How the contribution is calculated
The calculator first grows your current savings on their own, compounding monthly at the return rate you set, and checks whether that alone clears the goal. If there's a gap left over, it solves for the level monthly contribution that closes that gap by your target date, assuming each contribution lands at the end of the month and compounds along with everything else. That's the standard formula behind any "how much do I need to save" question, whether it's a retirement fund, a house down payment, or an emergency fund.
How to read the results
"Save each month" is the level contribution that gets you from where you are to the goal by the deadline, assuming the return rate holds steady the whole way. The chart plots your projected balance year by year against a dashed line marking the goal itself, so you can see roughly when the two lines meet. "Total contributed" adds up your starting balance plus every monthly contribution; "interest earned" is the difference between that and the final balance, in other words, growth you didn't have to save yourself.
When you're already on track
If your current savings are projected to clear the goal on their own before the deadline, the required contribution is zero, and the calculator says so plainly instead of showing a flat "$0" for a plan you'd need to keep working. You can still keep contributing if you want a bigger cushion or an earlier finish.
Caveats worth keeping in mind
This model assumes a constant annual return compounded every month, which real markets never actually deliver. A savings account or CD is close to guaranteed but low-yield; a stock or fund portfolio might average a higher return over long stretches but will bounce around year to year, sometimes losing value for a while. The further out your goal, the more that variance matters. Treat the result as a planning estimate, not a promised outcome, and revisit it periodically as your actual returns come in.