How strong is balsa wood?
Balsa's typical strength in compression, tension and shear, how it scales with density, and why those numbers are estimates and not guarantees.
Updated · 1 min read
Short answer: balsa is weak in absolute terms but strong for its weight. For typical hobby balsa at about 150 kg/m³ (0.15 times the density of water), our planning estimates are about 10 MPa in compression along the grain, 20 MPa in tension along the grain and 2 MPa in shear. Across the grain it is far weaker, around 1 MPa or less.
Planning estimates at 150 kg/m³
| Property | Estimate | Plausible range |
|---|---|---|
| Stiffness along the grain | 3.5 GPa | 2.0 to 4.5 GPa |
| Compression along the grain (short piece) | 10 MPa | 7 to 15 MPa |
| Tension along the grain | 20 MPa | 12 to 35 MPa |
| Shear along the grain | 2 MPa | 1.5 to 2.8 MPa |
| Compression across the grain (crushing) | 0.8 MPa | 0.4 to 1.2 MPa |
These come from published tests and wood references, gathered and reconciled in our balsa data sheet. Tension across the grain and rolling shear have no good balsa test that we could find, so those values are borrowed from general wood data.
Density changes everything
Strength and stiffness rise roughly in proportion to density, so a 200 kg/m³ stick is noticeably stronger than a 100 kg/m³ one. But samples of the same density still vary by 15 to 25% or more, so two sticks of the same weight are not equally strong.
What it means for a bridge
- A short stick is strong. A long thin stick in compression buckles at far less than these numbers. See truss basics.
- Balsa is weak across the grain, which is why joints and bearing points fail first. See why bridges fail.
- Moisture matters, with perhaps a 10% change over ordinary humidity ranges.
Treat these as estimates, not guarantees, and test your own sticks. See choosing sticks.