What the calculator computes each month
Each month, the calculator applies your expected annual return (converted to a monthly rate) to your remaining balance, adds that growth to the balance, then subtracts your fixed monthly withdrawal — repeating this for every month across your chosen withdrawal period. The result is a month-by-month picture of whether your balance grows, holds roughly steady, or shrinks toward zero.
This month-by-month mechanic is why SWP outcomes are sensitive to the relationship between your withdrawal amount and your return rate, not just either number viewed alone. A withdrawal rate below your return rate means the corpus can theoretically sustain withdrawals indefinitely; a withdrawal rate above it means the corpus is being drawn down and will eventually reach zero, however large it started.
The single number that matters most: withdrawal rate versus return rate
If your monthly withdrawal, annualised, is smaller than your expected annual return on the corpus, the balance can in principle last indefinitely, since growth each period outpaces what's being withdrawn. If your annualised withdrawal exceeds the expected return, you're drawing down principal, not just income, and the corpus has a finite lifespan that the calculator will show explicitly.
Take a ₹50,00,000 corpus at an assumed 8% annual return. A ₹30,000 monthly withdrawal (₹3,60,000 annually, or 7.2% of the corpus) is close to the return rate, so the balance holds roughly steady over a long period. Push the monthly withdrawal to ₹50,000 (₹6,00,000 annually, 12% of the corpus) instead, and the corpus depletes meaningfully faster, since withdrawals now clearly exceed what the assumed return generates each year.
Why sequence of returns matters more here than in accumulation
The calculator assumes a constant, smooth annual return for simplicity, but real markets don't move that evenly — and during withdrawal, the order in which good and bad years happen matters significantly more than it does while you're still contributing. A market downturn early in your withdrawal period forces you to sell a larger share of a shrunken corpus to fund the same withdrawal amount, which can permanently damage the corpus's ability to recover even if returns average out fine over the full period.
This is sequence-of-returns risk, and it's a real limitation of any constant-rate SWP projection — the calculator's smooth output represents an average-case scenario, not a guarantee, and an early downturn in your actual withdrawal years could deplete a corpus faster than the tool's clean curve suggests.
Using the calculator to find a sustainable withdrawal amount
Rather than picking a withdrawal amount based on desired monthly income alone, it's worth running the calculator at a few different withdrawal levels against your actual corpus and a conservative return assumption, to see which amount keeps the corpus intact for your full intended withdrawal period rather than depleting early.
A commonly referenced rule of thumb from retirement planning research suggests withdrawing around 4% of the corpus annually as a starting point for long, multi-decade withdrawal periods, though this figure was developed in a different market context and should be treated as a reference point to test against this calculator's output, not a rule to follow blindly.
Common mistakes when planning an SWP
Using an overly optimistic return assumption. A withdrawal plan built on an aggressive return assumption that doesn't materialise can deplete a corpus much faster than expected — a conservative assumption produces a more protective plan.
Ignoring inflation on the withdrawal amount. A fixed ₹30,000 monthly withdrawal today won't have the same purchasing power in 15 years — consider whether you'll need to increase withdrawals over time, and model that scenario separately if so.
Not stress-testing against a market downturn early in withdrawal. The calculator's constant-rate model doesn't show what happens if the first few years of withdrawal coincide with a market fall — worth mentally adding a margin of safety to whatever withdrawal rate the calculator suggests is sustainable.