The Janka hardness test measures the hardness of wood. It involves measuring the force required to embed an 11.28 mm (0.444 in) steel ball into wood to half its diameter. This method was chosen so that the result would leave an indention 100 mm² (0.16 sq in) in size. 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, perpendicular to the grain, the test is said to be of "side hardness." Testing the cut surface of a stump would be called a test of "end hardness."

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 pounds-force (lbf). In Sweden it is in kilograms-force (kgf), and in Australia, either in newtons (N) or kilonewtons (kN). Sometimes the results are treated as units, e.g., "660 Janka".

To convert the United States pound-force (lbf) units to newtons N multiply pound-force by 0.453 592 37 then multiply by 9.80665 (1 standard g in units of m/s²). Janka hardness N = (lbf x 0.453 592 37) x 9.80665 OR multiply by 4.44822161526. To get lbf from N, multiply N by 0.224808943099736.

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

The Janka Hardness test below is done in accordance with ASTM D 1037-7 testing methods. Please note, no flooring is ever tested. The test is done on raw material stock that can range from 1" to 2" thick. Note these numbers are an average! A standard deviation exists for each species, but these numbers do not get published. The chart is not an absolute; it is meant to help you understand which woods are harder than others. There are other factors that can affect how flooring performs: type of core (for engineered flooring), grain direction and thickness floor or top wear surface.