What the test is

1. The Janka test presses an 11.28 mm steel ball into a wood sample until it is embedded to half its diameter. The force required, in pounds-force, is the Janka rating. Ipe reads around 3,510 lbf. White oak reads about 1,
2. 3
3. 5
4. Eastern white pine reads near 380.

That is the whole test. One ball, one direction, one clean sample at a controlled moisture content.

What it predicts well

Dent and wear resistance on a floor. This is the test's home turf. A high-Janka species will take dropped tools, chair legs, and dog claws better than a low-Janka one.

How hard the wood is to machine. High Janka generally means dulled blades, pre-drilling for every fastener, and slower going at the miter saw.

Relative feel underfoot. Softer species read as warmer and more forgiving; harder ones read as more solid.

What it does not predict

Rot resistance. Hardness and decay resistance are unrelated properties. Decay resistance comes from extractives in the heartwood, not density. Black locust and western red cedar sit far apart on the Janka scale and both perform well outdoors. Hard maple is dense and rots quickly in ground contact.

Dimensional stability. How much a board moves across the grain with seasonal humidity is a function of tangential and radial shrinkage coefficients. Some very hard species move a great deal, which is why high-Janka decking still needs generous gapping.

Insect and marine borer resistance. Again, chemistry, not density.

Strength as a structural member. Span capacity depends on modulus of elasticity and bending strength. A Janka number tells you nothing about whether a joist will deflect.

Splitting behavior. Dense species are often more prone to splitting at fasteners, not less.

The properties to check instead

For an exterior deck, look at natural durability class and dimensional stability first, hardness third. For an interior floor in a busy house, Janka genuinely belongs near the top. For a table top, movement and finishing behavior matter more than dent resistance. For a structural beam, Janka is irrelevant — go to span tables.

How to use the number honestly

Treat Janka as one column in a comparison, not a ranking of quality. A species is not "better" because its number is higher; it is harder, which is sometimes exactly what you want and sometimes a liability at the saw.

Compare full property sets side by side in the species comparison tool, see the hardness scale in context on the Janka hardness chart, and read how we source these values.