The SWP mechanic
A systematic withdrawal plan is a SIP in reverse: instead of buying units monthly, you redeem a fixed rupee amount monthly. The model here applies each month's growth first, then takes the withdrawal at month-end:
balance = balance × (1 + i) − withdrawal, where i = annual return ÷ 12.
Everything about corpus longevity falls out of one comparison: the withdrawal versus that month's growth. When growth covers the withdrawal, the corpus holds or rises. When it does not, the shortfall comes out of principal, and next month's growth is computed on a smaller base, so the shortfall widens. Depletion is not linear; it is a slow leak that becomes a collapse in the final years.
The sustainable rate
A ₹50 lakh corpus at an 8% assumed return grows about ₹33,333 in a month (50,00,000 × 0.08 ÷ 12). Withdraw up to roughly ₹33,000 and the model sustains the corpus indefinitely, the growth replaces every rupee drawn. Push past it and the clock starts:
| Monthly withdrawal | Share of monthly growth | ₹50 lakh corpus at 8% lasts |
|---|---|---|
| ₹30,000 | 90% | Indefinitely, balance keeps rising |
| ₹33,000 | 99% | Indefinitely, balance roughly flat |
| ₹40,000 | 120% | Around 22 years, long, but not forever |
| ₹50,000 | 150% | Roughly 13 years |
Notice how sharp the cliff is: ₹33,000 lasts forever, ₹50,000 lasts 13 years. Near the tipping point, small changes in the withdrawal produce enormous changes in longevity, which is exactly why it is worth running your own numbers rather than borrowing a rule of thumb.
The caveat that matters: sequence of returns
The calculator assumes 8% arrives as a smooth 0.67% every month. No market does that. Real equity returns come as −15% years and +30% years that merely average to something like 8%, and for a corpus under withdrawal, the order of those years changes the outcome. A bad stretch early forces you to redeem more units at depressed prices; those units are gone and never participate in the recovery. The same bad stretch late in the plan is almost harmless. Treat the calculator's answer as the average-case trajectory, then defend against the bad case: keep one to two years of withdrawals in liquid or debt funds so a crash never forces equity redemptions, and prefer a withdrawal comfortably below the sustainable rate rather than exactly at it.
SWP vs FD interest for retirees
The traditional alternative is parking the corpus in fixed deposits and living on the interest. FDs win on certainty, the rate is contractual, the income arrives in any market. The SWP wins on flexibility, on long-run growth potential, and on tax: FD interest is fully taxable at your slab rate the year it accrues, while an SWP withdrawal is largely a return of your own capital, with only the gain slice taxed at capital-gains rates. A common structure is to do both, essential expenses covered by FD interest (see the FD calculator for what a ladder of deposits yields), discretionary spending drawn via SWP from a balanced or equity fund.
Taxation of withdrawals
Each SWP instalment is a redemption, split between capital and gain. Only the gain is taxed, for equity funds at 12.5% on long-term gains beyond the ₹1.25 lakh annual exemption, with units redeemed first-in-first-out. In the early years of an SWP most of each withdrawal is simply your own money coming back, so the effective tax on the cash received is modest and typically far lighter than slab-rate tax on an equivalent FD payout. The picture tightens over the years as the remaining units carry larger embedded gains.
Building the corpus first
The drawdown maths above assumes the corpus already exists. Getting there is the accumulation problem, the SIP calculator covers what a monthly investment grows into, and the inflation calculator tells you what target corpus your future expenses actually require. Working the two ends towards each other, required withdrawal, sustainable rate, hence required corpus, is retirement planning in one sentence.