The Janka hardness test measures the hardness of wood. It involves measuring the force required to embed an 11.28mm (0.444in) steel ball into wood to half its diameter.
This method was chosen so that the result would leave an indention 100mm² (0.16sqin) in size.
It is one of the measures of the ability of a wood species to withstand denting and wear.
The higher the Janka Rating, the harder the timber is.