Every solid matter, be it steel, cast iron, rubber or wood has a method of testing its properties. One of the important aspects is the hardness of a solid which determines its ability to withstand wear and abrasion. Every solid has its own methods of testing called by different names. The hardness of wood is classified by the Janka Hardness Test.

The Janka Hardness Test is a measure of the force required to insert a steel ball measuring 11.28 millimeters (0.444”) into wood up to half its diameter in depth. In doing so, the ball would have made an indentation of exactly 100 square millimeters in area.

There is some confusion in the outcome as different countries observe different units of measurement. In the United States, the measurement is in pounds force, abbreviated to lbf. Brazilian Cherry, for example is 3650 on the Janka hardness scale, which means it requires 3650 pounds force to be applied to the ball to leave an indentation of 100 square millimeters. When communicating between countries, a conversion factor is applied to bring it to a common denominator.

There are two conventions in measuring the hardness of wood. If it is taken across the grain or perpendicular to it, it is known as "side hardness" whereas if it is taken with the grain it is known as "end hardness," as would be the case when measuring the wood across the face of a stump.

Based on the hardness test the Janka Hardness Chart has been created for all common species of wood. This ranges from 4500 for Lignum Vitae, the hardest known wood, to just 100 for Balsa.