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Janka hardness

A measure of the hardness of wood, produced by a variation on the Brinell hardness test. The test measures the force required to push a steel ball with a diameter of 11.28 millimeters (0.444 inches) into the wood to a depth of half the ball’s diameter (the diameter was chosen to produce a circle with an area of 100 square millimeters). In Janka's original test, the results were expressed in units of pressure, but when the ASTM standardized the test (tentative issue in 1922, standard first formally adopted in 1927), it called for results in units of force.

The results are stated in various ways in different countries, which can lead to confusion, especially since the name of the actual unit employed is often not attached. In the United States, the measurement is in pounds-force. In Sweden it is apparently in kilogram-force (kgf), and in Australia, Janka hardness ratings are either in newtons (N) or kilonewtons (kN). Sometimes the results are treated as units, e.g., “360 janka.”

The hardness of wood usually varies with the direction of the grain. If testing is done on the surface of a plank, with the force exerted perpendicular to the grain, the test is said to be of “side hardness.” Side hardnesses of a block of wood measured in the direction of the tree's center (radially), and on a tangent to the tree's rings (tangentially), are typically very similar. End testing is also sometimes done (that is, testing the cut surface of a stump would be a test of end hardness). The side hardness of teak, for example, is in the range 3730 to 4800 newtons, while the end hardness is in the range 4150 to 4500 newtons.

The most common use of Janka hardness ratings is to determine whether a species is suitable for use as flooring.

Notes on the table

The pale blue background indicates species growing in North America. Common names are unreliable since two or more species often have the same common name. We show the common name preferred by the Forest Products Laboratory. Except as noted, all samples were tested at 12% moisture content, typical of air-dried wood. Wood varies; a sample picked at random is expected to have a side hardness between about 80% and 120% of the value shown. Blank cells occur where a wood is sold commercially but so far we have found no hardness data.

*Authorities differ significantly on the hardness of Pinus radiata, with published values as high as 792 lbs-force (3.5 kilonewtons) and as low as 625 lbf (2.8 kN).

sources

Forest Products Laboratory.
Wood Handbook: Wood as an Engineering Material.
Technical Report FPL-GTR-113.
Madison, WI: U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory, 1999.

Downloadable from www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.pdf  See also the 2010 edition, which is downloadable chapter by chapter. Hardness information is in chapter 5.

Harry A. Alden
Hardwoods of North America.
General Technical Report FPL-GTR-83.
Madison, WI: U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory, 1995.

Downloadable from www.fpl.fs.fed.us/publications/fplgtr83.pdf

Harry A. Alden
Softwoods of North America.
General Technical Report FPL-GTR-102.
Madison, WI: U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory, 1997.

Downloadable from www.fpl.fs.fed.us/publications/fplgtr102.pdf

B. Alan Bendtsen and Martin Chudnoff.
Properties of Seven Columbian Woods.
USDA, Forest Service, Forest Products Laboratory Research Note FPL-0242
Madison, WI: Forest Products Lab, January 1981.

Martin Chudnoff.
Tropical Timbers of the World.
Agriculture Handbook 607.
Madison, WI: U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory, 1979.

Downloadable from www.fpl.fs.fed.us/publications/chud_total.pdf

David W. Green, Marshall Begel and William Nelson.
Janka Hardness Using Nonstandard Specimens.
Research Note FPL-RN-0303.
Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 2006.

Wayne C. Lewis.
Hardness Modulus as an Alternate Measure of Hardness to the Standard Janka Ball for Wood and Wood-Base Materials.
USDA, Forest Service, Forest Products Laboratory Research Note FPL-0189
Madison, WI: Forest Products Laboratory, March 1968.

Michael C. Wiemann and David W. Green.
Estimating Janka Hardness from Specific Gravity for Tropical and Temperate Species.
USDA, Forest Service, Forest Products Laboratory Research Paper FPL-RP-643
Madison, WI: Forest Products Laboratory, September 2007.

Downloadable from www.fpl.fs.fed.us/documnts/fplrn/fpl_rn303.pdf

resources

Standards:
ASTM D1037-99. Standard Test Methods for Evaluating Properties of Wood-Base Fiber and Particle Panel Materials.
ASTM D143-94(2000)e1 Standard Methods of Testing Small Clear Specimens of Timber.
ISO 3350:1975. Wood–Determination of static hardness.
ISO 3351:1975. Wood–Determination of resistance to impact indentation.

Janka hardness ratings can also be found at sites selling flooring, such as:

www.wflooring.com/Technical_Info/Species_Tech_Info/species_hardness.htm

www.zoltanfloors.com/tech.html

Some information sources say or imply, incorrectly, that Janka hardness is measured in units of pressure, such as pounds per square inch, and that side hardness is a synonym for Janka hardness.

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Last revised: 29 August 2018.