This question already has an answer here:

I will be travelling from Katmandu to Istanbul with a 5 hour stopover in Delhi is a transit visa required.I have British passport and will not be leaving international airport.

marked as duplicate by Mark Mayo, Gayot Fow, Dirty-flow, Thorsten S., Relaxed Apr 22 '15 at 10:58

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.