What this means in plain English: Black Locust moves a moderate amount — a 24-inch flatsawn board will grow and shrink about 0.50 inches across its width between a humid summer and a dry winter. It stays flat beautifully — you can flatsaw it for wide panels without much worry.
How much it moves at common board widths
Find the width of your board on the left. Find your expected moisture swing along the top (8% is typical for a heated/cooled house; 12% is typical outdoors or in a shop without climate control). The number inside the table is how many inches the board will grow and shrink across its width. That’s your slot size.
| Board width | Δ4% MC | Δ6% MC | Δ8% MC | Δ12% MC |
|---|---|---|---|---|
| 6 in | 0.063" | 0.094" | 0.126" | 0.189" |
| 12 in | 0.126" | 0.189" | 0.252" | 0.377" |
| 18 in | 0.189" | 0.283" | 0.377" | 0.566" |
| 24 in | 0.252" | 0.377" | 0.503" | 0.755" |
| 36 in | 0.377" | 0.566" | 0.755" | 1.132" |
How to use it: find your board width in the left column and the expected moisture content change across the top. The cell is the total inches of cross-grain expansion and contraction you should plan for — elongated slots, breadboard clearances, and panel gaps all come from this number.
About Black Locust
- Color
- Yellow-green to dark brown, often with a greenish cast.
- Grain
- Straight to irregular, coarse texture with ring-porous structure.
- Workability
- Difficult due to hardness, dulls tools, excellent for turning.
- Durability
- Exceptionally durable, among the most rot-resistant domestic hardwoods.
- Common uses
- Fence posts, boat building, flooring, tool handles.
- Typical price
- $4-8/bf
- Specific gravity
- 0.69 (oven-dry)
- Modulus of elasticity
- 2,050 × 1,000 psi
The math, explained once
Three numbers multiplied together. That’s it.
width (in) × coefficient × ΔMC (%) = movement (in)
- Width is how wide the board is, measured across the grain.
- Coefficient is the number at the top of this page (tangential 0.00262 for flatsawn, radial 0.00166 for quartersawn).
- ΔMC is how many percentage points the moisture content will change between seasons. Indoor heated spaces: around 8. Shops, garages, or outdoor pieces: 10–14.
These coefficients come from the USDA Forest Products Laboratory and published wood science data. Dead On uses the same numbers on iPhone, iPad, and Apple Watch — your shop math matches your field math.
Calculate Black Locust movement in your project
Dead On has this species built in, alongside 29 others — with visual diagrams for dovetails, box joints, mortise & tenon, and more.
Download Dead On — FreeFrequently asked questions about Black Locust
How much does Black Locust move seasonally?
A 24" wide flatsawn Black Locust board will move about 0.50" across the grain with an 8% change in moisture content (typical indoor seasonal swing). The tangential coefficient is 0.00262 per 1% MC change, the radial coefficient is 0.00166.
Is Black Locust stable enough for wide tabletops?
Black Locust is very stable. The tangential-to-radial ratio is 1.58 (lower is more stable; values under 2.0 are ideal for wide panels). For the most stable results, use quartersawn stock or breadboard ends with elongated slots for seasonal movement.
What is Black Locust typically used for?
Fence posts, boat building, flooring, tool handles.
Is Black Locust a hardwood or softwood?
Black Locust is a hardwood. Its Janka hardness rating is 1700 lbf and specific gravity is 0.69.
Species with similar movement
Hardwoods with tangential coefficients closest to Black Locust’s 0.00262. If your project plan calls for Black Locust but availability is tight, these behave most like it through the seasons.