Linear measurement upon Barsoom has been defined, and redefined a couple of times within the Martian novels. It seems in earlier works some mistakes were made in the conversion of Barsoomian measurement into American. The following is the most 'definitive' version, as taken from the Burrough's Cyclopaedia.
A karad is a Martian unit of latitude, and measured at the equator it is 100 haads.
A haad is a Martian equivalent of a mile (1,949 feet), it is roughly 200 ads.
An ad is made up of 10 sofads (9.745 feet).
A sofad is the Martian equivalent of a foot, it is roughly 11.694 Earth inches.
A sof is a the Martian 'inch', which is approximately 1.1694 inches.