Solving for the Number You're Missing
Most compound interest calculators only run in one direction — you enter everything and get a future balance. This one works backward too: if you already know your target and timeline, it solves for the monthly contribution you'd need. If you know what you can realistically contribute, it solves for how long it will take instead.
Worked Example
Target: $500,000. Starting balance: $30,000. Timeline: 15 years. Expected return: 7%. Solving for the required contribution: about $1,308/month gets you there exactly on schedule.
If the Number Looks Unrealistic
A very high required monthly contribution usually means the timeline is too short for the goal size. Try extending the years, lowering the target, or checking how much a larger starting balance would change the math.
Frequently Asked Questions
How is this different from the Wealth Builder calculator?
Wealth Builder models growth across specific account types like HYSA, brokerage, and CDs with their own typical rates. This calculator is the simpler, generic version — it solves for whichever single number you're missing (contribution or time) using one overall expected return, without account-specific modeling.
Which should I solve for — contribution or time?
Solve for contribution if you have a fixed deadline (like a target retirement age) and need to know what to save monthly. Solve for time if you know what you can realistically contribute each month and want to know when you'll get there.
What if the required monthly contribution seems unrealistically high?
That usually means the timeline is too short for the goal. Try extending the years field, lowering the target amount, or checking whether a higher starting balance changes the math meaningfully.