How the math works
The key idea behind regular investing is that each contribution starts compounding the moment it is made. Money contributed earlier goes through more compounding periods, so the same total amount contributed over a different schedule can produce very different results.
This calculator assumes contributions at the start of each period with monthly compounding: a fixed amount goes in at the beginning of every month and compounds with the rest of the balance. That is close to how most monthly savings plans work. Real funds move in price every day, so your actual curve will not be this smooth.
Tip: be realistic about the annual rate you enter. It is an assumption for your own projection, not a forecast of future returns.
To avoid the most common mistakes in this kind of projection, read regular investing pitfalls. To understand why the curve keeps getting steeper, see what is compound interest.