What is conversion board feet to square feet or square feet to board feet?

you can't straight convert a linear measure to an area measure... missing info.

If the board is 12" wide, then the ratio is one to one. if the board is 6" wide, the linear footage needed would be double. It's a simple ratio.

width of board in inches/12 * length in feet = square feet.

length in feet = square feet * 12 / width