Wood Hardness Scale

The Janka hardness test is the wood hardness scale that measures the hardness of wood. The Janka wood hardness scale is determined by measuring the amount of force that is required to press a steel ball 11.28 mm (0.444 inches) in diameter into the wood to a depth of half the ball’s diameter. The harder the wood, the more force it will take to embed the ball into the wood.

This size of ball was chosen for the wood hardness scale because it gives an indentation with a 100 square mm surface area.

Different Janka wood hardness scales can have different results, which can be a little confusing, since the results of different tests on the same tree species can vary with the grain orientation the wood was tested under. Also different parts of the world use different units of measurements.

If a plank of wood is tested on the side of a plank and the test ball is pressed into the wood perpendicular to the grain, it is testing the side hardness. Within the side test there are other variables with orientation to the grain of the radial rings and the center of the wood. The grain orientation can be radial and perpendicular to the rings or in tangent with the rings, tangential. The results of these two tests will often be very similar. End testing is done on the cross cut section of the wood parallel to the grain. An example would be on the cross cut section of a log or the top of a stump. There can be significant differences between side hardness and end hardness. When comparing wood hardness scales each specimen should have been tested with the same grain orientation to get an accurate comparison.

The results of the Janka wood hardness test are expressed in different units in different parts of the world. In the US the Janka hardness scale is expressed in pounds of force.

Other Janka Hardness Measurements:

kilogram-force (kgf)

newtons (N) or

kilonewtons (kN).

The Janka hardness test is often used to determine the hardness of wood as an indicator of its durability for flooring. When a wood has a higher number on the Janka wood hardness it is considered to be more resistant to denting and wear. It can also be useful in determining how a species of wood will be for nailing, sawing, carving and working with.