4 X 4 6 X 2

You walk into the timber yard needing posts for a deck. Practically speaking, the guy behind the counter asks, "Four-by-four or six-by-two? " You hesitate. They sound similar. Think about it: both are chunky. That said, both get used for structural work. But they're not interchangeable — and picking the wrong one can cost you time, money, or a failed inspection.

I've seen this confusion more times than I can count. A 4×4 is a post. A 6×2 is a joist or a bearer. They're not. DIYers and even some builders treat these sizes like they're in the same family. They carry load differently, they span differently, and they sit in your structure in completely different orientations.

Let's clear this up once and for all.

What Is 4×4 and 6×2 Timber

First thing to know: the name lies. And a "6×2" isn't six inches by two inches. Think about it: a "4×4" isn't four inches by four inches. Those are nominal* sizes — what the board was called when it came off the saw rough-cut, before it was planed smooth and kiln-dried.

Actual dimensions you'll hold in your hand

That's a massive difference in cross-section. On top of that, a 6×2 gives you 6,300 mm². A 4×4 gives you 8,100 mm² of section area. But cross-section alone doesn't tell you how they behave — because orientation changes everything.

A 4×4 is square. It doesn't care which way you turn it. Still, a 6×2 is rectangular — deep and narrow. Stand it on edge (140 mm vertical) and it's stiff. Lay it flat (45 mm vertical) and it bends like a wet noodle.

Species and grades you'll actually encounter

In Australia and New Zealand, you're mostly looking at:

• MGP10 / MGP12 — machine-graded pine, the workhorse for framing. MGP12 is stiffer and stronger; MGP10 is cheaper and fine for non-critical spots.
• F5 / F7 / F8 — visual stress grades for hardwoods (spotted gum, ironbark, mixed hardwoods). F7 is the common decking bearer grade.
• H3 / H4 / H5 treatment — this is preservative treatment level, not strength. H3 is above-ground outdoor. H4 is in-ground contact (posts). H5 is critical structural in-ground (house stumps, retaining wall posts).

A 4×4 post for a deck must* be H4 at minimum. A 6×2 bearer sitting on stirrups above ground can be H3. Get this wrong and your posts rot in three years.

Why the Distinction Matters

People mix these up because both show up in deck builds. But they do different jobs.

Load path fundamentals

A 4×4 post carries axial load* — weight pushing straight down its length. The same post at 3.Think about it: 0 m drops to maybe 10 kN. 4 m H4 4×4 in MGP10 might carry 15–18 kN. A 2.The longer the post between restraints, the less load it can take. Even so, its job is to transfer roof, floor, or deck loads into the footing. Buckling is the failure mode. Slenderness kills capacity.

A 6×2 bearer or joist carries bending load* — weight pushing across* its span. A 140 mm deep joist is roughly 3.On top of that, 7× stiffer than a 90 mm deep one of the same width. Still, stiffness goes with the cube* of depth. Its job is to resist deflection. The deeper the section (140 mm vs 90 mm), the exponentially stiffer it is. That's why you don't use 4×4s as bearers spanning 2 metres — they'll sag.

Span tables don't lie

Open AS 1684 (the timber framing code) or your supplier's span tables. Look up:

• 90×90 MGP10 bearer, 1.8 m span, supporting 2.4 m floor load width → fails deflection limits
• 140×45 MGP10 bearer, same conditions → passes comfortably

Same timber grade. Same load. Even so, one works, one doesn't. Because depth matters more than width for bending.

Real-world consequence

I watched a mate build a low deck using 4×4s as bearers on 1.8 m spans. "They're chunky, they'll be fine." Six months later the deck had a visible crown in the middle. Because of that, the boards cupped. The screws popped. He ripped it out and redid it with 140×45 bearers. Cost him two weekends and $400 in timber.

Don't be that guy.

How to Choose Between Them

Use 4×4 (90×90) when:

• Building posts for decks, pergolas, carports, fences
• Stumps for subfloor (if H5 and engineered)
• Vertical struts in wall frames (sometimes)
• Anything where the load runs parallel* to the long axis

Use 6×2 (140×45) when:

• Bearers running perpendicular to joists
• Joists spanning between bearers
• Lintels over door/window openings (check span tables — often need 190×45 or 240×45 or LVL)
• Wall plates (top/bottom plates) — though 90×45 is more common there
• Anything where the load runs perpendicular* to the long axis and you need depth for stiffness

The grey zone: 90×90 as a short bearer

Can you use a 4×4 as a bearer on a short* span? Say 900 mm? And span tables might allow 90×90 MGP12 at 900 mm span with light load width. Maybe. But you're burning your safety margin. A 140×45 costs maybe 20% more and gives you 3× the stiffness. Just use the right piece.

Continue exploring with our guides on according to the fundamental theorem of algebra and solve the system of equations by gauss elimination method.

Common Mistakes People Make

1. Treating nominal size as actual size in calculations

You sketch your deck in SketchUp. And or your 140 mm bearer is 5. You draw 4×4 posts at 4″ × 4″. Plus, your bearer notches are 4″ wide. You get to site and the 90 mm post doesn't fit your 100 mm notch. 5″ not 6″ and your joist hanger doesn't sit flush. Worth keeping that in mind.

Always design in actual dimensions. 90, 140, 190, 240, 290 mm. Not 4, 6, 8, 10, 12 inches.

2. Assuming H3 is fine for in-ground posts

H3 is above ground only*. The treatment doesn't penetrate the core reliably enough for soil contact. That's why h4 uses a different preservative system (usually copper chrome arsenate or ACQ at higher retention) and a different penetration standard. If your 4×4 post goes in a hole, it's H4. No exceptions.

If it’s a house stump — the hidden trap

A 90 × 90 mm H4 stump may look solid on the plan, but the code is unforgiving when the bearing area is too small. AS 1684 – 2021 requires a minimum bearing length of 150 mm on concrete or masonry footings for timber in-ground elements. A 90 mm post can’t meet that, so you either:

• Step up to a 140 × 45 or 190 × 45 bearer that spans the footing and provides the required bearing width, or
• Add a steel plate or timber pad under the post to spread the load and satisfy the bearing length rule.

Skipping this step is a fast track to settlement, uneven floors, and a costly retrofit later on. And that's really what it comes down to.

3. Footing size matters more than you think

Even when the timber itself passes the span table, the footing can be the weak link. A 300 × 300 mm concrete pad under a 140 × 45 bearer is fine for most domestic decks, but a 200 × 200 mm pad under a 190 × 45 lintel over a large opening can flex enough to induce cracking in the surrounding masonry.

Rule of thumb:
Footprint area ≥ 1.5 × the tributary width of the timber.*
If a bearer carries 2 kN/m of load, aim for a footing at least 300 mm wide (or the equivalent steel plate) to keep bearing stress under 0.5 MPa.

4. The “double‑bearer” myth

Some builders think stacking two 90 × 90s together will give them the strength of a 140 × 45. Day to day, in reality, the composite section’s moment of inertia is not simply additive because the layers are not bonded. The result is a beam that looks stiffer on paper but still deflects under service loads, especially when the lower member is not adequately nailed or bolted to the upper one.

If you need extra depth, use a single member that meets the required nominal size or a engineered alternative (LVL, Glulam, or a glued‑in‑place plywood strip) rather than a makeshift sandwich.

5. When “bearing” isn’t bearing at all

A common mistake on site is treating a bearer as a decorative plate rather than a load‑bearing element. If you notch a 140 × 45 to receive joists but leave a 10 mm gap between the notch bottom and the bearer’s underside, you’ve effectively created a cantilever that will bow under the joist reactions. The fix is simple: keep the notch depth at least 5 mm shy of the full depth and verify with a feeler gauge that the joist sits flush on the bearer’s face.

6. Practical selection checklist

7. The hidden cost of “saving” a few millimetres

A builder once swapped a 140 × 45 for a 90 × 90 on a 2 m span to cut material costs by $30 per lineal metre. Six months later the deck showed a 10 mm sag in the middle, the roof tiles cracked, and the homeowner’s insurance refused the claim because the timber didn’t meet the approved engineering specifications.