It is a question we get asked a lot, so, here is the simple answer.
A one sheet movie poster is 27 inches x 40 inches (older ones are 27 inches x 41 inches) and is portrait format.
A quad movie poster is 40 inches x 30 inches and is landscape format.
A picture is worth a thousand words, so here is a simple graphic to illustrate the difference.
Here are two Star Wars Rogue One movie posters. First a one sheet and then a quad.
The quad format (or the 'crown quad' to give its full name) has its roots in British paper sizes from the early Twentieth Century.
Creation of the one sheet format is closely aligned to the development of the U.S. film industry and you can find out more about that in this blog post.
Hope that helps!
If you have any other movie poster related questions, do get in touch. We would be delighted to help.
You can find all of our fantastic one sheet and quad movie posters here.
Adam and the Art of the Movies team.