You tell the calculator the retirement amount you want in today’s money and it does the rest. First it grows that amount by inflation to find what it will actually cost by the time you retire. Then it grows your current savings at your expected return and subtracts it, so you only save for the shortfall. Finally it works out the monthly saving needed to cover that shortfall.
Wanting ₹50,00,000 (in today’s value) at retirement, aged 30 retiring at 60, with 6% inflation, 8% returns and ₹1,00,000 already saved: ₹50 lakh grows to an inflation-adjusted ₹2,87,17,456 by age 60. Your current savings grow to ₹10,93,573, leaving a remaining target of ₹2,76,23,883. Covering that needs a monthly saving of about ₹18,535 — ₹66,72,617 invested over 30 years, with the remaining ~₹2.2 crore coming from growth. The chart shows your corpus climbing to the inflation-adjusted target.