A Systematic Withdrawal Plan is the mirror image of a SIP: instead of investing a fixed amount every month, you redeem a fixed amount from an existing investment at regular intervals. The remaining units stay invested and continue to earn (or lose) with the market.

Each month the balance grows at the assumed monthly return, then the withdrawal is deducted. The loop continues until the balance is exhausted or 60 years pass. If your withdrawal is smaller than the interest the corpus earns each month, the balance never falls and the plan is shown as sustainable.

With monthly return i = annual rate ÷ 12 ÷ 100, each month: balancet+1 = balancet × (1 + i) − W, where W is the monthly withdrawal. The sustainable withdrawal reference is the first month's growth, corpus × i: withdrawing at or below it leaves the balance non-decreasing. When W exceeds it, the number of months the corpus lasts has a closed form — n = ln[W ÷ (W − corpus × i)] ÷ ln(1 + i) — which the month-by-month simulation reproduces while also producing the balance table.

At an illustrative 7% annual return (0.583% a month), ₹60 lakh earns about ₹35,000 in the first month. Withdrawing ₹40,000 — ₹5,000 more than the growth — eats into principal, and the corpus lasts roughly 358 months, just under 30 years. Change one input at a time:

A two-point difference in return changes the answer from 'forever' to 'under 20 years'. Real returns are not constant, so treat these as boundaries rather than predictions.

The gap between the monthly withdrawal and the corpus's monthly growth — a small change in either flips a plan from sustainable to finite. Because this tool holds the withdrawal fixed, inflation silently shrinks what it buys: ₹40,000 at 6% inflation has the purchasing power of about ₹22,000 after ten years (see the Inflation calculator). Early poor returns hurt more than late ones because withdrawals continue while the balance is depressed.

Setting the withdrawal equal to the fund's long-run average return looks safe on paper but ignores sequence risk and inflation. Two or three weak years early on, with withdrawals continuing, can leave a corpus that never recovers even if the long-run average is later met. Plan with a lower rate than you expect, and size the corpus first with the Retirement calculator, which grows withdrawals with inflation.

The first month's interest on the corpus is a useful reference point: withdrawing less than that means you are living off growth alone; withdrawing more means you are consuming principal, and the plan has a finite life. Inflation makes a fixed withdrawal buy less each year — plan with the retirement calculator for inflation-growing withdrawals.