When selecting timber or plywood for flooring, cabinetry, furniture or architectural applications, hardness is often considered a key performance indicator. Hardness describes how well a material resists indenting under a concentrated load. However hardness is only one aspect of performance and does not predict overall durability or structural strength.
The most widely used method of measuring timber hardness is the Janka hardness test. This article explains how the test works, how values are reported in Australia, how results vary between species and products, and how to interpret hardness when selecting solid timber and plywood.
The method was developed in 1906 by Gabriel Janka (1864–1932) while studying wood resistance to denting at the United States Forest Products Laboratory. The Janka test became internationally recognised by the 1920s and remains the primary hardness benchmark used in timber flooring and furniture industries worldwide.
What Is the Janka Hardness Test
The Janka hardness test measures the force required to embed an 11.28 millimetre steel ball halfway into the wood surface. The value represents resistance to denting under static pressure.
Although the concept appears simple, its usefulness depends on standardised testing conditions. Published values may differ significantly depending on moisture content, testing orientation and whether data came from laboratory testing or forestry reference tables.
How the Test Is Conducted
1. Sample Preparation
Wood is conditioned to a controlled moisture content, typically around 12 percent for published comparisons in Australia. Samples are clear, defect free and tested most commonly across the grain on side timber surfaces.
2. Force Application
A hydraulic testing machine presses the steel ball into the wood surface at a controlled rate.
3. Measuring the Result
The result is recorded when the equator of the ball becomes flush with the wood surface. Results are reported in kilonewtons in Australia.
4. Why the Result Matters
Higher values indicate greater resistance to denting from foot traffic, furniture, tools and general impact. Harder species maintain surface appearance longer but may be more difficult to machine or fasten.
Radiata Pine (1.6 kN) ▰▱▱▱▱ (Very Soft)
Poplar (2.4 kN) ▰▰▱▱▱ (Soft)
Hoop Pine (3.5 kN) ▰▰▱▱▱ (Moderate Soft)
Baltic Birch (5.6 kN) ▰▰▰▱▱ (Medium-Hard)
Tasmanian Oak (6.0 kN) ▰▰▰▱▱ (Medium-Hard)
American Oak (5.7 kN) ▰▰▰▱▱ (Medium-Hard)
Spotted Gum (8–14 kN) ▰▰▰▰▱ to ▰▰▰▰▰ (Very Hard / Variable)
Ironbark (14–16+ kN) ▰▰▰▰▰ (Extremely Hard)
Measurement Units: Australia vs USA
Australia uses kilonewtons (kN)
This aligns with metric standards including:
AS 1080.1 Timber Methods of Test
AS 2796 Timber Hardwood Sawn and Milled Products
Forestry datasets from ABARES and state agencies
The United States uses pounds force (lbf)
Most American flooring data is published in lbf.
Conversion
1 lbf equals 0.004448 kN
Because of unit differences and variations in moisture content, two published values for the same species may appear inconsistent.
How to Use Janka Hardness When Specifying
Flooring
Less than 5 kN: softer residential floors
6 to 10 kN: standard hardwood floors
More than 10 kN: high impact commercial applications
Interpretation requires additional factors such as coating system, substrate stiffness and installation method.
Furniture and Cabinetry
3 to 7 kN is suitable for most joinery as it balances dent resistance with machining efficiency.
Benchtops and Work Surfaces
More than 6 kN recommended, though stability and finish quality may be more important than raw hardness.
Plywood Selection
Face veneer hardness matters for surface appearance but structural suitability depends mainly on core properties and compliance with relevant plywood standards.
Limitations of the Janka Test
Janka hardness measures only resistance to indentation under a static load. It does not measure:
- scratch resistance
- bending strength
- impact toughness
- modulus of elasticity
- dimensional stability
- surface performance under coatings or finishes
A high Janka value does not guarantee a timber is suitable for high wear environments and a lower value does not indicate poor performance in structural or joinery applications.
For example:
Birch plywood is structurally strong despite lower hardness compared to spotted gum.
Strand woven bamboo has high hardness but is brittle and highly resinated.
Laminate flooring cannot be compared by Janka at all because hardness relates to the wear layer, not the substrate.
Janka Hardness and Plywood: Important Considerations
Janka values apply primarily to solid timber. Using these numbers to evaluate plywood requires caution for several reasons.
Face veneers are often thin, typically 0.3 to 0.6 millimetres, so the effective hardness of the surface relates more to the finish coating and substrate than to raw veneer hardness.
Cross laminated cores distribute pressure differently and resist indentation through layer interaction, not surface hardness.
Hardwood faced plywood can still dent easily when sanding wear reduces veneer thickness.
Plywood performance is a composite of:
- face species hardness
- core construction and thickness
- adhesive type
- modulus of elasticity and shear strength
- finish durability
Therefore Janka hardness is useful only for face selection but not sufficient to assess overall plywood durability.
