How it works
The calculation runs in four steps, and the table above shows each of them. Your income is multiplied by the share you are prepared to commit to instalments. Existing commitments come off that. What remains is the instalment available for a new loan. That instalment is then discounted back into a principal.
That last step is the EMI formula run backwards. The EMI calculator takes a principal and produces an instalment; this one takes an instalment and produces a principal. Both use the same engine, so a figure from here fed into the EMI calculator returns the instalment you started with.
More detail
Adding your down payment to the borrowable amount gives a property budget. That is the number worth carrying into a property search, since it is what you could actually put on the table.
The share of income is an input rather than a fixed rule because it is a decision, not a fact. Committing half your income to an instalment is arithmetically possible and may be a poor idea; committing a quarter is more comfortable and buys less. The calculator will compute either.
Once you have a loan amount in mind, the home loan EMI calculator.
To see what paying extra later would save, use the prepayment calculator.