If you have a valid US visa (F, H or B) on your passport or you are a green card holder and traveling to or from US, you don't need a transit visa in UK else you need one.  Click the link below and follow the "Transit without Visa Concession" section. 

http://www.ukba.homeoffice.gov.uk/visas-immigration/transitthroughtheuk/travelling-through-the-cta/#