A cord is an informal unit of volume for stacked firewood,  sometimes called a rick . 
Width and height is typically the same as a string , but the depth can vary.  The front face is the same as a string (4’x8 ‘), hence the name. The depth is generally 16 “(for use in residential fireplaces) but can be anything from 12 inches to 32 inches.
This product is a product of 32 to 85 cubic feet . Several states only allow to be sold by the cord or fractions of a cord, to avoid confusion among consumers.  
The wood should be stacked and stacked so that they are parallel and air gaps are minimized. It should not be cross-stacked (alternating directions), as this adds considerable empty space to the stack. 
Common volumes for a face cord
4 feet x 8 feet x 12 inches = 1/4 cord (32 cubic feet)
4 feet x 8 feet x 16 inches = 1/3 cord (42.66 cubic feet)
4 feet x 8 feet x 24 inches = 1/2 cord (64 cubic feet, 1.8 cubic meters)
4 feet x 8 feet x 32 inches = 2/3 cord (85 cubic feet)
- Jump up^ Cardarelli, F. (2003). Encyclopaedia of Scientific Units, Weights and Measures. Their SI Equivalences and Origins . London: Springer. p. 52. ISBN 978-1-4471-1122-1 .
- Jump up^ “Unusual units” . UNC . Retrieved 28 December 2016 .
- Jump up^ “Wood heating” . Utah State Univeresity, Forestry Extension . Retrieved 28 December 2016 .
- ^ Jump up to:a b “Firewood FAQ” . Lumberjax . Retrieved 28 December 2016 .
- Jump up^ “Consumer tips firewood” . Maryland Dept of Agriculture . Retrieved 28 December 2016 .