4x4 vs 6x6 Post: How Much Concrete for Each? (2026 Guide)

4×4 vs 6×6: concrete per post at a glance

The table assumes each post buried 30 in with a hole about twice the post's width in diameter. Concrete per post = hole volume minus the buried post, divided by bag yield and rounded up (80-lb ≈ 0.60 cu ft, 60-lb ≈ 0.45 cu ft).

Worked example: concrete for one 4×4 post

The buried post counts as a 3.5 × 3.5 in square (a nominal 4×4 is smaller than its name) and the hole as a cylinder. Work in inches, then divide by 1,728 cu in per cu ft.

Step 1 — hole volume. A 10-in hole has a 5-in radius: π × 5² × 30 = π × 25 × 30 ≈ 2,356 cu in, or 2,356 ÷ 1,728 ≈ 1.36 cu ft.

Step 2 — the post displaces concrete. The buried part is 3.5 × 3.5 × 30 ≈ 368 cu in, or 368 ÷ 1,728 ≈ 0.21 cu ft. Subtract it: 1.36 − 0.21 ≈ 1.15 cu ft of concrete per post.

Step 3 — bags. 80-lb: 1.15 ÷ 0.60 ≈ 1.9 → buy 2. 60-lb: 1.15 ÷ 0.45 ≈ 2.6 → buy 3. Always round up, and add one spare per multi-post job for spillage and ragged holes.

What changes for a 6×6?

A nominal 6×6 is about 5.5 × 5.5 in, so at a 12-in hole and 30-in depth the hole is π × 6² × 30 ≈ 3,393 cu in ≈ 1.96 cu ft, the post takes 5.5 × 5.5 × 30 ≈ 908 cu in ≈ 0.53 cu ft, and the concrete is 1.96 − 0.53 ≈ 1.43 cu ft → 3 bags of 80-lb (4 of 60-lb). The cross-section jumps from 12.25 to 30.25 sq in — about 2.5× the wood — the reason 6×6s are chosen for tall, windy or load-bearing posts.

Note on depth: many 6×6 jobs are poured 36 in deep to beat the frost line or brace a tall fence. At 36 in the same 12-in hole holds about 2.36 cu ft before the post; deduct the longer buried section (5.5 × 5.5 × 36 ≈ 0.63 cu ft) and the concrete is about 1.7 cu ft ≈ 3 bags of 80-lb (1.73 ÷ 0.60 ≈ 2.9). Our 6×6 post concrete guide sizes the deeper pour from the full hole volume — about 2.36 cu ft, ≈ 4 bags — conservative to cover spillage and oversized holes.

Getting the pour right

• Add 4–6 in of gravel at the bottom of the hole. Crushed gravel drains water away from the post's end grain and gives the concrete a clean, level base — treat it as drainage, not structure.
• Mound concrete above grade and slope it away. Stop the pour about 1 in above ground and slope the top so rainwater runs off instead of pooling at the wood line — the #1 cause of early post rot.
• Choose 6×6 when the load or wind says so. Stick with 4×4 for standard 4–6 ft fences, deck rails and pergolas in sheltered yards. Go 6×6 for fence runs over about 6 ft, exposed or high-wind areas, gate posts, and posts carrying deck or pergola beams.

Pouring a row? Multiply the per-post numbers across the run — see concrete for fence posts and the batch math in concrete for 10 fence posts.

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