In U.S. lumber, a standard 4×4 post measures 3.5 × 3.5 inches (89 × 89 mm) when dressed; rough-sawn or green stock can be closer to a full 4 × 4.
Quick Answer With Real Numbers
For regular, surfaced softwood sold across the U.S. and Canada, a 4×4 is 3.5 inches by 3.5 inches. In millimetres, that’s 89 by 89. This is the size you’ll see on store listings and on most structural drawings. You can verify this on a typical retailer’s spec page, such as Lowe’s listing that shows “Actual Measurements: 3.5-in × 3.5-in.”
4×4 Actual Size: Common Variations You’ll Meet
Stores and yards carry more than one cut. Moisture, surfacing, and region can change what lands on your bench. The table below sums up the usual cases so you can plan clearances, mortises, caps, and brackets without guesswork.
| Label On Rack | What You Get (Inches / mm) | Notes |
|---|---|---|
| 4×4 (S4S, kiln-dried) | 3.5 × 3.5 in / 89 × 89 mm | Standard dressed size for framing and posts. |
| 4×4 (pressure-treated S4S) | About 3.5 × 3.5 in / 89 × 89 mm | May feel a touch swollen when wet; design around the 3.5 in spec. |
| 4×4 (rough-sawn) | Near 4.0 × 4.0 in / ~102 × 102 mm | Sold “full” or close; expect saw marks and extra weight. |
| 100 × 100 mm (UK planed/PAR) | 95 × 95 mm / ~3.74 × 3.74 in | Planed finish reduces the section by about 5 mm each way. |
| 100 × 100 mm (UK sawn) | 100 × 100 mm / ~3.94 × 3.94 in | Sawn finish; check if regularised or eased. |
Why “4×4” Rarely Measures Four By Four
Lumber starts out larger. After drying and surfacing, the cross-section drops. The name sticks, the size shrinks. That convention comes from long-standing standards for softwood lumber. The rulebook behind those names is the U.S. PS 20 Standard, which sets how sizes are shown and how much material is removed at the mill. Industry groups and inspection bureaus build grading and labeling around that standard. That shared language keeps buyers, mills, and code officials aligned on what each label means.
Actual Size Of A 4×4 Post For Real Projects
When you’re setting hardware or laying out joinery, use 3.5 by 3.5 inches for surfaced stock. Many post caps, bases, and beam saddles are built around that reality. You’ll see the same figure on store listings that label the product 4×4 but list the measured size as 3.5 by 3.5 inches. It’s the safe number to put on drawings and cut lists unless you order rough-sawn material on purpose.
Metric Conversions That Won’t Bite Back
Designing in millimetres? Convert the measured size, not the nominal. The American Wood Council’s sizing note says to soft-convert the actual dimensions when switching units. See the AWC guidance on converting lumber sizes. So a dressed 4×4 becomes 89 by 89 mm, not 100 by 100 mm. That choice keeps hardware fit and gap lines correct.
Moisture, Treatment, And Small Swings
Two 4×4 posts can feel a touch different in hand. A pressure-treated stick may carry more moisture from the tank. A green rough-cut might be bulkier when it leaves the yard and then settle as it dries. These swings are normal, yet the nominal-to-actual step stays the same: surfaced equals 3.5 inches square.
Regional Notes So You Don’t Get Caught Out
Across the Atlantic, timber is sold in metric. A section marked 100 × 100 often arrives planed to 95 × 95 mm. Many UK merchants state this on the product page, and it’s common with PAR or PSE stock. If you need a full 100 × 100, choose sawn or “rough” and confirm before purchase. The pattern mirrors North America: planing trims a little off the stated size.
Choosing The Right Cut For The Job
Pick the finish that matches the next step in your build. Here’s how the common options compare in feel, prep time, and layout accuracy.
S4S: Ready To Lay Out
S4S means surfaced on four sides. Edges are straight, faces are square, and the stick is easy to mark. For decks, pergolas, rails, and posts that meet brackets, this is the smooth path. Good on most builds.
Rough-Sawn: Full Or Near Full
Rough pieces hold close to the label size and come with saw marks. Many craftspeople pick rough when they want full-size joinery or plan to mill faces later. Expect to plane or sand, and expect movement as the piece seasons.
Pressure-Treated: Built For Ground Contact
When posts meet soil or damp concrete, treatment adds the staying power you need. Treated 4×4 posts still measure around 3.5 inches square when dressed. Seal cut ends, space posts up off the slab with bases, and allow drainage around anchor points.
True Dimensions For 4×4 Lumber In Plans And Layouts
Good drawings start with the measured size. Use the figures below when you mark, drill, and assemble, and your parts will meet without shims.
Joinery And Clearances
For housed joints, cut to a true 3.5 inch width and test on scrap. For slip-fit caps and saddles, give yourself a small clearance.
Drilling And Hardware
Centerlines sit at 1.75 inches from each face. For through-bolts, start with a snug pilot, then ream to final size once the hardware is at hand.
Species, Shrinkage, And Why Two Posts Aren’t Twins
Two boards with the same label can land a touch different once they reach your site. Species shrink at different rates as they dry. Southern pine and Douglas-fir don’t move the same way as spruce or hemlock. The mill also sets knives a little differently for various runs. Those small changes add up to hairline gaps or tight fits when parts meet. This is normal. Design to the measured size, then allow a credit card of clearance for parts that must slide together.
Grading Stamps And What They Tell You
Every surfaced post carries a stamp that lists the species group, the grade, the mill or agency, and the moisture condition. That mark ties back to the inspection rules used on the line. A #2 grade 4×4 has more knots and slope of grain than a Select Structural post, yet both follow the same sizing rules once planed. Keep a stamp photo for reference.
When You Want A True Four By Four
Sometimes you need every bit of section. For traditional joinery or heavy trim, order rough-sawn or ask for “full size” stock from a yard that mills in house. Expect to pay more and expect to plane. The payoff is a thick shoulder, bold reveals, and joints that read with more presence.
Measuring, Marking, And Cutting Clean
Even with the right size in mind, small layout habits keep results sharp. Start with a true reference face and edge. Mark all lines from those two. Use a sharp pencil for rough work and a knife for tight shoulders. Square both ways on through cuts. When parts meet at right angles, mark centerlines on both faces and confirm that the marks cross at the same point. Little checks like these save time once fasteners come out.
Drilling Straight And True
Clamp a guide block with a pre-drilled hole when you want a straight bore through the center. Start from one side with a pilot. Flip the piece and finish from the other side so the exits are clean. For lag screws, wax the threads to reduce split risk near edges.
Site Tips That Save Rework
Layout strings help posts line up on long runs. Pull a line along the beam face, then set each post so the outside faces touch the string. Check plumb on two faces before the mix sets. Where posts sit on concrete, use bases that lift wood off the slab so water can drain. Tighten hardware after a day as the wood settles. When you notch for beams, keep the bearing flat and leave a clean inside corner so loads pass straight down.
Storage Before Install
Keep posts flat on stickers with a cover on top. Leave the sides open for airflow and lift bundles off the ground to keep the ends dry and clean.
Length, Tolerance, And Ordering With Confidence
Cross-section gets most of the attention, yet length has rules too. Softwood pieces are trimmed so the delivered length isn’t shorter than the name. Overlength up to a small margin is allowed by the same PS 20 rulebook. That’s why an 8-ft post can arrive a shade long and still meet spec. You may see pieces trimmed slightly long, sometimes up to 3 inches over the length name, which stays within standard tolerances.
4×4 Buying Checklist
Before you load a cart or place a yard order, run through this short list. It keeps mismatches out of your project and saves return trips.
- Confirm the finish: S4S, PSE/PAR, or rough-sawn.
- Read the “actual size” line on the listing or tally sheet.
- Match hardware to the measured size, not the label.
- Pick a moisture level that suits the install date.
- Order a spare if you plan heavy notching or deep checks.
Common Lengths And Typical Uses
Different projects lean on different spans. The table below lists the sizes you’ll bump into most often, plus common jobs where they shine. Treat the task column as a guide, not a rule.
| Stock Length | Measured Section | Typical Job |
|---|---|---|
| 6 ft / 1.83 m | 3.5 × 3.5 in S4S | Short porch posts, low rails, mailbox stands. |
| 8 ft / 2.44 m | 3.5 × 3.5 in S4S | Fence posts, small pergola posts, garden frames. |
| 10 ft / 3.05 m | 3.5 × 3.5 in S4S | Taller deck rails, light shade structures. |
| 12 ft / 3.66 m | 3.5 × 3.5 in S4S | Carport posts, deeper footing runs. |
| Rough-sawn any length | Near 4 × 4 in | Full-size joinery, hewn or rustic work. |
Troubleshooting Fit And Finish
Hardware won’t slip? Measure the post, not the box. If the post runs fat from moisture, let it dry or ease the inside of the cap. If the post runs shy because it’s planed, pad with thin shims at hidden faces. When a mortise feels loose on rough stock that dried down, reset your gauge to the actual face and tighten the fit on the next piece.
Care, Storage, And Movement
Wood moves. Stack posts flat on stickers, cover the top, and leave the sides open so air flows. Keep ends off wet concrete. Pre-drill near edges to reduce checks. For treated posts, coat fresh cuts with the matching end-seal product from your supplier.
Key Takeaways You Can Build With
One Number For Most Jobs
Plan for 3.5 × 3.5 inches when you see “4×4” on surfaced softwood in North America. That’s the figure hardware makers and store specs agree on.
Look Closely At Rough And Metric
Rough-sawn pieces often land near a full 4 inches square. In the UK, planed 100 × 100 comes in at 95 × 95 mm, while sawn stays near 100 × 100 mm. Read the small print on the product page and you’ll know which you’re getting.
Convert The Actual Size, Not The Label
When you switch units, convert 3.5 inches to 89 mm. That keeps parts, caps, and spans consistent across drawings, schedules, and site work.
Sources And Extra Reading
To see the standard behind the numbers, read the U.S. PS 20 American Softwood Lumber Standard. For a quick metric note that avoids sizing traps, check the American Wood Council’s tip on converting lumber sizes. For a retailer spec that confirms the dressed 3.5 × 3.5 in measurement, see this 4×4 product page.