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This question already has an answer here:

I am travelling from Mumbai to Houston via London and have a valid US visa. I have to wait in London for 14 hours for my connecting flight. Some of my friends were saying transit visa is not required if the layover time is less that 24 hours and others say you should have one if you stay more than 12 hours. Could you please help me with issue.

marked as duplicate by Karlson, Dirty-flow, Mark Mayo, Gagravarr, Aditya Somani Aug 9 '14 at 12:10

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The relevant threshold is 24 hours, as documented in point 2.5 and 2.6 of this document [PDF].

As you probably know, there are also other conditions, depending on your citizenship but travelling to the US with a US visa, it seems you fulfill them. Your friend might have needed a visa for one of these other reasons. See Is there a way to find out if I need a transit visa for a layover in the UK? for more info on all this.

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