What is the actual size of a 4×4?
A 4×4 actually measures 3-1/2 × 3-1/2 inches (about 89 × 89 mm) when dry and surfaced; a wet pressure-treated post can be a little over that until it dries.
How to work it out
- Nominal 4×4 → dressed dry to 3-1/2 × 3-1/2 in (PS 20 standard sizes)
- Billed: 4 × 4 × 8 ft ÷ 12 = 10.67 board feet · Actual: 3.5 × 3.5 × 8 ft ÷ 12 = 8.17 board feet (77%)
Size post brackets, bases and mortises for 3½ in, but measure the actual posts first: pressure-treated 4×4s are often sold wet and can be a little fat, then shrink and check (crack along the grain) as they dry.
Adjust for your project
More options: L · Qty
- Actual thickness (decimal)
- 3.5 in
- Actual width (decimal)
- 3.5 in
- Board feet (nominal — what a board-foot price is charged on)
- 10.67 board feet
- Board feet (actual volume)
- 8.17 board feet
- Wood you get vs. what you pay for (if priced by the board foot)
- 77%
- Actual volume
- 0.681 cu ft
- Posts and 4× beams are often sold green or wet, especially pressure-treated. PS 20’s green size for a 4×4 is a little larger — 3-9/16 × 3-9/16 in — until the wood dries.
- Green (unseasoned) and rough-sawn lumber is larger than these dry, surfaced sizes. Pressure-treated lumber is often sold wet and shrinks slightly as it dries.
A 4×4 at common post lengths
| Length | Board feet (nominal — what a board-foot price is charged on) | Board feet (actual volume) | Actual volume |
|---|---|---|---|
| 6 ft | 8 board feet | 6.13 board feet | 0.51 cu ft |
| 8 ft | 10.67 board feet | 8.17 board feet | 0.681 cu ft |
| 10 ft | 13.33 board feet | 10.21 board feet | 0.851 cu ft |
| 12 ft | 16 board feet | 12.25 board feet | 1.021 cu ft |
What to know about a 4×4’s actual size
- Dry vs green. The American Softwood Lumber Standard (PS 20) sets a dry 4× face at 3½ in and a green one at 3-9/16 in. Treated posts straight off the truck often sit between the two, which is why a bracket that fits one post can bind on the next.
- Two 2×4s are not a 4×4. Nailed together they make 3 × 3½ in, a half-inch thin in one direction. If a plan calls for a 4×4 post base or a 3½ in notch, a built-up post will not fill it.
- Checks are normal. As a wet post dries, cracks open along one face. They look alarming but rarely matter structurally; face the worst side away from view and set posts before they have fully dried.
An 8 ft 4×4 is billed as 10.67 board feet but holds about 8.17. Framing lumber follows the same half-inch rule — see what is the actual size of a 2x4.
Different size? Use the full Lumber Dimensions Calculator.
What you’ll need
- Digital calipersBudget price
For measuring actual board and plywood thickness to the thousandth — useful for sizing dadoes and checking planed stock.
See options on Amazon (opens in a new tab) - Post hole diggerMid-range price
The clamshell digger digs and cleans out post holes with straight sides. For many holes, rent a power auger and use this to finish.
See options on Amazon (opens in a new tab) - Speed square (7 in)Budget price
Marks square and angled crosscuts and doubles as a saw guide for a circular saw. The degree scale is handy for rafters and stair angles.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
What is the actual size of a 6×6 post?
A 6×6 measures 5-1/2 × 5-1/2 inches (about 140 × 140 mm). Timbers this size are dressed green, ½ in under nominal, so a fresh post can shrink a little below that as it dries.
What is the actual size of a 4×6?
A 4×6 measures 3-1/2 × 5-1/2 inches (about 89 × 140 mm) dry.
How many board feet is a 4×4×10 post, billed and actual?
A 4×4×10 is billed as 13.33 board feet and holds 10.21 board feet of wood.
Other common sizes
- What is the actual size of a 2x4 — 1-1/2 × 3-1/2 in
- What is the actual size of a 2x6 — 1-1/2 × 5-1/2 in
Sources
- NIST, Voluntary Product Standard PS 20-20, American Softwood Lumber Standard (Rev. 1, 2021) (opens in a new tab) — Table 3: minimum dry and green dressed sizes of boards, dimension and timbers (timbers 5 in and thicker: 1/2 in under nominal, green).
- National Hardwood Lumber Association, Rules for the Measurement & Inspection of Hardwood & Cypress (2019) (opens in a new tab) — Paragraph 14: standard surfaced (S2S) thicknesses — 4/4 to 13/16 in, 5/4 to 1-1/16, 6/4 to 1-5/16, 8/4 to 1-3/4, 12/4 to 2-3/4.
- American Hardwood Export Council, The Illustrated Guide to American Hardwood Lumber Grades (opens in a new tab) — Rough quarter thicknesses and the 3/16 in / 1/4 in surfacing allowance.
- NELMA, Standard Grading Rules, Section 4: Other Board Grades (2024) (opens in a new tab) — Standard Shop thicknesses: 5/4 surfaced to 1-5/32 in.