Actual Dimensions of a 4x4 Post
A 4x4 post is nominally 4 inches by 4 inches when first surfaced. After kiln drying, planing, and optional pressure treatment, North American standards typically result in actual dimensions around 3 1/2 inches by 3 1/2 inches (89 mm by 89 mm). This reduction occurs because surfaced stock is milled below the nominal size to ensure consistency and structural performance across species and grades.
Nominal vs Actual Size in Framing and Construction
In framing lumber, nominal sizes describe a board before milling; actual sizes describe the final dimension after processing. For a 4x4 post:
- Nominal size: 4" x 4"
- Typical surfaced (S4S) size: approx. 3 1/2" x 3 1/2" (89 mm)
- Common metric equivalent: 90 mm x 90 mm for treated utility posts
These values can vary by species, sawmilling practices, and whether the post is machine-planed or rough-sawn. Tolerance in size is normal; the key is verifying the actual dimensions in use rather than relying on nominal labeling alone.
Standard Lengths and Grading Influences
4x4 posts are commonly available in lengths ranging from 8 ft to 20 ft, with 10 ft and 12 ft being the most widely stocked sizes in North America. Heavier-duty treated posts intended for decks, docks, and ground-contact applications are often cut to these standard increments to simplify layout and minimize off-cut waste.
Grades and Dimension Consistency
The grade of a post reflects allowable knots, wane, and structural capacity rather than tight control of exact width or thickness. Select Structural and Better grades are milled more consistently and are common where precise connections are specified. Common grades may appear slightly undersized at typical milled finishes but still meet code-required load values when properly specified and installed.
Regional and Species Considerations
In North America, softwood species such as pressure-treated Southern Pine, Douglas Fir-Larch, and Hem-Fir are typical for 4x4 posts. Each species machines to slightly different surfaced dimensions; for example, Douglas Fir may finish closer to 3 3/8" x 3 3/8" when surfaced to a high standard, while Southern Pine treated posts commonly target 3 1/2" x 3 1/2".
Metric markets often define equivalent posts at nominally 90 mm x 90 mm finished or 100 mm x 100 mm for certain timber-frame applications. Confirming local product labeling and mill reports is best practice when exact fit is critical.
Practical Tolerance and Verification Guidance
When a 4x4 post is specified, contractors and designers should confirm the actual finished dimensions on site or via material test reports, especially when connections or embedment depths depend on consistent size. A reliable expectation for surfaced 4x4 posts in North America is approximately 3 1/2" x 3 1/2", with potential variation of roughly ±1/16" due to planer set and moisture content.
For critical applications, ask your supplier for the actual milled size and compare it to the design requirements. When installing where moisture or treatment chemicals may affect dimensions, allow tolerance for swelling or shrinkage and verify fit after acclimation.
Comparison: Framing Lumber vs Utility Posts
| Product | Nominal Size | Typical Finished Size | Common Use | Notes |
|---|---|---|---|---|
| 2x4 | 2" x 4" | 1 1/2" x 3 1/2" | Wall framing | Widely milled to finished sizes |
| 4x4 Post | 4" x 4" | 3 1/2" x 3 1/2" (approx.) | Deck posts, columns, perimeter bracing | Grades vary; surfaced finish often near 3 1/2" |
| 6x6 Post | 6" x 6" | 5 1/2" x 5 1/2" (approx.) | Heavy-duty supports, beams | Dimensioning similar logic as 4x4 |
Key Takeaways
- A 4x4 post is nominally 4 inches square; when surfaced in North America, it is commonly around 3 1/2 inches by 3 1/2 inches (89 mm).
- Always verify actual dimensions on site or via documentation, especially for connections and embedded work where tolerance matters.
- Lengths typically range from 8 ft to 20 ft, with 10 ft and 12 ft most common; species, grade, and treatment can influence final size and tolerance.