Compound Interest Calculator with Monthly Contributions
See how an initial amount plus regular monthly contributions grows with compound interest, and how much of the final balance is interest.
This calculator assumes a fixed rate of return; real investment returns vary and are not guaranteed. See our Disclaimer.
The formula
The starting balance grows as P × (1 + i)^m, where i is the equivalent monthly rate and m is the number of months. Each monthly contribution C forms an annuity worth C × ((1 + i)^m − 1) ÷ i at the end, multiplied by (1 + i) if contributions are made at the start of the month. The final balance is the sum of the two.
For monthly compounding, i is the annual rate divided by 12. For other frequencies, i is the monthly rate that gives the same yearly growth, (1 + r ÷ n)^(n ÷ 12) − 1.
Worked example
Start with 10,000, add 100 at the end of every month for 10 years at 5% compounded monthly. The 10,000 grows to 16,470.09 and the contributions grow to 15,528.23, so the final balance is 31,998.32. You contributed 22,000 in total, so interest earned is 9,998.32.
Limitations
It uses one fixed rate for the whole period and ignores taxes, fees and inflation. Contributions are level; it does not model raises in your monthly deposit.
How the compounding frequency is used
Deposits always happen monthly, whatever frequency you pick. The frequency only changes the monthly growth rate: monthly compounding uses the annual rate ÷ 12, and annual, quarterly or daily compounding uses the monthly rate that gives the same growth over a year as that compounding schedule. So the contribution schedule and the compounding schedule are set independently.
Comparing the options with the same inputs
10,000 to start, 100 a month for 10 years at 5%, all compounded monthly with deposits at the end of the month: 31,998.32. Deposits at the start of each month: 32,063.02, which is 64.70 more because every deposit earns one extra month. Compounded annually instead of monthly: 31,725.26. Compounded daily: 32,023.26. The gap between annual and daily compounding is exactly 298.00 after ten years, small next to what the deposits themselves add.
Saving from zero: 200 a month at 6% for 20 years grows to 92,408.18. You pay in 48,000 and interest earned is 44,408.18, about 48% of the final balance.
Edge cases
- Time is rounded to whole months. 2.5 years is 30 months. A time under half a month, such as 0.04 years, rounds to 0 months and returns zeros with no warning.
- A rate of 0 is accepted: 100 a month for 10 years is 12,000 with 0.00 interest.
- Leaving the monthly contribution empty shows an error, but entering 0 works and makes this a lump-sum calculator: 5,000 at 5% for 10 years gives 8,235.05.
- Negative amounts, a negative rate or a time of zero or less show "Please enter positive values."
Guide
- Compounding Frequency Compared: Annual to Daily — Worked examples of how much the compounding frequency changes a balance, why the gain flattens out, and when it matters more than it looks.
Frequently asked questions
Why does the end-of-month and start-of-month result differ?
A contribution made at the start of the month earns one extra month of interest, so the final balance is slightly higher.
Does this work in any currency?
Yes. The calculator is currency-neutral: enter amounts in whatever currency you use and the results are in the same currency.
How is this different from a lump-sum compound interest calculator?
A lump-sum calculator grows a single starting amount. This one also adds a regular monthly deposit, which is how most savings and retirement accounts work.
How much of my final balance will be interest?
It grows with time. In the 200-a-month example at 6%, interest is about 48% of the balance after 20 years. After 10 years the same deposits give 32,775.87, of which interest is 8,775.87, about 27%.
Does the compounding frequency matter much when I deposit monthly?
Less than the time and rate do. For 10,000 plus 100 a month at 5% over 10 years, annual and daily compounding differ by 298.00 on a balance of about 32,000.
Can I use a part of a year?
Yes. The time is multiplied by 12 and rounded to the nearest whole month, so 7.5 years is 90 months.