This section is concerned with buying lumber. There may be different guides for different parts of the world and different lumber vendors, e.g. hardware store versus lumber yard. Those guides are usually not geared towards full-time professional woodworking businesses, as those may have additional avenues of supply.
Prices are commonly given in boardfoot (imperial) or cubic meter (metric) depending on where you are in the world. Thickness in the US is in quarter steps of an inch, e.g. 4/4 or 8/4. This is nominal thickness, after drying, the boards will most likely be ⅛-¼" thinner. (According to Shannon Roger's research1 asking various sawyers, the quarters stem from notches the mill would snap into in steps of ¼" to set the desired thickness.)
A boardfoot is 1" x 12" x 12" i.e. 144 cubic inches. Given all dimensions in inches, it can therefore be calculated as
A board, 8/4 thick (2"), 12" wide and 6' (72") long would be 2"x12"x72"=1728 cubic inches divided by 144 equals 12 boardfoot. Or 6'x1' surface is 6 sft at 2" thick is 2x6 = 12 boardfoot.
Board thickness is usually given in quarters, i.e. 4/4 is a 1" thick board, 8/4 is 2".
For construction lumber, the cross section is often described by "x inches by y inches", e.g. 2 by 8 is a 2" thick board 8" wide.
A cubic meter is 1m x 1m x 1m = 1m³. You can calculate the cubic volume by multiplying the dimensions given in meter, e.g. a 30cm board 5cm thick and 2m long is .3m x .05m x 2m = 0,03m³. If the price of that quality and species is 1200€ per m³, that will end up at 36€.
Buying lumber is important for a good result in a project and can make the difference between a frustrating, bumpy experience and smooth sailing. It is valuable to know and keep the following aspects in mind when selecting and buying lumber.