It actually depends on the usage, but if you look at your local building code, you will probably find that the thickness of your cement garage floor should be six inches. This is a good size that will handle medium truck and car use. Indeed, if you only plan to house a car or two in your garage, you may find a variance that will allow you to pour a four-inch floor pad. If, on the other hand, your facility will be handling any heavy duty or medium duty trucks (semis and dumps, for example), then you will probably be best served by pouring a six to eight-inch concrete floor pad. Remember that you will also have to ensure that the cement is sealed properly so that it doesn’t crack.