I went to http://www.woodzone.com/tips/board_feet/board_feet.htm and found the following.
Calculating Board Feet
"Board Feet" is a measurement of lumber volume. A board foot is equal to 144 cubic inches of wood. Actually it's easy to calculate using the following formula:
(Thickness x Width x Length) / 144 = Board Feet
Note: Lumber is specified by its rough size. This is why a 1"x 4" board is actually 3/4" thick and a 2"x 4" board is actually 1-1/2" thick.
When you are figuring up board feet, keep in mind a waste factor. If you purchase good clear material add about 15% for waste, if you elect to use lower grade material you will have to allow for defects and more wasted material -add about 30%. Take a few boards and run the measurements and you will see how easy this works.
One 2 by 4, 16 feet long (that is 12 16 = 192 inches long) is
Length Thickness Width = 192 1.5 3.5 = 1008 cubic inches.
Hence one 16 foot 2 by 4 is
1008/144 = 7 board feet.
Thus to have 100 board feet you would need
1000/7 = 143
2 by 4's