The amount of money you will need on a regular basis for your daily livings costs plus unforeseen costs and costs incurred on luxuries should constitute your retirement corpus.
To estimate your retirement corpus
Draw up a list of your total expenses you currently incur
Identify goals post retirement
Make provision for a healthcare budget
Make an estimate of your expected income after retirement – this can include pension under EPS, income from fixed deposits etc.
You can either do all this manually, or can do it in a simpler way with the aid of an online retirement calculator.