Actual Dimensions of a 4x4 Post
A standard 4x4 post in North American framing and decking is nominally 4 inches by 4 inches when rough-sawn, but after surfacing and drying it typically measures about 3 1/2 inches by 3 1/2 inches (89 mm by 89 mm). Outside North America, softwood timber labeled 100 mm x 100 mm usually finishes around 90 mm by 90 mm (approximately 3 9/16 by 3 9/16 inches). These are working dimensions builders and contractors rely on for layout, joinery, and fastener spacing. For precise projects, always check the mill’s specific finished sizes and local codes.
Why Nominal and Finished Sizes Differ
The label 4x4 reflects the size of the rough sawn blank before jointing and planing. After surfacing, moisture equalization, and sometimes resawing, the actual dimensions shrink. In the United States, surfaced lumber is commonly 1 1/2 inches (38 mm) thinner and narrower in each direction than the nominal size when based on inch multiples, which is why a 4x4 becomes roughly 3 1/2 x 3 1/2 inches.
Drying and Shrinkage Considerations
Kiln-dried and properly stored timber holds closer to its finished dimensions, but continued acclimation can produce small additional changes. Engineered and laminated posts can be more consistent, while solid sawn timber may vary slightly with grain orientation and species. Use calipers or a tape for critical layouts, and allow tolerance for on-site conditions.
Species, Treatments, and Typical Uses
4x4 posts are commonly available in untreated Southern pine, cedar, redwood, and pressure-treated pine for ground contact. Species, grade, and treatment influence strength, durability, and fastening methods. Treated wood often carries a small size penalty due to preservatives and coatings, while some hardwood species are sold true to a surfaced 3 1/2 inch size.
Common Species at a Glance
| Species | Typical Nominal Size | Typical Finished Size | Common Uses |
|---|---|---|---|
| Southern Pine (treated) | 4x4 | 3 1/2 x 3 1/2 inches | Deck posts, fencing, structural framing |
| Western Red Cedar | 4x4 | 3 1/2 x 3 1/2 inches | Exterior railings, garden posts, mild climates |
| Doug Fir-Larch | 4x4 | 3 1/2 x 3 1/2 inches | Construction framing, heavy-duty supports |
| Hardwood (oak, maple) | 4x4 | Often true to 3 1/2 x 3 1/2 inches | Industrial, docks, heavy equipment bases |
| Engineered Laminated | 4x4 | Consistent 3 1/2 x 3 1/2 inches | Precise builds, predictable performance |
Metric Dimensions for International Use
Internationally, a 4x4 is often labeled 100 mm x 100 mm; after surfacing it measures roughly 90 mm x 90 mm (about 3 9/16 inches) per side. Regional tolerances and species-specific finishing practices can shift these values by a few millimeters, so confirm shop drawings and standards when working across markets. Some manufacturers offer 95 mm or 92 mm sizes to match local building traditions.
Practical Checks for Builders and DIYers
When accuracy is critical, use a tape or digital caliper to measure both sides and both ends, and note any twisting or bowing. For joinery, consider the actual dimensions rather than the nominal name, and adjust layout lines accordingly. Order materials with clear grade and treatment specifications, and verify that lengths and sizes meet project requirements before installation.
Quick Comparison: Nominal vs Finished Sizes
- Nominal 2x4: surfaced about 1 1/2 x 3 1/2 inches
- Nominal 4x4: surfaced about 3 1/2 x 3 1/2 inches
- Nominal 6x6: surfaced about 5 1/2 x 5 1/2 inches
- Metric 100 x 100: finished about 90 x 90 mm
Conclusion and Best Practices
Understanding the actual dimensions of a 4x4 post prevents surprises during layout, cutting, and assembly. In framing and decking, assume a surfaced 4x4 is about 3 1/2 by 3 1/2 inches, verify on-site, and plan for species-specific and treatment-related variations. Consistent measurement, clear specifications, and tolerance checks keep structures square, plumb, and code-compliant across projects and regions.