ReckonBoxHow the retirement corpus calculator works
A retirement corpus is the lump sum you need on your retirement day to fund your living expenses for the rest of your life, after inflation has raised those expenses and while the corpus keeps earning a return. This calculator sizes that number, then works backward to the monthly SIP that builds it. It runs three steps, and it shows every one.
Step one inflates your spending. Your ₹50,000 a month today is not ₹50,000 at 60: at 6 percent inflation over 30 years it becomes about ₹2.87 lakh a month, or ₹34.5 lakh a year. Step two turns that rising expense into a corpus that lasts through your retirement years. Step three divides the corpus by the growth of a monthly investment to find the SIP. On the default numbers, a 30-year-old retiring at 60 and planning to age 85 needs a corpus of about ₹7.71 crore, reachable with roughly ₹21,900 a month at a 12 percent return.
The corpus formula, done right
The corpus is the present value of a growing annuity: your inflated annual expense, rising each year with inflation, discounted at the return the corpus earns after retirement, over the number of retirement years. In symbols, Corpus = E x (1 - x^T) / (1 - x), where E is the first-year expense at retirement, T is the retirement years, and x = (1 + inflation) / (1 + post-retirement return).
Most calculators skip this and use a perpetuity, annual expense divided by (return minus inflation), which quietly assumes the money must last forever and ignores how long you actually expect to live. That overstates the corpus, and by more than most people would guess: the figures are further down. The growing-annuity version uses the precise real rate, (1 + return) / (1 + inflation) - 1, and lets the balance run down to near zero by your life expectancy, which is what a corpus is supposed to do.
One convention worth naming, since the answer moves on it. This tool takes each year's withdrawal at the start of the year, an annuity due, which is how a retiree actually spends. Taking it at year end instead gives ₹7.64 crore against ₹7.71 on the defaults below. There is a neat check hidden in it: when the post-retirement return equals inflation, the formula collapses to expense times the number of retirement years, because the corpus earns exactly what prices rise.
A worked example, start to finish
Take the defaults and follow the money. A 30-year-old spends ₹50,000 a month, or ₹6 lakh a year, and wants that lifestyle from 60 to 85.
| Step | Value |
|---|---|
| Current annual expense | ₹6,00,000 |
| Inflated to age 60 (6% for 30 years) | ₹34,46,096 a year |
| Monthly expense at retirement | ₹2,87,175 |
| Corpus required | ₹7,71,48,478 |
| Monthly SIP for 30 years at 12% | ₹21,856 |
The drawdown table on the tool shows something people rarely expect. Because the ₹7.71 crore keeps earning 7 percent, it does not fall from day one. It grows for the first decade or so, peaking above ₹8.3 crore around age 72, then the rising withdrawals overtake the returns and it drains to nearly nothing by 85. That single table is the honest picture of a retirement, and it is missing from every calculator we checked.
The 25x rule versus the real math
The 25x rule says your corpus should be 25 times your annual expenses at retirement, which is the flip side of a 4 percent withdrawal rate. It is a fine back-of-envelope check, and on these numbers it suggests ₹34.46 lakh times 25, about ₹8.62 crore. The precise finite-horizon math gives ₹7.71 crore, a little lower, because it credits the corpus with 25 more years of 7 percent growth that the flat multiple ignores.
Many Indian planners nudge the rule the other way, toward a 3 to 3.5 percent withdrawal, or 30 to 33 times expenses, to leave a bigger safety margin. Axis Max Life states it in those terms, and vrid settles on a 33X rule for India on the grounds that inflation here runs near 6 percent against 2 to 3 in the United States, where the 4 percent rule was derived. The field does not agree: Angel One treats 4 percent as internationally applicable and Aditya Birla builds its worked example on dividing by 0.04.
The rules of thumb are life expectancy in disguise
Every one of those multiples is a statement about how long you expect to live, and they line up almost exactly. Nobody on this search makes the connection, so here it is on the same ₹50,000 a month, 6 percent inflation and 7 percent post-retirement return.
| Live to | Corpus needed | Multiple of annual expense | First-year withdrawal |
|---|---|---|---|
| 85 | ₹7.71 crore | 22.4x | 4.47% |
| 90 | ₹9.05 crore | 26.3x | 3.81% |
| 95 | ₹10.33 crore | 30.0x | 3.34% |
| 100 | ₹11.55 crore | 33.5x | 2.98% |
Run the rules the other way and they meet in the middle: a 4 percent withdrawal is 25 times expenses, 3.5 percent is 28.6 times, and 3 percent is 33.3 times.
So the Indian 30 to 33x advice is arithmetically identical to planning to reach 95 or 100. It is not vague extra caution, it is longevity insurance with a number attached, and the life expectancy field above gets you there directly.
Which puts a flag on the default. This page starts at 85, and that produces a first-year withdrawal of 4.47 percent, higher than the American rule and outside the 3 to 3.5 percent band the India-specific sources recommend. Three of them plan for longer: Aditya Birla suggests 90 or 95, Moneyvesta uses 90 as its baseline, Axis Max Life says 85 to 90. Push the life expectancy field to 95 and the same plan lands inside the Indian guidance without changing anything else.
None of the six calculators checked on 17 September 2026 prints its implied withdrawal rate, which is one division away from figures already on their screens.
The shortcut formula that goes badly wrong here
One ranked calculator publishes its method as annual expenses at retirement divided by the post-retirement return minus inflation. That is the perpetuity formula, and it assumes the money never runs out because it is never meant to.
On these defaults the denominator is 7 percent minus 6, a single percentage point, and the answer is ₹34.46 crore against the ₹7.71 crore the finite calculation gives. Four and a half times larger.
It is also unstable in a way that should worry anyone relying on it. Because the denominator sits near zero, a small change in the assumed return moves the answer enormously: at 7.5 percent it drops to ₹22.97 crore, at 8 percent to ₹17.23 crore, at 9 percent to ₹11.49 crore. Half a point of optimism erases ₹11 crore of requirement.
The finite-horizon maths this tool runs does not have that problem, because it discounts a fixed number of years rather than an infinite one. The perpetuity version is worth knowing about mainly so you can recognise it when a number looks impossibly large.
Why inflation, especially medical, drives the number
Inflation is the lever that moves this result the most. A single percentage point on the inflation input compounds across decades, so the same expense at 7 percent inflation instead of 6 lands far higher at retirement. General Indian inflation is often planned at 6 percent, but healthcare is the outlier: Axis Max Life cites medical inflation of 13 to 14 percent, and inflationcalculator.in puts it at 8 to 10 percent, well above headline CPI.
Because medical costs land hardest late in retirement, exactly when the corpus is thinnest, testing a higher inflation rate is worth the few seconds it takes. The corpus that looks comfortable at 6 percent can look tight at 8.
What this does not promise
Every number here rests on assumptions you chose, and real returns and inflation will not follow a straight line, so treat the corpus as a target to aim at, not a guarantee. Markets swing, so the pre-retirement return that builds the corpus and the post-retirement return that sustains it are estimates, and a bad early retirement sequence can drain a corpus faster than the smooth table suggests. This is a planning tool and not investment advice, so a SEBI-registered adviser can pressure-test your plan against your real situation. To build the corpus, the SIP calculator projects a monthly investment forward, and to draw an income from it later, the SWP calculator models the withdrawals.
Frequently asked questions
What is a retirement corpus calculator? A retirement corpus calculator estimates the lump sum you need on your retirement day to fund your living expenses for the rest of your life, then the monthly SIP to build it. It inflates your current expenses to retirement, sizes the corpus so it lasts through your life expectancy, and works backward to the investment needed.
How is retirement corpus calculated? First inflate your current annual expense to retirement: expense times (1 plus inflation) raised to the years to retirement. Then size the corpus as the present value of those rising expenses over your retirement years, discounted at the post-retirement return. On ₹50,000 monthly expenses today, age 30 to 60, living to 85, at 6 percent inflation and 7 percent post-retirement return, the corpus works out to about ₹7.71 crore.
Why not just use annual expense divided by return minus inflation? Because on Indian assumptions it is wrong by roughly four and a half times. That perpetuity formula assumes the money must last forever, and its denominator here is 7 percent minus 6, a single point, which gives ₹34.46 crore against the ₹7.71 crore the finite calculation returns. It is unstable too: at an 8 percent assumed return the same formula says ₹17.23 crore. The correct version is a finite growing annuity over your actual retirement years, which lets the corpus deplete to near zero by your life expectancy.
What is the 25x rule for retirement? The 25x rule says your corpus should be 25 times your annual expenses at retirement, the inverse of a 4 percent withdrawal rate. It is a quick proxy, and many Indian planners prefer a more cautious 3 to 3.5 percent withdrawal, meaning 30 to 33 times. On our default numbers the 25x rule suggests about ₹8.62 crore against the ₹7.71 crore the precise math gives.
How much should I invest each month for retirement? The monthly SIP is the corpus solved backward through the future-value formula, over the years to retirement at your pre-retirement return. Building a ₹7.71 crore corpus over 30 years at a 12 percent return needs roughly ₹21,900 a month, and existing savings cut that figure.
What inflation rate should I assume? General Indian inflation is often planned at 6 percent, but medical inflation runs far higher, commonly cited between 8 and 14 percent. Since health costs rise fastest in retirement, testing the calculator at a higher inflation rate gives a more honest corpus for later years.
How long should my retirement corpus last? Plan for the corpus to last until your life expectancy, which Indian sources set higher than you might expect: Aditya Birla suggests 90 or 95, Moneyvesta uses 90 as a baseline, Axis Max Life says 85 to 90. The choice matters more than it looks, because it is the same decision as picking a withdrawal rate. On the defaults here, planning to 85 implies a 4.47 percent first-year withdrawal, to 95 implies 3.34 percent, and to 100 implies 2.98 percent.
What withdrawal rate do my inputs imply? Divide the first year of retirement expenses by the corpus. On the defaults here that is 4.47 percent, above the American 4 percent rule and outside the 3 to 3.5 percent that vrid and 1Finance both recommend for India, on the grounds that inflation here runs near 6 percent against 2 to 3 in the United States. None of the six calculators checked on 17 September 2026 shows this number, though all of them cite a rule expressed in its terms.
Does the calculator account for my existing savings? Yes. Enter what you have already saved and it grows at the pre-retirement return until you retire, then that future value is subtracted from the target, so the monthly SIP only has to build the gap.
Sources
Fetched 17 September 2026.
- vrid, why the 4 percent rule does not work in India, for the 33X rule
- 1Finance, the safe withdrawal rate, for the 3 to 3.5 percent band
- Axis Max Life retirement planning calculator, for the Rule of 25 against 30 to 33x
- Aditya Birla, how much retirement corpus you really need, which plans to 90 or 95
- Moneyvesta retirement corpus calculator, which publishes the perpetuity formula
- PrimeInvestor retirement calculator
- CalcWise retirement corpus calculator, which stress-tests living longer
- Angel One on the 4 percent rule, which treats it as internationally applicable
Carried over from the 10 July 2026 teardown and not re-fetched for this pass: