The most common test for testing wood hardness is known as the Janka hardness test. The actual number listed in the wood profile is the amount of pounds-force (lbf) or newtons (N) required to imbed a .444″ (11.28 mm) diameter steel ball into the wood to half the ball’s diameter.
Janka hardness testing
While most people would be looking for the hardest wood, just out of curiosity, here’s a list of the ten softest woods on the site. Keep in mind that five out of these ten woods (including the three softest) are considered hardwoods. This just goes to show that the terms hardwood and softwood merely refer to the botanical classification of the trees as either conifers (softwoods) or angiosperms (hardwoods). There’s no guarantee that any given hardwood will actually be hard!
It’s common knowledge, but Balsa is indeed the softest and lightest of all commercial woods. Nothing else even comes close. Useful for insulation, buoyancy, and other special applications.
Note: A hardwood named Quipo (Cavanillesia platanifolia) is commonly reported as the softest known wood, with an alleged Janka hardness of 22 lbf (98 N). However, the wood is omitted from this list for two main reasons. First, it is virtually unobtainable and not commercially available outside of its natural range in Central/South America. Secondly, the purported hardness seems highly questionable, especially in light of the fact that Quipo seems to be very susceptible to rot, and on one USDA test, it was remarked that “the results for quipo may have been influenced by the presence of considerable decay.” Furthermore, when comparing Quipo with Balsa, it has been shown that the two woods are virtually identical in hardness, with the absolute lowest recorded Janka hardness values, in the range of 20-35 lbf (89-156 N), were actually from Balsa, and not Quipo.