How the retirement corpus is estimated
The planner inflates your monthly expenses to the year you retire, then works out the lump sum needed to pay those expenses every year until your planning age. Each year the expense rises with inflation while the remaining corpus keeps earning your post-retirement return. That lump sum is your target corpus.
Closing the gap with monthly saving
What you have already saved grows at your pre-retirement return. Whatever it falls short of the target is the gap, and the planner shows the monthly SIP that fills it by your retirement age. Retiring two years later shrinks that figure from both ends: more years to save and fewer years to fund.
A worked example
A 30-year-old spending 3,000 a month, planning to retire at 60 and plan to age 85, at 4% inflation, will spend about 9,700 a month in their first year of retirement. With a 6% return after retirement, funding that for 25 years takes a corpus of about 2.3 million. With 20,000 already saved and a 10% return before retirement, the monthly saving needed is about 880.
Assumptions worth testing
Three inputs move the answer most: inflation, the return after retirement, and how long you plan for. Planning to 85 rather than 90 can understate the corpus if you live longer, and a one point drop in post-retirement return raises it noticeably. Run a cautious case alongside your base case.