Working backwards from a goal
How to turn a target and a date into a monthly amount, why the honest version inflates the goal first, and what to do when the number comes back unaffordable.
Last reviewed · 1,319 words
In short
- ₹50 lakh in ten years at 12% needs ₹21,520 a month. The same target over twenty years needs ₹5,004.
- Doubling the time roughly quarters the monthly amount, because contributions fall and compounding rises at the same time.
- Existing savings do more work than they appear to. ₹10 lakh already invested cuts a ten-year ₹50 lakh goal from ₹21,520 a month to ₹7,315.
- Inflate the goal before calculating. A target set in today's prices for a date twenty years away is not the target.
- If the number is unaffordable, the four levers are more time, a smaller goal, a higher return, and more income. Only three are under your control.
Most investment calculators run forwards: put in an amount, get a maturity figure. This one runs backwards. You state the target and the date, and it returns the monthly amount required.
That reversal is what makes it useful, because it turns a vague intention into a number you can either afford or cannot — and finding out which one, early, is the whole point.
The arithmetic
Solving the future value of an annuity due for the instalment:
P = FV × i ÷ [((1 + i)ⁿ − 1) × (1 + i)]
where i is the monthly return and n the number of months. The trailing (1 + i) reflects that the money goes in at the start of each month and earns for that month.
For a ₹50,00,000 target at 12%:
| Years | Monthly needed | Total contributed | From growth |
|---|---|---|---|
| 5 | ₹60,616 | ₹36,36,965 | 27% |
| 10 | ₹21,520 | ₹25,82,433 | 48% |
| 15 | ₹9,909 | ₹17,83,676 | 64% |
| 20 | ₹5,004 | ₹12,01,023 | 76% |
Doubling the horizon from ten years to twenty cuts the monthly amount by more than four times, and the total you actually contribute by more than half. Both happen at once: fewer rupees are needed because compounding supplies a larger share, and the share it supplies grows the longer the money is invested.
Time is by far the cheapest input into any goal. It is also the only one that runs out on its own.
What money already saved does
Existing savings compound on their own from day one, so they carry a disproportionate share of the target.
On the same ₹50,00,000 in ten years at 12%:
| Already invested | Grows to | Monthly still needed |
|---|---|---|
| ₹0 | ₹0 | ₹21,520 |
| ₹5,00,000 | ₹16,50,193 | ₹14,418 |
| ₹10,00,000 | ₹33,00,387 | ₹7,315 |
| ₹15,00,000 | ₹49,50,580 | ₹213 |
₹10 lakh already invested is not worth ₹10 lakh against the goal — it is worth ₹33 lakh, because it has ten years to grow. That is why the monthly requirement falls by two thirds rather than by a fifth.
The corollary is uncomfortable and worth stating: spending existing savings costs far more than the amount spent. Taking ₹5 lakh out of that pot for something else does not reduce the goal by ₹5 lakh; it adds ₹7,100 a month to the requirement for a decade.
Inflate the goal first
This is the step most people skip, and it invalidates everything that follows.
A goal expressed in today's prices is not the goal. A wedding that costs ₹20 lakh today costs about ₹36 lakh in ten years at 6% inflation. A college course at ₹25 lakh today, at education inflation of 10%, costs ₹64.8 lakh in ten years.
Two consistent approaches, and mixing them is the error:
Nominal. Inflate the goal to its future cost, then use a nominal return like 12%. The monthly amount comes out in today's rupees and should itself be increased annually.
Real. Leave the goal in today's prices and use the real return — roughly the nominal rate minus inflation, so about 6%. The answer is directly comparable to today's income.
Both give the same practical plan. Inflating the goal while also using the real return double-counts and produces a number nobody can afford; leaving the goal uninflated while using the nominal return produces one that quietly falls short.
When the number is unaffordable
It very often is, and that is useful information rather than a failure. There are exactly four levers.
More time. The strongest lever by a wide margin, as the first table shows. A goal moved out by five years usually becomes affordable without changing anything else.
A smaller goal. Less satisfying, and frequently the honest answer. A ₹15 lakh car instead of ₹25 lakh is a decision available today; a 15% return is not.
A higher return. The lever people reach for first and the one with the least give. Moving from 8% to 12% on a ten-year ₹50 lakh goal takes the monthly requirement from ₹27,149 to ₹21,520 — real, but not transformative, and it is bought with volatility rather than chosen. Beyond about 12% for equity you are not selecting a return, you are hoping for one.
More income directed at it. The only lever with no ceiling, and the slowest to move.
Note what is not on the list: waiting for a better entry point. Time out of the market costs more than a poor entry into it, and the first table quantifies why.
Match the instrument to the horizon
The return you enter is only credible if the money is somewhere that could plausibly produce it.
Under three years. A fixed deposit, a recurring deposit or a liquid fund. Use 6–7% and accept it. A three-year goal in equity can be 30% down on the date it is needed, and there is no time to recover.
Three to seven years. Hybrid or balanced funds, or a split across debt and equity. 8–10% is a defensible assumption.
Over seven years. Equity funds. 10–12% is reasonable as a long-run assumption, and it should still be tested at 8%.
The single most common planning error is using a 12% assumption for a three-year goal. The arithmetic is fine and the instrument that would deliver it cannot be relied on over that period.
Review it annually
A goal calculation is a snapshot of assumptions that will all turn out to be slightly wrong. Returns will differ from the estimate, income will change, and the cost of the goal will move.
Once a year, put the actual accumulated amount in as the existing savings, recompute the monthly requirement for the remaining term, and adjust. Corrections made annually are small. The same correction discovered in the final year is not a correction at all — there is no time left for it to work.
Running several goals at once
Most people have three or four goals with different dates, and the natural instinct — one large pot, sorted out later — is the one that fails.
Keep them separate, because the horizon determines the instrument. A three-year car and a twenty-year retirement cannot sit in the same fund at the same risk level without one of them being wrong. Separate folios, or at minimum separate records, also stop the long-horizon money quietly funding the short-horizon goal.
Order matters too. An emergency fund comes before every investment goal, because without one the first unplanned expense is met by breaking a goal — and breaking a goal costs the compounding as well as the money, as the table above shows. High-interest debt comes next: paying off a card at 40% is a guaranteed 40% return, which nothing on the investment side can match.
Then rank the remainder by consequence rather than by date. A child's education has a fixed date and no substitute. A car can be smaller or later. Retirement is the goal with no external deadline and the largest number, which is exactly why it is the one people fund last and should not.
Automating it
The requirement this calculator produces only works if it actually happens every month, and intention is a poor mechanism.
Set the debit for the day after salary credit, so the money leaves before it is available to spend. Increase the amount annually with income — a contribution fixed in rupees shrinks in real terms every year, and a 10% annual step-up roughly matches the effect of adding several years to the horizon.
The gap between a plan that is calculated and a plan that is automated is larger than any assumption in the model.
What this calculator assumes
- Contributions are made at the start of each month and earn for that month.
- The return is constant across the whole term, which no market provides.
- Existing savings are invested at the same rate as new contributions.
- The target is the amount you enter — inflate it yourself if it is expressed in today's prices.
- Figures are before tax, expense ratio and exit load, all of which reduce the return you actually get.