Actual post sizes
| Nominal | Actual size | Actual size (mm) | End area (actual) | Board feet per linear foot |
|---|---|---|---|---|
| 4x4 | 3-1/2" x 3-1/2" | 89 x 89 mm | 12.25 sq in | 1.333 |
| 4x6 | 3-1/2" x 5-1/2" | 89 x 140 mm | 19.25 sq in | 2 |
| 6x6 | 5-1/2" x 5-1/2" | 140 x 140 mm | 30.25 sq in | 3 |
These are the dressed sizes from the American Softwood Lumber Standard (PS 20). Going into a hole, a bracket or a post base? Measure the real piece. The dry size runs under the name, and some products, treated posts among them, can differ a little on top of that.
Board feet in posts
Board feet use the nominal size: 4 × 4 × length ÷ 12 for a 4x4, for instance.
| Length | 4x4 | 4x6 | 6x6 |
|---|---|---|---|
| 8 ft | 10.67 | 16.00 | 24.00 |
| 10 ft | 13.33 | 20.00 | 30.00 |
| 12 ft | 16.00 | 24.00 | 36.00 |
| 16 ft | 21.33 | 32.00 | 48.00 |
A 4x4 is 1.333 board feet per linear foot, a 4x6 is 2 and a 6x6 is 3. A 10 ft 6x6 comes to 6 × 6 × 10 ÷ 12 = 30 board feet.
Posts compared with 2x lumber
| Piece | End area | Compared with a 4x4 |
|---|---|---|
| 4x4 | 12.25 sq in | 1.00 |
| 4x6 | 19.25 sq in | 1.57 |
| 6x6 | 30.25 sq in | 2.47 |
| Two 2x4s side by side | 10.50 sq in | 0.86 |
A 6x6 carries more than double the end area of a 4x4, which is why you see it under heavier posts. For anything that holds a load, a deck or porch post, the size you need comes from your local code and the load on it, not from this table.
Outdoors, use the right wood
A post set in the ground or left out in the weather needs lumber treated for that exposure. See treated vs untreated lumber for the use categories, and the lumber dimensions chart for every common size.
Frequently asked questions
What is the actual size of a 4x4?
3-1/2 by 3-1/2 inches, or about 89 by 89 mm.
What is the actual size of a 6x6?
5-1/2 by 5-1/2 inches, or about 140 by 140 mm.
What is the actual size of a 4x6?
3-1/2 by 5-1/2 inches, or about 89 by 140 mm.
How many board feet are in a 4x4x8?
10.67 board feet: 4 x 4 x 8 divided by 12.
Is a 4x4 really 4 inches?
No, a dry dressed 4x4 is 3-1/2 inches in each direction.