Toggle between projecting an end amount and solving backward for the starting amount or monthly contribution a target goal requires.

How to Use the Investment Calculator

Set the annual return and years first — both carry across all three modes. End Amount mode then asks for a starting amount and monthly contribution and projects where they land in the future; Starting Amount mode and Monthly Contribution mode each swap one of those out for an end amount goal and solve backward for whichever figure is missing.

Every version of this calculator runs on the same two building blocks, just pointed in different directions. A growth factor tells you what a dollar invested today becomes by the end of the period; an annuity factor does the same job for a dollar contributed every month along the way.

Growth Factor = (1 + r)ⁿ
Annuity Factor = (Growth Factor − 1) ÷ r
Future Value = Starting Amount × Growth Factor + Monthly Contribution × Annuity Factor

r is the monthly return (annual rate ÷ 12) and n the number of months. End Amount mode stops there — plug in a starting amount and contribution, read off the projection. Run the defaults — $10,000 to start, $300 a month, 7% a year for 20 years — and it lands just under $196,700.

The other two modes rearrange that same equation to isolate a different variable:

Starting Amount = (Goal − Monthly Contribution × Annuity Factor) ÷ Growth Factor
Monthly Contribution = (Goal − Starting Amount × Growth Factor) ÷ Annuity Factor

Push the numbers far enough and one of those can come out negative — those same defaults aimed at a $200,000 goal in Monthly Contribution mode only ask for about six dollars more than the default $300 a month, and a bigger contribution or longer horizon could clear the goal on its own. Rather than show a negative starting amount or contribution, the calculator floors the result at zero and flags it with a note.

Total Contributions and Total Growth appear underneath the headline result in every mode. Contributions is just the cash you supplied yourself; Growth is what's left once that's subtracted from the goal or projected total — the part the market did on its own, and usually the more interesting of the two once compounding has had a couple of decades to work.