Balsa — wood data
- Janka
- —
- Density
- 11.2lb/ft³
- T/R ratio
- 2.5
- Spec. gravity
- 0.16
Balsa (Ochroma pyramidale) is an imported species. It has a specific gravity of 0.16, and an air-dry density near 11 lb/ft³ (179 kg/m³) (mechanical values at 12% moisture content). It shrinks 7.6% tangentially and 3% radially from green to oven-dry (T/R ratio 2.5), so a 12-inch flatsawn board moves about 0.18 in (≈ 3/16 in) across a 6-point moisture swing (about 6%→12% MC).
Properties
| Specific gravity | 0.16 |
|---|---|
| Janka hardness (side) | — |
| Density (air-dry, 12% MC) | 11.2 lb/ft³ (179 kg/m³) ovendry-weight basis: 10 lb/ft³ |
| Weight per board foot (air-dry) | 0.93 lb |
| Radial shrinkage (green→oven-dry) | 3% |
| Tangential shrinkage (green→oven-dry) | 7.6% |
| Volumetric shrinkage (green→oven-dry) | 10.8% |
| T/R ratio (stability indicator) | 2.5 |
Shrinkage is total green-to-oven-dry (Table 4-4). Specific gravity basis: estimated average clear wood SG (ovendry weight); volume basis per source, FPL-GTR-282 Table 5-5b. Primary density is air-dry at 12% MC: 62.4 × specific gravity × 1.12 (includes the ~12% bound-water mass — how dried lumber is actually weighed). The ovendry-weight figure (62.4 × SG, wood substance only) is shown beneath it. Values shown as “—” have no matched entry in the Wood Handbook strength tables (5-3b/5-5b).
Data: USDA Forest Products Laboratory, Wood Handbook FPL-GTR-282 (2021), public domain.
Estimated movement by board width
| Board width | Flatsawn (tangential) | Quartersawn (radial) |
|---|---|---|
| 4″ | 0.061″ (≈ 1/16 in) | 0.024″ (≈ 1/32 in) |
| 6″ | 0.091″ (≈ 3/32 in) | 0.036″ (≈ 1/32 in) |
| 8″ | 0.122″ (≈ 1/8 in) | 0.048″ (≈ 1/16 in) |
| 12″ | 0.182″ (≈ 3/16 in) | 0.072″ (≈ 1/16 in) |
Need a different width or moisture swing? Run the movement calculator preloaded with Balsa. A T/R ratio near 1 means the wood moves evenly in both directions and tends to stay flat; higher ratios cup and distort more as moisture changes.
Seasonal movement by city
| City | Driest month (EMC) | Most-humid month (EMC) | EMC swing | 12″ board movement |
|---|---|---|---|---|
| Phoenix, AZ | Jun · 4.4% | Dec · 9% | 4.6 pts | 0.14″ (≈ 1/8 in) |
| Chicago, IL | May · 12.3% | Dec · 15.1% | 2.8 pts | 0.085″ (≈ 3/32 in) |
| Miami, FL | Apr · 12.2% | Sep · 14.4% | 2.2 pts | 0.067″ (≈ 1/16 in) |
Computed with ΔD = D × (S ÷ 30 ÷ 100) × ΔEMC, where S is Balsa's total green-to-oven-dry tangential shrinkage (7.6%) and 30% is the fiber-saturation-point convention. The EMC figures are USDA Table 13-1 monthly equilibrium moisture content for outdoor exposure (through 2010); a climate-controlled interior swings less, so treat these as an upper-bound seasonal range for each location.
Sources & method
Shrinkage, specific gravity, and hardness values follow the USDA Forest Products Laboratory Wood Handbook (FPL-GTR-282, 2021); derived figures (movement, weight per board foot, T/R ratio, kg/m³, density from specific gravity) are computed with the formulas documented on the methodology page.