WORKBENCH GUIDE
Plywood or MDF? Choosing a sheet material
Plywood is stiffer and lighter; MDF is uniform and machines cleanly. Compare density, stiffness and sheet weight before you buy.
How the panels are made
Plywood is built from veneers glued with the grain of alternate layers at right angles, which gives it strength and stiffness in both directions. MDF is a dry-process fibreboard: wood fibres are refined, mixed with resin and pressed into a uniform panel without grain (Wood Handbook, chapter 11).
Particleboard and OSB sit in between: particleboard has fine particles on the faces and coarser ones in the core, while OSB uses long strands aligned along the panel. Wood Handbook Table 12-1 compares typical properties; the table below adds the weight of an 18 mm sheet of 2,440 × 1,220 mm at each end of the density range.
| Panel | Specific gravity | Stiffness (MOE) | Bending strength (MOR) | Weight of the sheet |
|---|---|---|---|---|
| Plywood | 0.4–0.6 | 6.96–8.55 GPa | 33.72–42.61 MPa | 21.4–32.1 kg |
| OSB | 0.5–0.8 | 4.41–6.28 GPa | 21.8–34.7 MPa | 26.8–42.9 kg |
| MDF | 0.7–0.9 | 3.59 GPa | 35.85 MPa | 37.5–48.2 kg |
| Particleboard | 0.6–0.8 | 2.76–4.14 GPa | 15.17–24.13 MPa | 32.1–42.9 kg |
| Hardboard | 0.9–1 | 3.1–5.52 GPa | 31.02–56.54 MPa | 48.2–53.6 kg |
Stiffness and sag
For shelves, doors and long spans, stiffness (modulus of elasticity) matters more than strength. In Table 12-1 plywood reaches 6.96–8.55 GPa, OSB 4.41–6.28 GPa, MDF 3.59 GPa and particleboard 2.76–4.14 GPa. Deflection is inversely proportional to stiffness, so an MDF shelf bends roughly twice as much as a plywood shelf of the same size under the same load.
Bending strength tells a slightly different story: MDF reaches 35.85 MPa, comparable with plywood at 33.72–42.61 MPa, while particleboard stays at 15.17–24.13 MPa. MDF carries a similar load before breaking but bends more on the way.
Weight, edges and screws
MDF is the heaviest common panel after hardboard: an 18 mm sheet of 2,440 × 1,220 mm weighs about 37–48 kg at the Wood Handbook densities, against about 36 kg for birch plywood at 680 kg/m³ and 25–28 kg for conifer plywood at the Finnish handbook’s densities.
MDF edges can be shaped and finished directly, while plywood shows its layers at the edge, which some designs use as a feature and others cover with banding. Screws need care in particle panels: in particleboard the Wood Handbook warns that over-tightening strips the threads and recommends modest tightening.
Which one to use
Choose plywood where stiffness or low weight matters: shelves, carcasses that will be moved, jigs. Choose MDF where flat, uniform faces and machined edges matter: painted doors, mouldings and profiled parts. Either way, enter the real thickness in the sheet-cut calculator, which only places parts on stock of the same material and thickness.
A 900 mm shelf in MDF instead of plywood: with a stiffness of 3.59 GPa against 6.96 GPa, it sags about 6.96 / 3.59 ≈ 1.9 times as much under the same load.