Compound Interest Calculator with Monthly Contributions
Project the future value of savings or an investment with compound interest and optional monthly contributions.
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About this calculator
Compound interest means you earn interest on your interest. Over long periods this snowball effect is the main driver of savings growth, which is why starting early and contributing steadily matters more than chasing high returns.
How it works
Balancet+1 = Balancet × (1 + rm) + monthly contribution
rm = (1 + annual rate ÷ k)k/12 − 1
k = compounding periods per year. The calculation runs month by month, so contributions are handled exactly rather than by an approximate closed-form formula.
How to read your result
Test the effect of time: doubling the number of years usually does far more than doubling the monthly deposit. Also try a lower rate: investment returns are never guaranteed, so plan with conservative assumptions.
Worked example
10,000 invested at 7% per year, adding 500 every month for 20 years, compounded monthly.
| Future value | 300,851 |
| Total contributed | 130,000 Starting amount + monthly deposits |
| Interest earned | 170,851 57% of the final balance |
Frequently asked questions
What is compound interest?
Interest calculated on both the original principal and the accumulated interest from earlier periods.
Does compounding frequency matter?
Slightly. Monthly compounding earns a little more than annual at the same quoted rate, but the effect is far smaller than the effect of time and contributions.
What is the Rule of 72?
Divide 72 by the annual rate to estimate years to double your money. At 7%, about 10 years.
Is this a guarantee of returns?
No. It is a projection with a constant rate. Actual returns vary and can be negative.
Limitations and when not to use
Educational projection only, not investment advice. Ignores inflation, fees and taxes.
Sources and references
- Standard compound interest and future-value-of-annuity mathematics.
Last updated: 2026-10-09