How to use
Enter an initial amount, a monthly contribution, a nominal annual rate and a duration. Use 0 explicitly when there is no contribution or no assumed growth. Choose whether monthly contributions occur at the beginning or end of each month.
The calculator uses monthly compounding and whole-month durations. Years are accepted when they correspond to a whole number of months. All money uses one currency; selecting another currency changes the label without converting the amounts.
Calculation method
Let P be the initial amount, D the monthly contribution, n the number of months and r the nominal annual percentage rate divided by 1,200. For end-of-month contributions, future value is P × (1 + r)^n + D × ((1 + r)^n − 1) / r. Beginning-of-month contributions multiply the contribution term by one additional (1 + r).
At 0%, the balance is P + nD. A zero duration includes the initial amount only. Growth is calculated without rounding the balance to cents every month; the displayed ending balance rounds to cents, and displayed growth is the rounded balance minus contributions.
Worked example
With an initial 1,000, monthly contribution 100, nominal rate 12% and two months, r is 1%. End-of-month contributions give 1,221.10: 1,200 contributed and 21.10 growth. Beginning-of-month contributions give 1,223.11 because each contribution receives an extra month’s modeled growth.
For the longer example of 1,000 initially plus 100 monthly at 5% for ten years, the end-of-month projection is approximately 17,175.24, including 13,000 contributed.
Understand the assumption
This is a constant-rate mathematical projection, not guaranteed investment income or a bank statement. It excludes withdrawals, variable or negative rates, taxes, fees and inflation. An APY is not automatically a nominal annual rate: the tool shows its derived effective annual rate separately.
Use the percentage calculator to inspect a relative change, and the loan payment calculator for a reducing-balance repayment schedule.
For the measurement and planning details, read monthly compound growth: rates, contributions and timing.