Compound Interest Calculator
The rate is the one you type. The ending number is a scenario, not a promised return. Contributions and growth stay in separate columns.
Put in a starting amount, or open the sample. The sample is $10,000 plus $200 a month, at 7%, for 10 years.
Share the setup
This link carries the years, the rates, how often money goes in, how often interest posts, and whether the contribution is at the beginning or the end. It does not carry the starting amount or the contribution.
/tools/compound-interest-calculator?years=10&rate=7&inflation=3&contrib=monthly&compound=monthly&when=end&also=5%2C9
$34,000 in, $20,713.57 from the rate
The sample starts at $10,000 and adds $200 at the end of every month, at 7%, compounded monthly, for 10 years. You put in $34,000. The rate adds $20,713.57. The balance is $54,713.57.
The first month's interest is $58.33, one twelfth of 7% of $10,000, rounded half up to the cent. The $200 lands after that interest, so the balance is $10,258.33. The $200 does not earn interest until a later month.
Two other rates, same deposits
The same $10,000 and the same $200 a month, at 5%, end at $47,526.51. At 9% they end at $63,216.48. The deposits do not change. The growth does. Those three rates are three scenarios. None of them is a forecast.
A contribution at the start of the period
Switch the timing to the beginning and the same 7% plan ends at $54,915.57, which is $202.00 more. Each $200 is in the balance before that month's interest posts. A zero rate ignores the timing. $10,000 plus $200 times 120 months is $34,000, and the interest column is $0.00.
Today's dollars are your assumption
If you type 3% inflation, the page divides the ending balance by 1.03 raised to the tenth power. $54,713.57 becomes $40,712.03. That line moves when you change the inflation box. It does not call a price index. Past dollars, from a published index, belong on the Inflation Calculator. A loan going the other way, with extra payments, belongs on the Amortization Schedule.
Questions
Is the ending number a promised return?
No. The 7% in the sample is the number in the rate box. Change it and the ending balance changes. Markets do not owe you that rate. The page runs the arithmetic and stops.
Why does the beginning contribution finish higher?
A contribution at the beginning is already in the balance when interest posts, so it earns this period. A contribution at the end is added after the posting, so it waits. On the sample, $10,000 plus $200 a month at 7% for 10 years, the beginning timing finishes $202.00 higher than the end timing.
What does today's dollars mean here?
It is the ending balance divided by your inflation assumption, compounded over the years. At 3% for 10 years the sample's $54,713.57 becomes $40,712.03. That is not a CPI lookup. The Inflation Calculator is the page that uses the price index for a past month.
Do the amounts leave this tab?
The starting amount and the contribution stay in the tab. Close it and they are gone. This browser remembers the years, the rates, the frequencies, and the timing, under softery.compound-interest-calculator.settings. Clear saved setup removes that. The share link carries those knobs and not the dollars. Mixpanel and Google Analytics load with Softery.io. They see that someone opened the tool. They get a size band and a row count. They do not get the amounts.