What is Janka Hardness?
Janka hardness is the standard method for measuring how hard a wood is. It answers the practical question: "Will this wood dent, scratch, or wear out easily?"
The Test Method
How it works:
- A steel ball (11.28 mm diameter) is pressed into the wood
- Force is gradually increased until ball is embedded halfway
- The maximum force required is the Janka rating
- Measured in pounds-force (lbf) or newtons (N)
Test specimen:
- Minimum 2 inches × 2 inches × 6 inches
- Tests done on radial, tangential, and end-grain surfaces
- Side hardness most commonly reported (average of radial/tangential)
- End-grain hardness typically 1.5-2x higher
Standardization:
- ASTM D143 (American standard)
- ISO 3350 (International standard)
- Results from different labs should be comparable
- Wood must be at 12% moisture content
Understanding the Numbers
Hardness Scale
Very Soft (< 500 lbf):
- Balsa (67 lbf) - softest commercial wood
- Easily dented by hand pressure
- Poor for flooring, good for carving
Soft (500-999 lbf):
- Most pines (380-870 lbf)
- White cedar (320 lbf)
- Suitable for walls, ceilings, light furniture
- Not recommended for high-traffic floors
Medium (1,000-1,499 lbf):
- Black walnut (1,010 lbf)
- Teak (1,155 lbf)
- Good balance of workability and durability
- Suitable for most applications
Hard (1,500-2,499 lbf):
- Red oak (1,290 lbf) - benchmark hardness
- White oak (1,360 lbf)
- Hard maple (1,450 lbf)
- Excellent for flooring, furniture, decking
Very Hard (2,500-3,999 lbf):
- Santos mahogany (2,200 lbf)
- Wenge (1,630 lbf)
- Ipe (3,680 lbf)
- Extremely durable, challenging to work
Ultra Hard (≥ 4,000 lbf):
- Lignum vitae (4,500 lbf)
- Australian Buloke (5,060 lbf) - hardest in the world
- Difficult to machine, self-lubricating
- Specialty applications
Costa Rican Woods
Ultra-Hard Costa Rican Species:
Guayacán Real (Guaiacum sanctum): ~4,390 lbf
- One of the world's hardest woods
- Lignum vitae ("tree of life")
- So hard it sinks in water
- Self-lubricating due to resin content
- Historic use in ship bearings
- Now CITES protected
Cocobolo (Dalbergia retusa): ~2,960 lbf
- Rosewood family
- Beautiful color (red-orange to deep purple)
- Excellent for knife handles, turned items
- Natural oils resist water and insects
- CITES Appendix II
Cristóbal (Platymiscium pinnatum): ~2,350 lbf
- Rosewood relative
- Orange-red heartwood
- Fine furniture and cabinetry
- Good dimensional stability
Ron Ron (Astronium graveolens): ~2,200 lbf
- "Aroeira" or "Goncalo alves"
- Striking grain patterns
- Very durable and dense
- Decking, flooring, tool handles
Medium-Hard Costa Rican Species:
Teca (Tectona grandis): ~1,155 lbf
- Teak (introduced but widely planted)
- Natural oils make it water-resistant
- Boat building standard
- Stable, minimal warping
Cedro Amargo (Cedrela odorata): ~600 lbf
- Spanish cedar (not a true cedar)
- Softer, easier to work
- Aromatic (cigar boxes)
- Furniture, veneers
Caoba (Swietenia macrophylla): ~800-1,000 lbf
- True mahogany
- Beautiful reddish-brown
- Fine furniture, musical instruments
- CITES protected due to overharvesting
Melina (Gmelina arborea): ~560 lbf
- Plantation species
- Fast-growing
- Light construction, pulp
- Not suitable for flooring
Practical Applications
Flooring Selection
High-traffic residential (≥1,200 lbf):
- Living rooms, kitchens, hallways
- Resists denting from furniture and foot traffic
- Examples: Oak, maple, most tropical hardwoods
Commercial/heavy traffic (≥1,500 lbf):
- Retail stores, restaurants, offices
- Must withstand rolling carts, high heel shoes
- Examples: Ipe, santos mahogany, hickory
Low-traffic (800-1,199 lbf):
- Bedrooms, closets, studies
- Softer woods acceptable
- Examples: Walnut, cherry, cedar
Never for flooring (< 800 lbf):
- Will dent and wear quickly
- Pine, fir, cedar (unless in very low traffic)
- Better for walls and ceilings
Furniture and Cabinetry
Tabletops and desktops (≥1,000 lbf):
- Resist denting from objects being set down
- Walnut, oak, maple, tropical hardwoods
Chair seats and arms (≥1,200 lbf):
- Constant pressure and friction
- Need extra durability
Cabinet doors (800-1,500 lbf):
- Moderate hardness prevents dings
- Still workable with standard tools
- Cherry, maple, oak
Drawer bottoms and backs (< 800 lbf):
- Hidden, low-stress areas
- Softer woods save money
- Pine, poplar acceptable
Outdoor Applications
Decking (≥1,500 lbf recommended):
- Foot traffic, furniture, weather exposure
- Ipe (3,680 lbf) - "ironwood," lasts 50+ years
- Teak (1,155 lbf) - classic choice, naturally rot-resistant
- Cumaru, massaranduba also excellent
Outdoor furniture (≥1,200 lbf):
- Must resist weather and use
- Teak standard for good reason
- White oak traditional choice
- Eucalyptus increasingly popular
Fence posts (varies):
- Below-ground needs rot resistance more than hardness
- Above-ground can benefit from hardness
- Cocobolo, lignum vitae historic choices (now protected)
Limitations of Janka Testing
What it DOES measure:
- Resistance to denting/indentation
- Side hardness (radial/tangential)
- Comparative hardness between species
- Suitability for wear applications
What it DOESN'T measure:
Dimensional stability:
- Some hard woods swell/shrink dramatically
- Teak is stable despite moderate hardness
- Test separately for moisture movement
Workability:
- Hardness ≠ difficulty to machine
- Grain structure matters more
- Silica content dulls tools (iroko, teak)
- Resin content affects sanding
Durability/rot resistance:
- Balsa is soft but resists decay in some conditions
- Hardness doesn't predict insect/fungal resistance
- Chemical composition more important
Strength:
- Hardness ≠ bending strength or stiffness
- Balsa is soft but strong for its weight
- Structural calculations need different tests
Appearance:
- Says nothing about color, grain, figure
- Beauty is independent of hardness
Variability Factors
Within-tree variation:
- Heartwood harder than sapwood (20-30% difference)
- Base of tree often harder than top
- Compression wood much harder
- Can vary 10-15% within same board
Between-tree variation:
- Growing conditions affect hardness
- Soil, climate, growth rate
- Fast-grown softer than slow-grown
- Can vary 20% or more
Moisture content:
- Standard test at 12% moisture
- Wet wood tests softer (20-40% softer)
- Kiln-dried may test slightly harder
- Always compare at same moisture %
Grain orientation:
- End grain 1.5-2x harder than side grain
- Test orientation must be specified
- Side hardness standard for flooring
Misconceptions
"Harder is always better"
Not true because:
- Harder woods more difficult to work
- Tools dull faster
- Harder to nail/screw without pilot holes
- More expensive
- Heavier to transport and install
When softer is better:
- Carving and turning
- Musical instrument soundboards (need flexibility)
- Thermal insulation
- Weight-sensitive applications
- Budget constraints
"Tropical hardwoods are always harder"
Reality:
- Many tropical species are softer than oak
- Balsa is tropical and the softest
- Cedar, mahogany moderate hardness
- "Hardwood" refers to leaf type (broadleaf), not hardness
"Engineered flooring doesn't matter"
Truth:
- Only the wear layer hardness matters
- 3mm wear layer of ipe (3,680 lbf) very durable
- 1mm wear layer of same wood wears quickly
- Total thickness includes plywood/HDF substrate
Sustainability Considerations
Overharvested Hard Species
CITES-listed hardwoods:
- Lignum vitae (Guayacán) - Appendix II
- Mahogany - Appendix II
- Rosewoods (including Cocobolo) - Appendix II
- Legal harvest requires permits
- Illegal logging still widespread
Alternatives to endangered species:
- Domestic hardwoods (oak, maple, hickory)
- Plantation teak (sustainably managed)
- Bamboo (not wood but hard, 1,380 lbf)
- Thermally modified softer woods
- Engineered products
Plantation vs. Old-Growth
Plantation-grown typically:
- Fast-grown (10-25 years to harvest)
- Lower Janka hardness (20-30% softer)
- More uniform, less character
- Environmentally preferable
- More affordable
Old-growth typically:
- Slow-grown (50-200+ years)
- Higher Janka hardness
- More color variation and character
- Often illegal/unsustainable harvest
- Increasingly unavailable
Costa Rican Context
Traditional Hardwood Use
Colonial construction:
- Guayacán for mill gears (self-lubricating)
- Cocobolo for decorative elements
- Cristóbal for furniture
- Ron Ron for ox cart wheels
Modern sustainability:
- Forest Law restricts harvest
- Reforestation incentives (PSA program)
- CITES enforcement at ports
- Shift to plantation species
Available alternatives:
- Melina (fast-growing, soft)
- Teak (plantations, medium-hard)
- Eucalyptus (fast-growing, hard)
- Pochote (native, moderate)
Certification Programs
FSC (Forest Stewardship Council):
- Verifies sustainable forest management
- Available for some Costa Rican operations
- Guarantees legal, responsible harvest
SFI (Sustainable Forestry Initiative):
- North American standard
- Some imports certified
Look for certified products when possible
Choosing the Right Hardness
Decision Matrix
For flooring:
- Identify traffic level (low/medium/high/commercial)
- Select minimum Janka (800/1,200/1,500/2,000 lbf)
- Consider pets (add 200-300 lbf for dogs)
- Factor in appearance, budget
- Verify moisture stability
For furniture:
- Determine use (light/moderate/heavy)
- Select range (800-1,000/1,000-1,500/1,500+ lbf)
- Balance workability vs. durability
- Consider aesthetic requirements
- Check availability and cost
For outdoor:
- Rot resistance MORE important than hardness
- Janka ≥1,200 lbf recommended
- Check natural oils/preservatives
- Consider weathering color change
- Verify local availability
Related Concepts
- Brinell hardness: Similar test for metals, wood less than metal
- Density: Related to hardness but not identical
- Specific gravity: Weight compared to water, correlates with hardness
- Compression strength: Force to crush wood parallel to grain
- Bending strength (MOR): Resistance to breaking under load
- Stiffness (MOE): Resistance to deflection under load
- Shear strength: Resistance to sliding failure