The Janka Hardness Test measures the capacity of a wood to withhold pressure. This is done by measuring the amount of force required to insert a 11.28 millimeter (.444 inches) diameter steel ball half its diameter deep into the wood. Doing so creates a circular indention with an area of 100 square millimeters.
These particular data are expressed in pounds-force (lbf), and are side hardness data. This means that the testing was done on the surface of a plank, with the force exerted perpendicular to the grain.
The Janka Hardness Scale for Hardwood species
