Concrete Bags for Post Holes

Formula

V = π × (d/2)² × h. bags_per_hole = ceil(V × (1+waste) / yield).

Units

Diameter and depth in inches → convert to feet before πr²h. Or work entirely in inches and convert in³ to ft³ (÷ 1728).

Step-by-step

1. Measure hole diameter and depth (or sonotube size).
2. Convert to feet; compute cylinder volume.
3. Subtract post displacement only if the post remains in the pour and you want net concrete — many builders ignore it for small posts and keep waste instead.
4. Apply bag yield and waste; multiply by hole count.

Example — 12″ diameter × 36″ deep, 60 lb @ 0.45 ft³

r = 0.5 ft, h = 3 ft. V = 3.1416 × 0.25 × 3 ≈ 2.36 ft³. Bags ≈ 2.36/0.45 ≈ 5.2 → 6 before waste. With 15% waste for ragged holes → ~7 bags per hole.

Example — eight 10″ × 30″ holes, 80 lb @ 0.60 ft³

r = 5/12 ft ≈ 0.417 ft, h = 2.5 ft. V ≈ 1.36 ft³/hole. Eight holes ≈ 10.9 ft³. With 10% waste ≈ 12.0 ft³ → ceil(12/0.60) = 20 bags total.

Common mistakes

• Using diameter as radius in πr².
• Underestimating hand-dug hole irregularity (bellied sides).

Practical notes

• Frost depth and diameter rules are code/local — size the hole before bag math.
• Gravel below the pour is separate volume.

Related calculator

Use the free Concrete Bag Calculator to run the same math with your field numbers.

Related tools

• Concrete Bag Calculator — How many bags of concrete for slabs, footings, and round pours.
• Concrete Volume Calculator — Cubic feet and yards for slabs, walls, columns, and round forms.

Related guides

• How Many Bags of Concrete Do I Need? — Estimate bag count from pour volume and bag yield.
• 80 lb vs 60 lb Concrete Bags — How bag size changes count for the same pour.
• Concrete Slab Volume Basics — Cubic feet and yards for rectangular slabs.