Compound interest calculations become more useful when they show how much you contributed and how long each deposit had to grow. Our compound interest calculator with monthly contributions separates deposits from modeled growth and lets you compare contribution timing.
Start with the meaning of the rate
A nominal annual rate divided into monthly periods is different from an effective annual yield. At a nominal 12% compounded monthly, the monthly rate is 1% and the effective annual growth factor is 1.0112, approximately 1.126825. A 12% effective annual yield instead has monthly equivalent (1.12)1/12 − 1.
In the calculator, choose annual compounding when treating an entered rate as an effective annual yield. For other frequencies, the input is nominal. The model uses equivalent monthly growth for fractional periods; bank posting and daily-balance methods can differ.
Worked end-of-month example
Begin with 1,000, add 100 at the end of each month, and use 12% nominal interest compounded monthly for one year. Month one ends at 1,000 × 1.01 + 100 = 1,110. Month two ends at 1,110 × 1.01 + 100 = 1,221.10.
After 12 months, the balance is approximately 2,395.08. Your contributions total 2,200, so modeled growth is 195.08. The final deposit receives no growth during that final month because it arrives at the month’s end.
Move the deposit to the beginning
Now add the monthly 100 before growth. Month one becomes (1,000 + 100) × 1.01 = 1,111. Over the same year, the ending balance is about 2,407.76. The 12.68 difference comes from each contribution receiving one extra month of growth.
This also explains a common calculation error: a deposit made late in a year should not automatically receive a whole year’s interest merely because the account uses annual compounding.
Use scenarios rather than a promised return
Try a lower rate, a shorter horizon and a smaller contribution. At 0%, the ending balance should equal starting money plus deposits; this is a useful check. Market investments do not produce a constant guaranteed return, and fees or taxes can reduce the amount retained.
The Investor.gov compound interest tool is an official reference for exploring savings inputs. Our worked examples above describe the specific monthly timing used on MG Calculator.
For purchasing-power comparisons, use the retirement savings calculator with inflation. Its withdrawal figure is an illustration, not a guarantee of sustainable retirement income.
Content reviewed on 9 October 2026. Worked examples are hypothetical. The original article URL is retained.