First, you have to provide your average monthly expense. This includes your basic household expenses, utility bills, lifestyle expenses, etc.
Next, provide the number of years for which you plan on working. This depends on your current age and retirement age. For instance, if you are 30 and you plan to work till 65, the number of years till retirement will be 35 years.
You also need to mention for how many years you plan your retirement corpus to last. In other words, after retirement, how long do you want your corpus to pay for your lifestyle expenses?
Enter the expected inflation rate, which you can get based on the past inflationary trends.
Also, provide the expected annual return on your savings