The thickness of lumber is read in quarters of inches.

Four-quarters 4/4
Five-quarters 5/4
Six-quarters 6/4
Eight-quarters 8/4
Etc.

In rough lumber (also called nominal thickness or size) that has not been surfaced (planed smooth)

4/4 rough = 1″ board thickness
5/4 rough = 1-1/4″ board thickness
6/4 rough = 1-1/2″ board thickness
8/4 rough = 2″ board thickness

But surfaced lumber that has been planed smooth may vary slightly in thickness.

4/4 surfaced = 3/4″ to 13/16″
5/4 surfaced = 1″ to 1-1/8″
6/4 surfaced = 1-1/4″ to 1-3/8″
8/4 surfaced = 1-3/4″ etc.

Commercial Lumber sizes in 4/4 lumber are as follows
1 x 2 = 3/4″ x 1-1/2″
1 x 4 = 3/4″ x 3-1/2″
1 x 6 = 3/4″ x 5-1/2″
1 x 8 = 3/4″ x 7-1/4″
1 x 10 = 3/4″ x 9-1/4″
1 x 12 = 3/4″ x 11-1/4″

A Board Foot

1 foot square by 1 inch thick
or
144 cubic inches

Either multiply the length in feet times the thickness in inches times the width in inches and then divide by 12

Length’ x thickness” x width” / 12 =1 bf

or

Multiply the length in inches times the thickness in inches times the width in inches and then divide by 144

Length” x thickness” x width” / 144 = 1 bf