A standard 4×4 in North America is 3.5 in × 3.5 in (≈89 × 89 mm); rough-sawn or UK metric posts often finish near 95 × 95 mm.
Why A 4×4 Isn’t Four By Four
Wood is sold by a name size and a measured size. The name, or nominal size, is the rough cut before drying and surfacing. The measured, or actual size, is what reaches your hands after the mill dries the stock and planes all faces smooth. Drying shrinks the piece a little; planing removes more. That’s why a 4×4 ends up smaller than four inches by four inches.
Most North American posts are surfaced on four sides (S4S) and kiln-dried or surfaced dry. In common retail racks that finished size is 3.5 inches by 3.5 inches. You’ll see the same pattern across the rack: a 2×4 is 1.5 × 3.5, a 6×6 is 5.5 × 5.5, and so on. The pattern keeps hardware and framing layouts consistent.
Nominal Vs Actual: Quick Reference
Use this chart for fast checks when you’re sketching a cut list or laying out hardware on a 4×4, 6×6, and matching framing sizes.
| Nominal Size | Actual Size (inches) | Metric (mm) |
|---|---|---|
| 4×4 | 3.5 × 3.5 | ≈ 89 × 89 |
| 4×6 | 3.5 × 5.5 | ≈ 89 × 140 |
| 6×6 | 5.5 × 5.5 | ≈ 140 × 140 |
| 2×4 | 1.5 × 3.5 | ≈ 38 × 89 |
| 2×6 | 1.5 × 5.5 | ≈ 38 × 140 |
| 1×4 | 0.75 × 3.5 | ≈ 19 × 89 |
Actual Size Of A 4×4 Post: Shop-Ready Guide
Grab a tape and check the faces. A surfaced 4×4 will read about 3-1/2 inches each way. That’s the figure to use for joinery, notches, fence post caps, and post bases. When you order hardware labeled “for 4×4,” it’s designed for this 3.5-inch reality.
Look for the grade stamp on one face. Besides species and grade, it often tells you if the piece was kiln-dried (KD), surfaced dry (S-DRY), or surfaced green (S-GRN). KD and S-DRY stock holds that 3.5-inch target well. S-GRN can shrink more on site as it dries, so leave a breath of tolerance in tight housings and notches.
Why Mills Use Nominal Names
Sawmills rough cut to a set of easy names that carpenters and codes share. Drying and surfacing make the pieces uniform and ready for layout. The name stays the same for ordering and design, while the measured size stays consistent for fit-up and hardware. That’s the whole point of standards in softwood dimensional lumber.
For planning, think in two tracks. Use the nominal when you mark plans and order lists. Use the actual when a hole, notch, or sleeve must match the piece in your hands.
What Size Is A 4×4 In Real Builds
On a deck, pergola, or fence, assume 3.5 inches unless you’re buying rough-sawn posts. Rough stock can land closer to the name, and sometimes near a full 4 inches per side. That’s common from small mills or when the tag says “rough” or “resawn.” If your connector set expects 3.5 inches and your post arrives chunkier, the fit will be off. Flip that mismatch and a loose bracket can happen. A quick check at pickup saves a return trip.
Metric markets name these pieces as 100 × 100 mm, yet planed and regularised stock often finishes at 95 × 95 mm. That’s the same idea as a 4×4 finishing at 3.5 × 3.5 inches. When you move between inch and metric racks, treat the “100” as a name, not a promise, and measure the finished faces.
Metric Conversions You’ll Use Often
Here are the conversions you’ll reach for when you switch drawings or hardware between inch and metric workflows.
- 3.5 inches ≈ 88.9 mm (commonly rounded to 89 mm)
- 5.5 inches ≈ 139.7 mm (commonly rounded to 140 mm)
- 1.5 inches ≈ 38.1 mm (commonly rounded to 38 mm)
Standards That Keep Sizes Consistent
North American softwood follows the American Softwood Lumber Standard, which sets the naming, measurement, and labeling system that mills and retailers use. Industry groups such as the Western Wood Products Association publish guidance and training so yards and builders speak the same sizing language. In the UK and Ireland, trade bodies map sizes to BS EN 336 tolerances so planed 100 × 100 sections finish close to 95 × 95 mm.
What This Means For Hardware And Layout
Post bases, beam seats, and caps for a “4×4” are built around a 3.5-inch target. The same goes for notches, dados, and mortises. When a detail calls for a bolt through the center of a 4×4, the centerline sits at 1-3/4 inches from either face, not two inches. Small shifts like that keep holes aligned and plates seated.
Layout Tip
When you draw a centerline on a post, mark 1-3/4 inches from one face and confirm the overall with a quick diagonal check. That tiny habit keeps brackets square and keeps rails flush.
4×4 By Region And Finish
The measured size can vary by source and by how smooth the stock is sold. Use this table to set expectations before you buy, then measure your actual delivery.
| Market / Condition | Expected Finished Size | Notes |
|---|---|---|
| US/Canada, S4S KD or S-DRY | 3.5 in × 3.5 in | Most retail yard posts and pressure-treated stock |
| US/Canada, Rough-Sawn | about 3-7/8 in to near 4 in | Faces are not planed; thickness varies by mill |
| UK/EU, Planed/Regularised 100 × 100 | about 95 mm × 95 mm | Sold as “PSE” or “PAR”; same idea as S4S |
| UK/EU, Rough-Sawn 100 × 100 | near 100 mm × 100 mm | Often dries down a touch after install |
How To Measure A 4×4 For A Clean Fit
Pick a face you want to show. Hook your tape and read the near and far faces. Mark from one reference edge for all joinery on that piece. That keeps errors from stacking. If you’re cutting a batch, measure each post and group them by width. Small sort piles make hardware fit predictable.
Picking The Right Connector
Match the connector to the measured size. A bracket stamped for a 4×4 expects 3.5 inches. Some fence caps list both fits: 89 mm for planed metric posts, 3.5 inches for North American stock. If your post measures heavy, pick a cap with room or shim as the manufacturer allows. If it measures light, a cap made for 3.5 inches will seat better than one made for a rough post.
Clearance For Finish And Movement
Paint, stain, and wrap materials take space. Leave a hair of clearance inside sleeves and sockets so finish coats don’t bind. Wood moves with moisture swings, so a snug bracket today can pinch later. A fraction of wiggle keeps fasteners from squealing and keeps posts plumb.
Cutting, Notching, And Drilling A 4×4
Mark the face side and a top reference. Make your depth marks from the face side, not the back. For through-bolts, drill halfway from each side to control tear-out. For notches that seat beams on top of a post, set the shoulder back by a hair and ease edges. That keeps split risk down and helps plates seat flush.
When A True 4×4 Matters
Some traditional joinery calls for stock that’s close to the name size. If your plan needs a post near 4 inches per side, ask the yard for rough-sawn or “full-sawn” stock. Expect a heavier post, more planing work at the bench, and a little variation piece to piece. Price and supply vary by region.
Moisture, Shrinkage, And Your Layout
Stock stamped S-GRN can lose width after install as it dries. On a tall fence run, those tiny changes can stack. Lay out rails and panels with a touch of slack and set screws where you can retighten later. KD and S-DRY stay closer to shelf size but still need sealer on cut ends to limit checking.
Reading A Grade Stamp
A quick guide when you scan the stamp: species group (like SPF, SYP, DF), the grade, the mill or agency, and a moisture note (KD, S-DRY, or S-GRN). Agencies publish the rules that mills follow, which keeps a 4×4 consistent from different yards. When you mix species on one job, keep fastener guidance and corrosion ratings in view, since treatment and density vary.
Metric Drawings, Inch Hardware: Make Them Play Nice
Deck kit in millimeters and post caps in inches? Treat the 4×4 as 89 mm when you model. If the job uses metric 100 × 100 posts planed to 95 × 95, call that out on the materials list so crew and buyer pick the right caps and bases. Clear notes beat guesswork on site.
Common Cut-List Checks
- Centerlines on a 4×4 sit at 1-3/4 inches from a face
- A 45° miter across a 4×4 spans about 4-15/16 inches corner to corner
- A lap joint cut half-depth on a 4×4 removes 1-3/4 inches
Buying Tips That Save Hassle
Pull posts from one bunk for a match in color and crown. Sight down each piece and pick straighter stock for rails and gate posts. If you need a batch with tight width control, bring calipers and sort. Mark sizes on painter’s tape so the lot stays organized in the truck.
Pressure-Treated 4×4 Notes
Treatment adds moisture, which can swell faces a touch right after purchase. Plan your tight-fit work a few days after cutting and sealing. Use fasteners rated for the treatment used by your yard, and seal every cut end before install.
Edge Radius, Corners, And Bearing Area
Surfaced posts arrive with eased edges. That small radius trims contact area where parts meet. When a beam seats on a notched post, knock back the inside corner or add a shallow fillet so the beam rides on a flat. For railing work, eased corners make handholds smooth; plan cap profiles with that round in mind so lines stay crisp.
Square Vs. Eased: When It Matters
Joinery that counts on a sharp corner calls for rough-sawn or custom-planed stock. If you need crisp shoulders for a housed joint, specify square edges when you order or plan for a light pass with a hand plane after layout. For fence caps, the ease can leave a tiny gap at the corners; a cap with a hem or a small bead hides that daylight and keeps water away from end grain.
Species And Treatment Can Shift Measurements
Spruce-Pine-Fir and Douglas-fir fill many racks, while southern yellow pine dominates treated lines. Width holds close to 3.5 inches, yet weight and hardness vary. Pre-drill near end grain on dense posts, and match drivers to fastener coatings.
Freshly treated posts can read a touch heavy right after pickup. After a few warm days, faces settle back toward shelf size. Seal cut ends the same day to control checks and keep posts looking uniform.
Length, Straightness, And Tolerance Notes
Length deserves the same care. Posts often arrive a bit long, which helps when you square ends on site. Make a shallow chop, square across, then cut to final. Sight every post for bow, crown, and twist; place the straightest at gates and corners.
Key Takeaways For A 4×4
A shelf 4×4 in North America measures 3.5 inches per side. Planed metric posts sold as 100 × 100 often land at 95 × 95 mm. Rough-sawn posts can run heavier. Hardware marked “for 4×4” is built for the 3.5-inch reality. Measure the faces you have, then lay out joinery, holes, and hardware to match those faces. That simple habit keeps builds square and parts happy on every single job site.