The top of Willis Tower is the highest point in Illinois. The tip of its highest antenna is 1,729 feet (527 m) above street level or 2,325 feet (708.7 m) above sea level, its roof is 1,450 feet 7 inches (442.14 m) above street level or 2,046 feet (623.6 m) above sea level, the 103rd floor observation deck (The Sky deck) is 1,353 feet (412 m) above street level or 1,948 feet (593.8 m) above sea level, the Franklin Street entrance is 595 feet (181.4 m) above sea level. (The highest natural point in Illinois is the Charles Mound, at 1,235 feet (376.4 m) above sea level.)
The building leans about 4 inches (10.2 cm) towards the west due to its slightly asymmetrical design, placing unequal loads on its foundation.
The design for Willis Tower incorporates nine steel-unit square tubes in a 3 tube by 3 tube arrangement, with each tube having the footprint of 75 ft × 75 ft (22.9 m × 22.9 m). Willis Tower was the first building for which this design was used. The design allows future growth of extra height to the tower if wanted or needed.
The restrooms on the 103rd floor, at 1,353 feet (412.4 m) high, are the highest (relative to street level) in the Western Hemisphere.
The design was inspired by an advertisement for a package of cigarettes.