How to use it
Enter your starting balance and how much you can add each month, then drag the sliders for your expected annual return and how many years you will stay invested. The projected balance updates instantly, and the line underneath shows how much you contributed versus how much came from growth.
The math behind it
This calculator compounds monthly and adds your contribution at the end of each month. For every month in the horizon it does two things:
- Multiply the current balance by (1 + annual return ÷ 12) to add that month's growth.
- Add your monthly contribution to the balance.
Repeating that for every month is what makes the curve bend upward: early contributions have the most time to compound, which is why starting sooner beats contributing more later. The "growth" figure is simply the projected balance minus everything you put in.
Why the early years matter most
Because each month's growth is calculated on the whole balance, the dollars you invest first do the most work — they compound the longest. That is the entire argument for starting early even with small amounts, and it is also why fees that skim a percentage every year do so much damage over decades.
Frequently asked questions
Does it include monthly contributions? Yes — it models a starting balance plus a recurring monthly deposit, not just a one-time lump sum.
Are the returns guaranteed? No. Real returns vary year to year; this is a projection to compare scenarios, not a promise.