This calculator runs the retirement-savings problem backward: instead of projecting a balance from a fixed contribution, it solves for the contribution needed to hit a specific target by a specific date.
The current savings are first projected forward on their own; whatever gap remains between that projection and the goal is what the monthly contributions need to fill, factoring in that those contributions also earn returns along the way.
Why the required amount is lower than a naive division
Simply dividing the goal by the number of months ignores that both the current savings and every contribution made along the way keep earning returns. The earlier a contribution is made, the longer it has to grow, so contributions made in year one do more work than contributions made in the final year. Over a long enough timeframe and at a reasonable rate of return, this effect can meaningfully lower the monthly amount actually required compared with simple division.
Sensitivity to the assumed rate of return
This calculation is more sensitive to the rate of return than people expect, especially over longer periods. A goal that requires $380/month at an assumed 7% return might require $460/month at 4% — the same goal, the same timeframe, a meaningfully different required monthly amount. Because future returns are never actually guaranteed, it is sensible to run this calculation at a conservative rate rather than an optimistic one, and treat the result as a lower bound on what you should actually be setting aside.
Adjusting the plan over time
This is a one-time calculation based on the numbers entered today. In practice, actual returns will differ from the assumption in any given year, so revisiting the calculation periodically — checking the current balance against where the plan assumed it would be — lets you catch a shortfall early and adjust the monthly contribution while there is still time to compound the correction, rather than discovering the gap only when the goal date arrives.