Janka Hardness Rating
The janka hardness rating gives a numerical representation of the wood’s durability. This is obviously very important for matching material to its intended usage. Higher levels of foot traffic would require a more durable species and thus one with a higher Janka rating.
The Janka test basically involves measuring the amount of force, in pounds per square inch (psi), required to embed half the diameter of a 0.444-inch steel ball into the face of a wood sample. Of course, several tests are performed to find the proper amount of force, and then a series of tests are averaged to determine the rating.Technically, the average includes samples of tangential (plain-sawn) and radial (quarter-sawn) material.
If you are a do-it-yourselfer, there is another point you need to keep in mind about the hardness of wood. The harder the wood, the more difficult it will be to saw or nail. Unless you have top-of-the-line equipment and a lot of wood flooring experience, you may not want to select the species with the highest Janka rating.
Here is a comparative reference on different species: