How the corpus is worked out
Your expenses grow with inflation until you retire and keep growing after. The corpus has to pay them every year of retirement while the remaining money earns a (usually lower) post-retirement return.
The calculator grows today's expenses to your retirement age, finds the corpus that funds them until the age you plan for, subtracts what your existing savings will grow to, and works out the monthly SIP that closes the gap.
Results are estimates for illustration only, not financial advice. Banks and schemes may round, apply TDS or use slightly different day counts — confirm figures with your bank or employer.