1. Current age, retirement age, plan-until age — define the saving phase and the withdrawal phase.
2. Monthly expenses today — what retirement life would cost now; the tool inflates it to your retirement year.
3. Inflation assumption — applied both before and after retirement.
4. Return before / after retirement — typically a higher growth assumption while saving and a more conservative one while withdrawing.
5. Existing retirement savings — EPF, PPF, NPS or investments already earmarked; grown to retirement and deducted.

You get the corpus needed at retirement, the monthly SIP (or single lump sum) required to reach it, and a year-by-year view.

Three inputs dominate: the gap between post-retirement return and inflation (the 'real' return that decides how long a corpus lasts), the number of years in retirement, and how early you start saving. Retiring at 55 instead of 60 means five fewer years of saving and five more years of withdrawals — a double hit that can raise the required SIP by half or more. A one-point increase in assumed inflation typically raises the corpus needed by 15–25% over a 30-year retirement. Treat the output as a scenario to revisit every few years, not a one-time target.

Age 30, retiring at 60, planning to 85, with monthly expenses of ₹40,000 today, 6% inflation, an illustrative 12% return while saving and 7% after retirement, and no existing savings. Expenses at retirement inflate to about ₹2.3 lakh a month; the corpus needed is roughly ₹6.1 crore, and the monthly SIP to build it from scratch is about ₹17,300. Two single-input changes:

Retiring five years earlier needs a smaller corpus (fewer years of inflation before retirement) yet a far larger SIP, because there are fewer years to build it. All figures are illustrations of the stated assumptions, not projections.

Sizing a corpus as '25 years × today's annual expenses' ignores that costs keep rising after retirement too — the ₹40,000 above is ₹2.3 lakh by year one of retirement and about ₹9.9 lakh a month by 85 at 6%. Test the inflation input with the Inflation calculator, and treat any guaranteed component (for example a PPF balance) as existing savings rather than as a separate plan.

Three steps. First, today's monthly expenses are inflated to the year you retire. Second, the corpus is sized so that inflation-growing monthly withdrawals can be paid from it, while the remaining balance earns your post-retirement return, until your life-expectancy age. Third, the SIP formula is reversed to find the monthly investment that reaches that corpus by retirement.

Expenses at retirement: E × (1 + inflation)years to retire

Corpus (growing annuity due, monthly): Eret × [1 − qn] / [1 − q], where q = (1 + g) / (1 + r), g monthly inflation, r monthly post-retirement return, n months in retirement.

SIP needed: the goal planner formula applied to the corpus, after growing any existing savings.

Before retirement most people can hold growth assets for decades; after retirement the portfolio usually shifts towards stability and lower expected returns. Using one number for both phases hides that change. The gap between post-retirement return and inflation is what determines how fast a corpus erodes — model withdrawals in detail with the SWP calculator.