Lumpsum Calculator

How a lumpsum actually compounds

One-time investing has the simplest arithmetic in personal finance. There is one cash flow, and it sits there:

FV = P × (1 + r)ⁿ

P is what you put in, r is the annual return as a decimal, and n is the number of years. Put ₹10 lakh into something returning 12% a year and leave it for fifteen years and you end with ₹54,73,566 — the ₹10 lakh you invested plus ₹44,73,566 you did not, which is 5.47 times your money.

The lopsidedness is the point. In the first year that ₹10 lakh earns ₹1,20,000. In the fifteenth year the same 12% earns ₹5,86,453, because it is being applied to a much larger base. Nothing about your behaviour changed; only the base did. On this example the final five years add ₹23,67,718 while the first ten added ₹21,05,848 — at equity-like rates the last third of a long holding period out-earns the first two thirds combined. At deposit-like rates it does not: the lopsidedness comes from the rate, not from time alone. Either way it is why the most expensive decision a lumpsum investor makes is usually selling early rather than buying badly.

Note the compounding basis. This page compounds once a year, which is the right convention when the return is expressed as a CAGR — the way equity and mutual fund returns are quoted. Bank deposits are different: they compound quarterly, so an FD at the same headline rate matures slightly higher than annual compounding would suggest. Use the FD calculator for those.

The rule of 72, and where it stops working

Divide 72 by the annual return and you get the number of years the money takes to double. At 12%, that is six years. The exact answer is 6.12 years, so the shortcut is off by about six weeks. At 6% the rule says twelve years against an exact 11.9. At 18% it says four years against an exact 4.19, and by 25% the error is wide enough to matter.

The rule is at its most accurate around 8% and drifts as rates rise, but its use is not precision. It is a way of checking whether a number you have been given is plausible without reaching for a calculator. Fifteen years at 12% is two and a half doublings, so roughly 5.7 times your money — against the exact 5.47 above. If somebody projects that a fund will turn ₹10 lakh into ₹2 crore in fifteen years, the rule of 72 tells you instantly that this requires more than four doublings — a compounded return of about 22% a year, held for fifteen unbroken years. That is not a forecast; it is a sales pitch.

All at once, or stagger it through an STP?

This is the real decision facing anyone with a large sum in hand — a maturity, a bonus, a property sale, a retirement payout. The alternative to investing it in one go is a Systematic Transfer Plan: park the money in a liquid or arbitrage fund and move a fixed amount into the equity fund every week or month until it is deployed.

The arithmetic answer favours investing at once, and the reason is mechanical rather than historical. Every rupee still sitting in the parking fund is earning the parking fund's return, not the equity return you bought the equity fund for. If you did not expect equity to out-return a liquid fund over your horizon you would not be buying it, and deploying slowly while holding that expectation is inconsistent. Staggering is therefore, on its own terms, a small expected drag — you are paying a premium for the option to buy later.

The honest qualification is that "on average" is not the experience of the person who deployed everything three weeks before a 30% drawdown. When a large sum arrives at a point in the cycle nobody can identify in advance, staggering does not improve the expected outcome — it narrows the range of outcomes and, more usefully, reduces the chance of a decision you cannot live with. An investor who staggers and stays invested beats one who goes all in and capitulates in month four. If you do stagger, keep it short: three to six months, not three years. Every extra month is another month of the corpus earning the wrong rate.

There is a mechanical detail that generic dollar-cost-averaging calculators get wrong in the Indian context. An SIP instalment is a purchase. An STP instalment is a redemption from the source scheme, so each transfer is a capital gains event with its own acquisition cost and holding period. Twelve monthly STP transfers leave twelve small tax lots to track and report, and some source schemes levy an exit load on early transfers. Staggering is not free, and this calculator does not model it.

Why the inflation-adjusted figure is the one to look at

The headline number flatters. Take the ₹1 crore that so many long-horizon plans are built around: at 6% inflation, ₹1 crore received twenty-five years from now buys roughly what ₹23,29,986 buys today. Not ₹1 crore. Around ₹23 lakh.

The same correction applies to the worked example. That ₹54,73,566 after fifteen years is worth ₹22,83,928 in today's money at 6% inflation. Your money did not multiply 5.47 times in purchasing power; it multiplied 2.28 times. Both figures are true, but only one of them tells you what you will be able to buy.

Note also that real returns divide rather than subtract. A 12% nominal return against 6% inflation is not 6% real — it is 5.66%, because you deflate the whole compounded amount, not the rate. The gap widens as inflation rises. It is also worth holding the risk-free comparison in view: small savings rates are notified quarterly by the Ministry of Finance, and PPF currently pays 7.1% a year. Against 6% inflation, that is barely a point of real return, which is the argument for taking equity risk over long horizons and the reason the inflation box on this page is not decorative.

What this calculator cannot tell you

It cannot tell you the return. Everything downstream of that box is arithmetic; the box itself is a guess, and no amount of decimal places in the output makes it less so.

It also cannot show you the path. Two funds can both compound at 12% over fifteen years and feel entirely different to hold — one drifting upward, the other falling 35% in year eleven and recovering. If you might need the money at a fixed date, the path matters more than the average, because you redeem on a date rather than on an average. Nor does the calculator model tax: gains crystallise when you redeem, and the treatment turns on the fund category and holding period, so check the rate applicable to your fund before you assume the end figure is what reaches your bank account.

What the calculator is genuinely good for is comparing shapes: how much a longer horizon is worth against a higher return, how much inflation quietly removes, and whether a target is within reach of the sum you actually hold. Those comparisons hold up even when the specific numbers do not.