The Janka chart is the most-consulted species selection tool in the trade. Every woodworker has one bookmarked. You use it for flooring, workbench tops, cutting boards, tool handles. Higher number, harder wood, more durable. That's the logic.
The logic is wrong about two of those three.
The Janka test is simple. A steel ball, 0.444 inches in diameter, is pressed halfway into the wood's face. The load required is the number. One slow push, one measurement. That's it.
What that means: Janka measures resistance to slow, concentrated indentation. A furniture leg sitting on a floor for eight years. A clamp screw biting into a bench top. A stiletto heel. These are Janka questions. The chart handles them well.
What that doesn't mean: Janka measures durability in general.
Three failure modes it doesn't track.
Abrasion. Flooring wears from foot traffic grinding wood fibers sideways, not from pressing objects straight in. The failure is surface-level and repeated. There are high-Janka species that have worn faster in real floor installations than lower-ranked ones, because abrasion follows different physics than indentation. The chart won't tell you this.
Dynamic impact. Drop a hammer on a bench top. The load is fast, and the failure mode is governed by material properties that slow-press testing doesn't capture. High Janka helps, but Janka isn't the measurement you want here.
Machining. How a species takes a hand plane, a router, a chisel: that's anatomy, moisture content, and grain angle as much as it is density. Interlocked grain in wenge makes it harder to work than its Janka rank suggests. Hard maple machines cleanly relative to its hardness. The chart tells you neither of these things.
The Janka number correlates with density so tightly that it's essentially a density proxy. Specific gravity to roughly the 2.25 power. Dense wood is hard to indent slowly. That's a useful fact. It just isn't a general statement about durability.
The fix is one question before you open the chart: what's the failure mode?
Static indentation: Janka is the right number. Abrasion: find species-level wear data from flooring applications. The information exists and it isn't the same list as the Janka rankings. Dynamic impact: ask someone who's dropped tools on the species in question. Machining: same thing.
The chart is good at what it does. It doesn't do everything.