To convert lineal feet to board feet, multiply the lineal feet by:

size	factor	size	factor	size	factor	size	factor
1 x 2	0.1667	1 1/4 x 2	0.2083	1 1/2 x 2	0.25	2 x 2	0.3333
1 x 3	0.25	1 1/4 x 3	0.3125	1 1/2 x 3	0.375	2 x 3	0.5
1 x 4	0.3333	1 1/4 x 4	0.4167	1 1/2 x 4	0.5	2 x 4	0.6667
1 x 5	0.4167	1 1/4 x 5	0.5208	1 1/2 x 5	0.625	2 x 5	0.8333
1 x 6	0.5	1 1/4 x 6	0.625	1 1/2 x 6	0.75	2 x 6	1
1 x 8	0.6667	1 1/4 x 8	0.8333	1 1/2 x 8	1	2 x 8	1.333
1 x 10	0.8333	1 1/4 x 10	1.0417	1 1/2 x 10	1.25	2 x 10	1.6667
1 x 12	1	1 1/4 x 12	1.25	1 1/2 x 12	1.5	2 x 12	2

Sources:  NHLA, Architectural Woodwork Association,
Kitchen Cabinet Manufacturers Association