Janka Wood Hardness Rating

The Janka test measures the force required to embed a 11.28 millimeter (0.444 inch) steel ball into wood to half its diameter. This method was chosen so that the result would leave an indention 100 square millimeters 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.

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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."

Douglas Fir, a relatively soft wood, has a Janka hardness rating of 660 lbf. Brazilian cherry, a very hard wood, has a rating of 2350 lbf, while Brazilian walnut has a hardness rating of 3800 lbf.

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