance.
That is the ordinary way a monthly plan is described, and it is the default on the page. The status line says so in one sentence, next to the ending number, so you do not have to scroll to find out which way the $200 was applied.
If your own account posts the deposit first and the interest second, switch the timing before you compare the table to a statement. The two timings are not the same number with a rounding error. On this sample they are $202.00 apart.
Beginning finishes $202.00 higher
Switch the same $10,000, the same $200, the same 7%, and the same 10 years to the beginning of each period. Each $200 is in the balance before that month's interest posts, so it earns the month. The ending balance is $54,915.57.
$54,915.57 minus $54,713.57 is $202.00. That gap is ten years of one month's interest on each $200, rounded to the cent along the way. It is not a fee and it is not a bonus the page invented. It is the difference between "the deposit is already there" and "the deposit arrives after the posting."
A statement that does not say which one it uses cannot be checked against either column. Ask the statement, then set the box.
Once a year is a different clock
Interest can post monthly, quarterly, or once a year. The contribution can arrive on its own clock. A monthly deposit with annual interest means eleven months go by with deposits and no interest posting. On the twelfth month the annual rate is applied to the balance, and the timing box decides whether that month's deposit is inside the balance or still outside it.
A contribution at the beginning of the last month of the year is already in the balance when the annual interest posts. A contribution at the end of that month is added after the posting. I say that on the page because "annual compounding" is easy to picture as "everyone earns a twelfth every month," and that is a different rule. This page posts the annual interest once.
Quarterly works the same way, every three months. The row count is still one row per month, so you can see the months where interest is $0.00 because nothing posted.
Today's dollars are the inflation box
Type 3% and the page divides each balance by 1.03 raised to the years so far. At the end of year 10, $54,713.57 becomes $40,712.03. The 5% scenario becomes $35,364.19. The 9% scenario becomes $47,039.00.
That division uses the percent you typed. It does not look up a price index, and it does not predict next year's prices. If you type 0, the today's-dollars column matches the balance column. If you type 3 and then 4, the column moves. The nominal balance does not, because inflation in this table does not change how the interest is computed. It only restates the ending pile in older dollars.
Past prices, from a published index, are a different question. The Inflation Calculator answers that one, month against month. This page will not pretend a typed 3% is that index.
The faint line is the money you put in
The chart has two lines. The faint one is the running total of the starting amount plus every contribution. On the sample it climbs in a straight stair from $10,000 to $34,000. The solid line is the balance, which starts at the same place and pulls away as interest posts.
If the solid line and the faint line stay together, the rate is zero or the term is one month. If the solid line is above, the gap is the growth column. I draw both because a single rising line hides whether you got there by saving or by the rate. $54,713.57 with $34,000 put in is a different story from $54,713.57 that was mostly the rate. The columns say which.
A withdrawal is not in the table. Once you start taking money out, the balance can fall, and this page does not model that. A loan, with a payment and extra principal, is the other direction. That table is the Amortization Schedule.
The cells are the cents, as text
The CSV writes 10258.33 for the first balance and 54713.57 for the last. The cells are text, so a spreadsheet does not turn them into a binary float and then round them again when you reopen the file. The header row names the columns: month, contribution, interest, contributed, balance, real balance.
There are 120 month rows on a 10-year plan, and 600 on a 50-year plan. Fifty years is the cap. The starting amount and the contribution each stop at $10,000,000. The rate stops at 30. The inflation assumption stops at 20. Past those, the box asks you to come back inside the range rather than printing a number I will not stand behind.
The calculation is the click. There is no progress bar, because 600 rows of cent rounding do not need one.
The link does not carry the dollars
The share link has the years, the rate, the inflation assumption, how often you add money, how often interest posts, the timing, and the two comparison rates. It does not have $10,000 and it does not have $200. The URL is on the page before you copy it.
This browser remembers those knobs under softery.compound-interest-calculator.settings. Clear saved setup removes them. Close the tab and the starting amount is gone either way. A ?v= on the end of the address does not replace a setup you already saved.
Mixpanel and Google Analytics load with Softery.io. They see that someone opened the tool. A download reports a size band and a row count. They do not get the amounts.
Open the sample before you type your own
- Open the compound interest calculator and click Use the sample.
- Read the status line. Ending $54,713.57. You put in $34,000.00. Growth $20,713.57. At 3% inflation, today's dollars are $40,712.03.
- Find $58.33 in the first interest cell and $10,258.33 in the first balance cell.
- Read the rate table. 5% ends at $47,526.51. 9% ends at $63,216.48. The deposits did not change.
- Download the CSV. The first balance is
10258.33. - If you copy the setup link, read it. Years, rates, frequencies, timing. No starting amount.
Then put your own numbers in. If a statement disagrees, believe the statement's timing and its rate, and use this page to see what those choices add up to. The ending number is the scenario you typed.
Last updated: September 22, 2026 | Reading time: 10 minutes
Written by Softery.io, which split $34,000 of deposits from $20,713.57 of growth and left both on the page.