4/4 = 13/16"
5/4 = 1 1/16"
6/4 = 1 7/16"
8/4 = 1 13/16"
All lumber at OP Hardwoods comes in non-dimensional, so no two boards are alike. When calculating board feet it's important to keep in mind that this is a volume measurement, not a surface area measurement, so for thicker boards there's more footage.
Board Feet = Length(inches) x Width(inches) / 144
That equation will give you the board feet for a standard size 4/4 board.
For example, if you found a 6" wide red oak board that's 96" long(8ft.) you've got : 6" x 96" = 576 (now divide that by 144) and you get 4 board feet.
When dealing with thicker boards you must compensate for the thickness. With 5/4 you add 25%, 6/4 you add 50%, and for 8/4 you add 100% (or just double it)
For example, if you found that same board 6"x96" and it's 6/4 size (1.5" thick) you've got; 6" x 96" = 576 (divide by 144) gives you 4. Add 50% to get 6 board feet.
If that board were 8/4 (1.75" thick) it would be 8 board feet, because you double the footage.
The following list is all domestic hardwoods made available at OP Hardwoods: