Purpose: There are three formats for arranging English text for vertical reading―upright letters are arranged vertically (marquee), or horizontal text is rotated 90° to the right or left. Previous research has shown that reading speed is slower for all three vertical formats, with marquee slower than either rotated formats (Byrne, 2002). It has been proposed that the size of the visual span―the number of letters recognized with high accuracy without moving the eyes―is a visual factor limiting reading speed. We predicted that reduced visual-span size for vertical text is correlated with the slower vertical reading, and that differences in visual-span size are associated with the differences in reading speed across the three vertical formats. Methods: Twenty-four normally sighted young adults were randomly assigned to groups tested with either lowercase or uppercase text. Visual span profiles, plots of letter-recognition accuracy as a function of letter position left or right (horizontal orientation) and above or below (vertical orientation) the midline, were measured with trigrams (strings of three random letters). Size of the visual span was defined as the area under this profile, converted to bits of information transmitted. Reading performance was measured using two presentation methods―RSVP (Rapid-Serial-Visual-Presentation) and flashcard (a block of text on four lines)―for horizontal, marquee, rotated right, and rotated left conditions. Results: Sizes of the visual span and reading speeds showed a qualitatively similar dependence on text formats for lowercase and uppercase and both RSVP and flashcard reading. Correlations were computed between reading speed and visual-span size across the four test conditions. Averaging across subjects and reading tasks, the correlation accounted for 83% of the variance in reading speeds. Conclusions: Our results are consistent with the view that horizontal/vertical differences in reading speed are due to changes in the size of the visual span.