How Does a Systematic Withdrawal Plan (SWP) Work?

A Systematic Withdrawal Plan (SWP) is the mirror image of an SIP. Instead of adding a fixed amount to an investment every month, you take a fixed amount out every month - a self-made pension drawn from a lump sum you already hold. The money you have not yet withdrawn stays invested and keeps earning returns, so the corpus is being pulled in two directions at once: your withdrawals shrink it, and market returns grow it. Whether the balance lasts for decades or runs dry early depends entirely on which force wins. This guide explains exactly how that tug-of-war is resolved month by month, so you can reproduce and sanity-check every figure in the SWP Calculator.

The month-by-month mechanics

An SWP is not a single formula you plug numbers into - it is a repeating monthly cycle. Each month two things happen in order: the whole remaining balance earns one month of return, and then your fixed withdrawal is taken out. Written as a step:

• New balance = (Previous balance × (1 + i)) − W
• where i is the monthly return (annual rate ÷ 12 ÷ 100) and W is your fixed monthly withdrawal.

That single line is applied over and over - once for every month in your chosen period. Because the withdrawal comes out after the growth, each month you draw from a balance that has just earned a little, and next month's growth is calculated on the slightly smaller balance that remains. This is why an SWP has to be worked out iteratively rather than in one shot, and it is exactly the loop the SWP Calculator runs internally.

A worked example

Take the calculator's default scenario: a corpus of 10,00,000, a monthly withdrawal of 10,000, an expected return of 8% a year, over 10 years. First find the monthly return: 8 ÷ 12 ÷ 100 = 0.006667. Now walk the first month:

• Month 1 growth: 10,00,000 × 1.006667 ≈ 10,06,667.
• Month 1 after withdrawal: 10,06,667 − 10,000 = 9,96,667.
• Month 2 growth: 9,96,667 × 1.006667 ≈ 10,03,311, then − 10,000 = 9,93,311.

Repeat that 120 times and the balance after 10 years lands at about 3,90,180 - and along the way you will have drawn out 10,000 × 120 = 12,00,000. So a 10,00,000 corpus paid you 12,00,000 in income and still left roughly 3,90,180 on the table, because the returns did a large part of the heavy lifting. There is a neat closed-form check for the ending balance: Final = P × (1 + i)^n − W × (((1 + i)^n − 1) ÷ i), where n is the number of months. Plugging in P = 10,00,000, i = 0.006667, n = 120, W = 10,000 gives 3,90,180 - matching the month-by-month result to the rupee. Try your own corpus, withdrawal and rate in the SWP Calculator to see the ending balance move.

When does the corpus run out?

An SWP does not always end with money left over. If your monthly withdrawal is larger than the return the corpus earns, the balance falls a little every month, and eventually a withdrawal takes it to zero. Keep the same 10,00,000 corpus and 8% return but raise the withdrawal to 15,000 a month: the corpus now depletes in month 89 - about 7 years and 5 months - well short of a 20-year plan. The calculator watches for exactly this and flags roughly when the money runs out, so you are not caught assuming an income that stops early. The lesson is that the headline period you choose is only achievable if the withdrawal is sustainable; otherwise the plan quietly ends sooner.

Sizing a withdrawal that lasts

The pivotal number is the monthly return your corpus earns in rupees. On 10,00,000 at 8% a year, one month of return is about 10,00,000 × 0.006667 ≈ 6,667. If you withdraw less than that, the corpus keeps growing even as it pays you; if you withdraw exactly that, it roughly holds steady; withdraw more, and it erodes. So a rough rule for a corpus you want to preserve indefinitely is to keep the monthly withdrawal at or below the monthly return - here, around 6,667. Drawing 10,000 (above 6,667) is why the default scenario shrinks over the decade rather than growing, yet it still lasts because the gap is small. If you need a specific income, you can work backwards: a higher corpus or a lower withdrawal both push the plan toward lasting longer.

SWP versus a fixed deposit or dividend

People often compare an SWP with simply parking money in a deposit and living off the interest, or holding funds for dividends. The difference is control and tax treatment. With an SWP you decide the exact amount and date of each payout regardless of what the fund distributes, and because each withdrawal is partly your own capital and partly gains, only the gains portion is taxed - often making it more tax-efficient than fully-taxed interest income. A deposit pays a contractually fixed rate with no market risk but no growth beyond it, while an SWP keeps the balance invested for potential growth at the cost of return uncertainty. If you are drawing a regular income from a lump sum and want flexibility with some growth, an SWP is the tool; if you cannot tolerate any fall in the balance, a deposit is safer. You can contrast the growth side of the same money in the SIP Calculator.

What the estimate assumes

The calculation is precise, but it rests on a few assumptions worth naming. It uses a single constant monthly return, whereas real fund returns swing above and below that average - a bad run of early months hurts an SWP more than a good run helps, because you are withdrawing from a temporarily smaller balance (sequence-of-returns risk). It assumes withdrawals are taken at the end of each month and never change, though in practice you might raise them for inflation, which shortens how long the corpus lasts. It also shows pre-tax figures and ignores fund exit loads and expense ratios. Treat the output as a well-grounded planning estimate rather than a guarantee, revisit it whenever your return expectation or income need changes, and use the SWP Calculator to compare scenarios rather than to predict an exact future balance.