How Much Must You Invest Each Month for a Future Goal?

The calculation, and the number it hides

The required monthly contribution comes out of four inputs: what the goal will cost on the day it arrives, what is already saved towards it, how long there is, and what the money will earn along the way.

Three of those are knowable to a useful approximation. The cost can be researched and inflated — the method is here. The existing assets are a fact. The date is a decision.

The fourth is a guess about the future, it is the one input nobody can check, and because it is compounded over the whole period it does more to the answer than the other three combined. A calculator that returns a figure to the rupee is presenting a guess with three decimal places of confidence attached.

So the useful question is not what the required contribution is. It is how much the answer moves when the guess is wrong, and that can be measured rather than asserted.

How wrong the guess can be

Take a saver who invested a fixed amount every month into the Indian index and held for fifteen years, then sold and paid capital gains tax. Run that over every fifteen-year period in the record — 147 of them, one starting each trading day.

The middle period returned +12.46% a year after tax. But the middle eight periods in ten spanned +10.85% to +14.77%, and across the whole record the range ran from +6.22% to +16.45%.

That spread is the thing the calculator conceals. Two savers doing exactly the same thing, for the same fifteen years, differing only in which fifteen years they got, ended up a long way apart. And because these are annualised figures compounding over fifteen years, a gap of a few percentage points a year is a very large gap in the final amount.

The required-contribution figure is therefore not a target that guarantees the goal. It is the contribution that would have worked if the return had been exactly the assumed one, which is the one outcome that will not happen.

Two limits on those numbers, both of which matter. The periods overlap heavily — thousands of fifteen-year windows drawn from twenty-seven years of history are nowhere near thousands of independent observations. And this is one market over one stretch, described more fully in the uncertainty of long-term equity returns.

What to do with a range instead of a number

The response is not to abandon the calculation. It is to run it three times and read the answers differently.

Compute the required contribution at a pessimistic return, a central one and an optimistic one. The spread above is a reasonable guide to how far apart those should be for a goal funded from equity; for a goal funded from something stable, the spread is narrower and the exercise is quicker.

Then look at the three contributions, and the useful finding is which of two situations Kavita is in.

If the three figures are close together, the return assumption barely matters. She can fund the central one, stop thinking about it, and get on with the rest of her life. This happens when the horizon is short or the goal is largely funded already.

If they are far apart, she has learned that this goal's outcome is mostly determined by something nobody controls. That is not a reason to give up; it is a reason to fund closer to the pessimistic figure if she can, to review more often, and to identify in advance which part of the goal flexes if the poor case arrives.

When the answer is unaffordable

The calculation frequently produces a contribution the household cannot make, and this is where the real decision is.

There are five honest levers and they are all uncomfortable. Contribute more now. Contribute more later, through a step-up tied to income rises — but committed rather than merely intended. Extend the date. Reduce the goal. Or fund part of it from somewhere else entirely.

There is a sixth thing that is not a lever, and it is the one most often reached for: raising the assumed return until the contribution becomes affordable. This changes the number on the screen and nothing about what Kavita will actually have. Worse, it usually arrives with a change in the portfolio to justify it, which means she has taken on more risk in order to make a spreadsheet agree with her budget — and the additional risk is concentrated in the outcomes where the goal was already going to fail.

For Kavita, starting seriously in her forties, this trap is particularly live, because the honest arithmetic of a shorter horizon is genuinely less forgiving and the temptation to close the gap with an assumption is correspondingly stronger. The honest answer may be that the goal changes, and that is a legitimate outcome rather than a failure.

Details that change the answer more than they should

Three mechanical points, each of which will make two calculators disagree.

Whether contributions are made at the start or the end of the period. Start-of-period payments get an extra period of growth each, and over a long horizon the difference is not trivial.

Whether the figure quoted is before or after tax and costs. A goal is funded from what is left after both, and a required contribution computed on gross returns is too small.

And whether a step-up is assumed. A calculation that quietly assumes contributions rise every year produces a much smaller starting figure than one that does not, and if the increases do not actually happen the plan fails late, when there is no time left to correct it.

Test it against the things that go wrong

Before accepting a contribution figure, it is worth checking that the plan survives the ordinary bad year rather than only the expected one.

What happens if contributions pause for six months — a job change, an illness, a family expense? What if the goal arrives a year earlier than planned? What if the cost turns out higher than the estimate? And, for a goal funded from equity, what if the poor years land immediately before the money is needed rather than at the start — which matters more than the average, for reasons demonstrated in the sequence-risk experiment.

That last one is the argument for reducing risk as the date approaches, which is the glide path, and it is the part of goal planning most often left out of the contribution calculation entirely.

What to take away

The required monthly contribution is a cash-flow test, not a promise. Three of its inputs are yours and the fourth is a guess, and over a long horizon the guess dominates.

Measured across every fifteen-year period in the Indian record, a steady monthly investor got somewhere between +6.22% and +16.45% a year after tax, with the middle eight in ten between +10.85% and +14.77%. Run the contribution at the low, middle and high end, and find out whether your goal is sensitive to that or not — because that finding is more useful than any single figure.

If the answer is unaffordable, change the amount, the date, the contribution or the funding source. Do not change the return assumption, which alters the spreadsheet and nothing else.