This number is incredibly useful in directly determining how well a wood will withstand dents, dings, and wear—as well as indirectly predicting the difficulty in nailing, screwing, sanding, or sawing a given wood species.
The actual number listed in the wood profile is the amount of pounds-force (lbf) or newtons (N) required to imbed a .444″ (11.28 mm) diameter steel ball into the wood to half the ball’s diameter. This number is given for wood that has been dried to a 12% moisture content, unless otherwise noted.
For reference, White Oak has a Janka hardness of 1,360 lbf (6,000 N), while the super-hard Lignum Vitae has a hardness of an astounding 4,500 lbf (20,000 N). (Who could imagine a wood species that is over three times harder than White Oak?) On the lower end of the spectrum, Basswood has a hardness of around 410 lbf (1,800 N).
Also, in some instances, I’ve estimated the Janka hardness value using equations that use the wood’s basic specific gravity, as found in the paper, “Estimating Janka Hardness from Specific Gravity for Tropical and Temperate Species.”