Does muscle weigh less than fat?

No: Muscle has a greater density than fat. Weight is a relative term, but i know what you mean. Muscle would sink in water and fat would float.

See below: The question you are really asking is about density which is weight per volume. For instance one pound of muscle equals one pound of fat which equals one pound of feathers. All weigh one pound. However, fat is less dense than muscle so if you put a certain volume of fat, say one half liter of fat and compared it to one half liter of muscle, the muscle would weigh more. Hope that helps.