Nominal Vs Actual 4x4 6x6 Posts
A "4x4" post can measure a true 3.5 inches S4S, a rough 4 inches full-sawn, or something between — three different milling paths share one nominal name.
A nominal 4x4 post measures 3.5" x 3.5" actual and a nominal 6x6 measures 5.5" x 5.5" — but posts are the one corner of the nominal-vs-actual system where the actual size is genuinely less consistent than dimensional lumber, because post stock is milled, treated, and sold through more than one manufacturing path, and not all of those paths land on the same number.
Why posts don't follow one clean rule
A framing 2x4 is almost always kiln-dried and S4S-surfaced before it reaches a lumberyard, so 1.5" x 3.5" is close to universal. A "4x4" post, by contrast, might be: a standard S4S dimensional post cut the same way as a 2x, landing at the familiar 3.5" x 3.5"; a rough-sawn or "full-sawn" landscape timber that was never surfaced at all, which can measure close to a true 4" x 4" or land somewhere between rough and dressed depending on the mill; or pressure-treated post stock, where the treating process itself adds moisture that's still drying out at the point of sale, so the actual dimension on the day you buy it can run slightly over the dry S4S figure and shrink further after installation. None of this is a labeling error — it reflects three genuinely different products that all get called "4x4" in casual conversation.
The numbers that do hold steady
| Nominal | Actual, S4S dry (in) | Category |
|---|---|---|
| 4x4 | 3.5 x 3.5 | Post |
| 4x6 | 3.5 x 5.5 | Post |
| 6x6 | 5.5 x 5.5 | Post |
For standard dry S4S stock, these are consistent, and they're what a board-foot or weight calculation should use unless you've measured your specific material and found it different. An 8'-long 4x4 post in the S4S dimension comes to 8.17 board feet, confirmed directly by the board foot calculator, against a naive "4 inches by 4 inches" figure of 10.67 board feet — an overstatement of about 30.6% if the nominal number is used instead of the real one. That 30.6% gap is meaningfully smaller than the ~52% gap on a 2x4, because a post loses proportionally less material in surfacing relative to its larger starting size — the same absolute half-inch loss per face matters less against a 4" starting number than against a 2" one.
Weight, where treatment matters as much as species
Figuring an 8' actual 4x4 in Douglas-fir at its 32 lb/ft³ milled density gives about 21.8 lb dry — but pressure-treated posts carry retained moisture from the treating chemicals and process that a plain kiln-dried post doesn't, so a PT 4x4 straight off the rack at a home center will weigh noticeably more than that dry figure and lighten as it dries in place over the following months. If you're estimating shipping weight or load for a delivery, weigh treated stock as-bought rather than trusting a dry-wood density figure; if you're estimating for a finished, seasoned structure, the dry figure is the more honest number to design around.
Where the actual size matters most
Post-to-beam and post-to-footing hardware (post bases, caps, brackets) is manufactured to fit the S4S actual dimension — a bracket sized for a true 3.5" post won't seat correctly on a rough-sawn post running closer to a true 4", and the reverse leaves a S4S post loose in hardware sized generously for rough stock. Confirm which category of "4x4" you've actually bought before ordering hardware, not after it arrives and doesn't fit. The same logic applies to any structural notch or joint cut into a post — a birdsmouth-style bearing notch (see /joinery/birdsmouth-joint/) or a mortise for a rail needs to be laid out from the real, measured dimension of the specific post in hand, exactly as it would for a 2x board, just with a wider tolerance for which "actual" you're starting from.
The 6x6 case specifically
A 6x6 loses proportionally even less than a 4x4 to surfacing — the 30.6% board-foot overstatement on a 4x4 drops to about 19.0% on a 6x6, since the same half-inch-per-face loss is a smaller fraction of a 6" starting figure than a 4" one. That makes a 6x6 the size in the whole nominal-vs-actual system where the label comes closest to the truth in relative terms, even though the absolute gap (a full inch off each face, 6" nominal to 5.5" actual) is the largest anywhere on this site's tables. Structural post-and-beam framing, deck support posts carrying real vertical load, and heavy timber-frame joinery are the usual reasons to reach for 6x6 over 4x4 rather than for a cosmetic size preference — at that load, the difference between a true 5.5" and a true 4" actual dimension is a real structural decision, not a stylistic one.
Buying rough timber instead
If a project calls for genuinely rough, full-dimension timber rather than S4S post stock — a rustic pergola post, for instance, meant to show visible saw texture — that material is priced and measured by an entirely separate convention from either the dimensional-lumber system on this page or the hardwood quarter system at /reference/rough-lumber-quarter-system/. Confirm actual dimensions with the specific sawyer or yard rather than assuming any of the tables on this site apply; rough-sawn timber sizing is the least standardized corner of the whole nominal-vs-actual landscape.