The Janka test measures the force required to embed a 11.28 millimeter (.444 inch) steel ball into wood to half its diameter. It is one of the best measures of the ability of a wood species to withstand denting and wear. It is also a good indicator of how hard a species is to saw or nail.

The hardness of wood usually varies with the direction of the wood grain. If testing is done on the surface of a plank, the test is said to be of "side hardness." Testing the cut surface of a stump would be called a test of "end hardness." In materials science, hardness is the characteristic of a solid material expressing its resistance to permanent deformation. ... Wood grain describes the alignment, texture and appearance of the wood fibres. ...

The results are stated in various ways, which can lead to confusion, especially when the name of the actual units employed is often not attached. In the United States, the measurement is in pound-force. In Sweden it is in kilogram-force (kgf), and in Australia, either in newtons (N) or kilonewtons (kN). Sometimes the results are treated as units, e.g., "660 Janka." The pound-force is a non-SI unit of force or weight (properly abbreviated lbf or lbf). The pound-force is equal to a mass of one pound multiplied by the standard acceleration due to gravity on Earth (which is defined as exactly 9. ... The deprecated unit kilogram-force (kgf) or kilopond (kp) is the force exerted by one kilogram of mass in standard Earth gravity (defined as exactly 9. ... This article is about the SI unit of force. ...

Douglas Fir, a relatively soft wood, has a Janka hardness rating of 660 (using pound-force). Brazilian cherry, a very hard wood, has a rating of 2350 (also using pound-force). Species See text. ...

A common use of Janka hardness ratings is to determine whether a species is suitable for use as flooring.