It measures board feet for saw logs, given specific diameter and length of log from 8 ft to 18 foot in length. This folding ruler is good up to 48″ diameter. To get an 8′ calculation use the 16′ scale and divide it by two. This was formulated around the year 1825. Doyle measures (Scribner’s considered too high) for smaller logs and Scribner measures (Doyle’s considered too high) for larger logs. Hud-Son and lumbermen believe that Doyle loss allows for saw kerf of the old circular saw blades. Hud-Son Portable sawmill owner’s average 15 to 25% more yield from this scale using narrow kerf band blades.