Plan an investment around whichever variable you don't yet know. Enter your starting amount, return rate, contributions, and time horizon — then pick a tab to solve for the End Balance, the contribution required, the return rate required, the starting amount required, or how long it takes to reach a target. Every result comes with a balance breakdown and a full accumulation schedule.
What each tab solves for
| Tab | You provide | It solves for |
|---|---|---|
| End Amount | start, rate, contribution, length | the final balance |
| Contribution | start, rate, length, target | the contribution needed |
| Return Rate | start, contribution, length, target | the rate needed |
| Starting Amount | rate, contribution, length, target | the start needed |
| Invest Length | start, rate, contribution, target | the time needed |
Examples
$20,000 start · 6% · $1,000/month · 10 years
Starting with $20,000 and adding $1,000 at the end of every month for 10 years at a 6% return (compounded annually) grows to $198,290.40. Of that, $20,000 is your starting amount, $120,000 is contributions, and $58,290.40 is interest.
How much must I contribute to reach $300,000?
Same start, rate, and 10-year horizon, but targeting $300,000: the calculator's Contribution tab requires about $1,626.01 per month, paid at each month's end. Enter the target as End Balance and select monthly contributions and annual compounding. The solver uses an unrounded contribution internally; paying the displayed cent-rounded amount can leave a small final difference.
Formula
The balance grows each period and each contribution is added at the period's start or end. For a starting amount P, a per-period rate i, M periods, and a contribution C added at the end of each period:
End balance = P × (1 + i)^M + C × [ (1 + i)^M − 1 ] / i
The per-period rate i comes from your nominal return rate R: first the effective
annual rate for the compounding frequency, EAR = (1 + R/n)^n − 1, then the per-deposit
rate i = (1 + EAR)^(1/p) − 1 for p deposits per year (12 monthly, 1 yearly).
Compare a lower return assumption with a higher contribution. A required return tells you what the equation needs; increasing deposits is a separate scenario, and the tool does not establish which investments can deliver a rate.
Assumptions and limits
The calculator applies one constant nonnegative nominal return, regular contributions, and the selected timing for the full horizon. It cannot model negative-return periods, withdrawals, volatility, sequence risk, fees, taxes, or inflation. Return-rate mode searches from 0% to 100%; length mode finds the first whole contribution period that reaches the target, up to 100 years. Zero required contribution or starting amount means the other inputs already meet or exceed the target, rather than that a withdrawal has been calculated.
A required-return result tells you what rate would satisfy the equation; it does not say that rate is realistic or available. The SEC compound-interest tool is a separate reference for the core growth relationship. For separate deposits and withdrawals, use the Compound Interest Calculator.