Janka Hardness Rating
The Janka Hardness Rating gives a numerical representation of the durability of various types of wood, essential for matching material to its intended usage, ie: higher levels of foot traffic need a more durable species with a higher Janka rating.
The Janka test 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. 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-your-selfer’, remember that the harder the wood, the more difficult it will be to saw or nail, so unless you have top-of-the-line equipment and a lot of wood flooring experience, you may want to select the species with a lower Janka rating.
This comparative reference on different species in order from highest to lowest Janka Rating, may be of assistance: