The tread depth should be at least 10 inches (254 mm). The tread depth is measured horizontally between the vertical planes of the foremost projection of adjacent treads and at a right angle to the tread’s leading edge. The greatest tread depth within any flight of stairs or stairway should not exceed the smallest by more than 3/8 inch (9.5 mm). The riser height is at most 7 and 3/4 inches (196 mm). The riser is measured vertically between leading edges of the adjacent treads. The greatest riser height within any flight of stairs or stairway shall not exceed the smallest by more than 3/8 inch (9.5 mm).