A board foot (as discussed in What is a Board Foot?) is a volume measurement that is impacted by three dimensions:  thickness, width and length.  A square foot, on the other hand, is an area measurement that involves just two dimensions:  width and length. 

The # of square feet associated with a particular board or timber can be calculated by means of the following formula:

# of square feet  =  ( Width in inches x Length in feet ) / 12


It would be close to accurate to say that boards that are 1" thick have the same # of square feet as they do board feet.  The reason that this statement is not completely accurate is that board footage calculations generally use nominal board dimensions, while square footage calculations typically use actual widths.  Take, for example, a board which is 1" thick (actual 3/4") x 6" wide (actual 5 1/2") x 12' long.  This board has 6 board feet ( (1 x 6 x 12) / 12 ) but only 5.5 square feet ( ( 5.5 x 12) / 12 ).

The square foot unit of measurement is most often used for products like flooring and siding where a certain amount of area needs to be covered. 

Note that:

(1) One can convert board feet into square feet by dividing the # of board feet by the thickness (in nominal inches), then taking into account any nominal width (by multiplying by a factor of (actual width / nominal width.))  For example, 100 board feet of 4" (3.5" actual) x 12" (11.25" actual) boards would translate into 23.4375 square feet ((100 / 4) x (11.25/12)).

(2) One can convert square feet into board feet by multiplying the # of square feet by the thickness (in nominal inches), then adjusting for any nominal width (by multiplying by a factor of (nominal width / actual width.))  For example, 100 square feet of 4" (3.5" actual) x 12" (11.25" actual) boards would translate into 426.67 board feet ((100 x 4) x (12/11.25)).

(3) The orientation of a board or timber does not impact its board footage, but does impact the square footage it covers (unless thickness = width.)  A 6" x 12" (assume full dimensions for ease of calculation) board, for example, used with the 6" dimension as its thickness covers twice as much area (has twice the square footage) as the same board used with the 12" dimension as its thickness.

See Trestlewood's Board Footage / Square Footage calculator for an automated way to calculate board footages and square footages associated with specific lumber and timber dimensions.