Black locust (Robinia pseudoacacia) in numbers: hardness, weight, strength, shrinkage, movement and decay resistance, taken from the tables of the USDA Forest Products Laboratory’s Wood Handbook, followed by what the wood is like to work and what it is used for. The values are averages from small, clear, straight-grained test specimens; real boards with knots, sloping grain or a different moisture content will differ.
| Property | Black locust |
|---|---|
| Janka side hardness, 12% moisture | 1,700 lbf (7,562 N) |
| Janka side hardness, green | 1,570 lbf |
| Hardness rank | harder than 133 of the 168 woods with a 12% hardness value in our database |
| Density at 12% moisture | 48.2 lb/ft³ (773 kg/m³) |
| Weight per board foot (12%) | 4.02 lb |
| Specific gravity (ovendry weight, green volume) | 0.66 |
| Specific gravity at 12% moisture | 0.69 |
| Bending strength (modulus of rupture), 12% | 19,400 psi |
| Stiffness (modulus of elasticity), 12% | 2.05 million psi |
| Crushing strength (compression parallel to grain), 12% | 10,180 psi |
| Shrinkage green to ovendry: radial / tangential / volumetric | 4.6% / 7.2% / 10.2% |
| Tangential / radial shrinkage ratio | 1.57 |
| Dimensional change per 1% moisture (radial / tangential) | 0.00158 / 0.00252 |
| Impact bending (drop height to failure), 12% | 57 in. |
| Heartwood decay resistance | very resistant |
The species in this group
Lumber is often sold under a group name. These are the species the Wood Handbook reports for this group, with the same properties side by side.
| Species | Janka (lbf) | Density (lb/ft³) | Bending (psi) | Stiffness (Mpsi) | Crushing (psi) | Shrink T/R (%) | Decay resistance |
|---|---|---|---|---|---|---|---|
| Black locust Robinia pseudoacacia | 1,700 | 48.2 | 19,400 | 2.05 | 10,180 | 7.2 / 4.6 | very resistant |
| Honeylocust Gleditsia triacanthos | 1,580 | 44.9* | 14,700 | 1.63 | 7,500 | 6.6 / 4.2 | moderately resistant |
What it is
Black locust (Robinia pseudoacacia), sometimes called yellow locust, grows from Pennsylvania along the Appalachian Mountains to northern Georgia and Alabama, and is also native to western Arkansas and southern Missouri. Most black locust timber is produced in Tennessee, Kentucky, West Virginia and Virginia. The sapwood is narrow and creamy white; the heartwood, when freshly cut, varies from greenish yellow to dark brown.
Honeylocust (Gleditsia triacanthos) is a different tree, shown in the table for comparison. The Handbook credits it with attractive figure and color, hardness and strength, but says it is little used because it is scarce. Its wide, yellowish sapwood contrasts with light red to reddish-brown heartwood.
Working properties
Black locust is very heavy, very hard, very resistant to shock and very strong and stiff, with moderately low shrinkage. Its heartwood has high decay resistance and is in the very resistant class of the Handbook’s durability table (Table 14-1). Honeylocust is also very heavy, very hard, strong in bending, stiff and resistant to shock; its species description calls it durable in contact with the ground, although Table 14-1 places its heartwood one class lower, in moderately resistant.
Weight and density
At 12% moisture content black locust weighs about 48.2 lb per cubic foot, so one board foot (144 cubic inches) weighs about 4.02 lb. An 8 ft board of actual size 3/4 × 5-1/2 in. (a 1×6) holds 0.229 cubic feet and weighs about 11.0 lb; an 8 ft piece 1-1/2 × 3-1/2 in. (a 2×4 size) weighs about 14.1 lb. Green wood is much heavier because of the water it holds.
Density here is ovendry weight plus 12% water, divided by volume at 12% moisture content (Wood Handbook Eq. 4-12: density = specific gravity at 12% × 62.43 lb/ft³ × 1.12).
Wood movement
The Wood Handbook gives dimensional change coefficients for each species: the fraction of its width a board gains or loses for every 1% change in moisture content, between about 6% and 14%. For black locust they are 0.00252 across a flatsawn face (tangential) and 0.00158 across a quartersawn face (radial).
Worked example: a flatsawn board 7-1/4 in. wide (a nominal 1×8) that goes from 6% moisture content in a heated winter house to 12% in a humid summer changes width by 7.25 × 0.00252 × 6 = 0.11 in. (about 1/8 in.). The same board quartersawn moves 0.069 in. (about 1/16 in.). Allow for this in panels, tabletops and floors; the wood movement calculator works it out for any size and moisture range.
How it compares
| Species | Janka (lbf) | Density (lb/ft³) | Bending (psi) | Stiffness (Mpsi) | Tangential shrink (%) | Decay resistance |
|---|---|---|---|---|---|---|
| Black locust | 1,700 | 48.2 | 19,400 | 2.05 | 7.2 | very resistant |
| Shagbark hickory | 1,880 | 50.3 | 20,200 | 2.16 | 10.5 | slightly or nonresistant |
| White oak | 1,360 | 47.5 | 15,200 | 1.78 | 10.5 | resistant |
| Ipe | 3,680 | 64.0† | 25,400 | 3.14 | 8.0 | very resistant |
| Western redcedar | 350 | 22.4 | 7,500 | 1.11 | 5.0 | resistant |
| Sugar maple | 1,450 | 44.1 | 15,800 | 1.83 | 9.9 | slightly or nonresistant |
Uses
Black locust: round, hewn or split mine timbers, fence posts, poles, railroad crossties, stakes and fuel, plus rough construction and crating. Historically it was important for insulator pins and the wooden pegs used in building ships, for which its strength, decay resistance and moderate shrinkage and swelling suited it well. Honeylocust, where available, is used locally for fence posts and general construction and occasionally for pallets and crating.
Frequently asked questions
How hard is black locust?
Black locust averages 1,700 lbf (7,562 N) on the Janka test at 12% moisture content and 1,570 lbf green, harder than white oak (1,360 lbf) and hard maple (1,450 lbf).
Is black locust rot resistant?
Yes. Its heartwood is in the very resistant decay class of Wood Handbook Table 14-1, the class the Handbook says generally performs satisfactorily in contact with the ground. The narrow sapwood is not durable.
Is honeylocust the same as black locust?
No, they are different trees. Honeylocust is a little softer (1,580 lbf against 1,700 lbf), and Table 14-1 rates its heartwood moderately resistant to decay rather than very resistant, although the Handbook’s species description calls it durable in contact with the ground.
Why was black locust used for wooden ship pegs?
The Handbook says its strength, decay resistance and moderate shrinkage and swelling made it well suited to the wooden pegs used in building ships, and to insulator pins.
Sources
- USDA Forest Products Laboratory. Wood Handbook: Wood as an Engineering Material. General Technical Report FPL-GTR-190, 2010. Chapter 2 (species descriptions), Chapter 4 (Tables 4-3, 4-4 shrinkage; Eq. 4-7, 4-9, 4-12 density), Chapter 5 (Tables 5-3b, 5-5b strength and hardness), Chapter 13 (Table 13-5 dimensional change coefficients), Chapter 14 (Table 14-1 decay resistance)
- USDA Forest Products Laboratory. Wood Handbook, 2021 edition (FPL-GTR-282)
Last reviewed: 24 October 2026. Numbers are generated from our parsed copy of the Handbook tables, so every page uses the same data. Values marked * are estimates computed with the Handbook’s own equations. Spotted an error? Please let us know.