How this is calculated
Without interest, it's simple: (goal − what you've already saved) ÷ number of months remaining. If you add an expected annual return, the calculator accounts for compounding on both your existing savings and each new monthly deposit, so the required monthly amount comes out a little lower the higher the rate you enter.
A note on estimates
Investment returns aren't guaranteed — treat the interest-adjusted number as an optimistic estimate, not a promise. For pure cash savings (a bank account), leave the rate at 0.
Track your progress
Outlay lets you set real budgets and see your actual monthly surplus, so you know if this target is realistic against what you actually spend.