The four questions people actually ask
Almost every investing question is one of the same four, asked with a different variable missing. Given what I can start with, add monthly and expect to earn, what will I end up with? That is the everyday projection, useful for sanity-checking a plan already in motion. Given a target and a deadline, how much must I save each month? That is the one to reach for once a goal has a firm date attached, a house deposit or a fixed retirement year among them. Given a target and what I can already save, how long will it take? That suits an open deadline, where the amount matters more than exactly when it arrives. And given a target, a timeline and what I can save, what return would I actually need? That last one is less a plan than a reality check: it turns a hope into a number you can judge against how markets have actually behaved.
This calculator is built around that choice rather than around one of the four questions alone. The seg control above the form picks which variable is missing; the other four stay as inputs, and the fifth is worked out properly, not guessed at or left for you to iterate towards by hand.
The formula behind every mode
All four modes share one formula: a starting amount compounding on its own, plus a monthly contribution added at the end of every month and left to compound for whatever time remains.
Contributions here are always monthly, so the calculator works entirely in months and turns whatever annual return you enter into a monthly rate by plain division, r = annual rate ÷ 12. That is the standard reading of a nominal annual rate, the same division used throughout personal finance, and it means a return compounds monthly here and nowhere else; there is no separate compounding-frequency choice to make. When r works out to exactly zero the divide-by-r term above is undefined, so the calculator switches to plain addition instead, FV = P + C × n, with the same fallback used in every mode.
Solving for the other four ways
End amount needs nothing more than substitution: every other variable is known, so the formula above is evaluated directly. Monthly contribution needed is solved by rearranging the same formula algebraically, moving every other term to one side until the contribution stands alone:
Years needed looks harder, since n sits inside an exponent, but it still has a genuine closed form. Multiplying through and isolating the exponential term turns the problem into one a logarithm solves outright:
That works whether or not there is a monthly contribution in the mix, not only for a lump sum left to grow alone. Return needed is the one exception: r appears both inside and outside a power in the same equation, and no algebraic rearrangement isolates it. The calculator instead searches for it, narrowing a bracket between 0% and 30% a year, checking the midpoint against the target and discarding whichever half cannot contain the answer, a hundred times over until the bracket is far narrower than a rounding error. It is the same bisection approach this site's interest-rate calculator uses to recover an implied APR from a loan payment, applied here to a savings return instead of a borrowing cost.
When the return needed is not a realistic one
Solving for a return does not make that return likely. Diversified stock market indices have historically averaged somewhere around 7 to 10% a year before inflation over long multi-decade stretches, built from individual years that swing far above and far below that average in either direction. A plan that needs a sustained return past that range, year after year, is not a plan that has found a better investment; it is a plan that needs a bigger monthly contribution, a longer timeline, or a smaller target instead. This calculator still shows a required return above that range rather than refusing to, but it flags it for exactly that reason, and shows plainly when even a generous 30% ceiling still is not enough to reach the target as entered.
One honest note on inflation
Every figure this calculator shows is nominal, the balance an account would actually display without adjusting for rising prices. What that balance can actually buy in the future is smaller than the number itself, so treat these results as what the account will hold, not what it will be worth.
Reading the chart, the table and the plan
Whichever mode is running, the results settle back into the same shape: a headline answer, then the full plan behind it. The chart draws two lines from year zero to the end of the plan, a solid line for the balance and a dashed line for total contributions with nothing added by growth; the widening gap between them is growth doing its work. In the three modes built around a target, an amber marker adds a third reference line so you can see how the balance climbs to meet it, not just that it eventually does. The year-by-year table underneath carries the same numbers row by row, thinning itself to a readable set of years on very long plans while always keeping the final row, and a small sensitivity table beside it shows what moving the one variable each mode holds fixed, years in three modes, the assumed return in the fourth, would do to the answer a couple of steps either side of yours.
Which calculator actually fits your question
This page is built around choosing the unknown rather than assuming it. If the question is always "what will my starting amount and monthly top-up grow to", with a compounding-frequency choice and a doubling-time check built in, the Compound Interest Calculator is the more direct fit for that single, everyday framing. Reach for this page instead the moment the question changes shape: not just where a plan ends up, but what it would take to make it hit a number you have already decided on.
Questions people ask
Which of the four modes should I use?
Pick whichever variable you do not already know. If you have a savings pattern in mind and want to see where it leads, solve for the end amount. If you already have a target and a deadline, solve for the monthly contribution or, if the deadline is flexible, the years needed instead. If you know what you can save and by when, solve for the return needed and judge whether that return is realistic.
Why does this calculator only compound monthly?
Contributions here are monthly, so working entirely in months keeps the contribution and the growth on the same clock without a separate compounding-frequency setting to reconcile. An annual rate is turned into a monthly one by simple division, annual rate divided by 12, the standard reading of a nominal annual rate.
How is the required monthly contribution worked out?
The future value formula is rearranged algebraically to put the monthly contribution on its own on one side, using your starting amount, return and years as knowns and the target as the amount to reach. It is a direct rearrangement, not a search, so it lands on the exact figure needed in one step.
Why does the return needed take longer to work out than the years needed?
Years needed can still be solved directly, using a logarithm, because the growth in the formula is exponential and logarithms undo exponents cleanly. Return needed has no such shortcut: the rate sits both inside and outside a power in the same equation, so the calculator instead searches for it, narrowing a bracket between 0% and 30% a year until the two sides of the formula match.
What if the return I would need is unrealistically high?
The calculator still shows it, but flags it. Diversified stock market indices have historically averaged somewhere around 7 to 10% a year before inflation over long stretches, with plenty of individual years far above or below that. A required return well past that range is a sign to save more, extend the timeline, or lower the target, not a return to bank on.
Does this account for inflation?
No, every figure here is nominal, the growth an account would show without adjusting for rising prices. A pound or dollar in the future buys less than one today, so treat these figures as what the account will hold, not what that amount is actually worth once inflation is stripped back out.
What if my target is already covered by my starting amount or contributions alone?
In contribution and return modes, the calculator says so plainly rather than asking for a contribution or a return of more than nothing when even less would still work. In return mode specifically, if contributions alone add up to the target or more, even a small loss could still reach it, so the honest answer is that no positive return is required at all.
This calculator projects growth at a constant assumed return for illustration. It is not financial advice. Real investments compound unevenly, may charge fees, and may be subject to tax; speak to a regulated financial adviser before making investment decisions.