`"Head Joint Width"_"Decimal Inch" = "selection"["5/16"]`

0.3125

This equation provides the decimal equivalent brick width in inches from a selection of standard head joint sizes.  This equation is an EiE support equation to a number of other masonry equations which need the head joint width selection as an input. 

This equation assumes the size of the joint is between 1/4 inch and 3/4 inch.  Smaller than 1/4 inch joints are difficult to properly finish (strike) and larger than three quarters uses an excessive amount of mortar.

The input to this equation is the selection from the following head joint widths:

1/4, 5/16, 3/8, 7/16, 1/2, 9/16, 5/8, 11/16, 3/4