How many bags per post?

For a 12 × 32 in hole, about 3 bags of 80 lb.

How deep should posts be?

One third of the above-ground height, at least 24 in.

Dry-pour concrete in the hole?

Works for light posts; mix it for loaded ones.

What hole size for a 4×4 post?

About 10–12 in — three times the post width.

One third of the post's exposed height, at least 24 in.

Concrete or mortar for posts?

Concrete — mortar lacks coarse aggregate.

Post hole concrete calculator

Number of posts, hole diameter and depth, and post diameter: you get concrete per hole, bags and drainage gravel for the bottom.

Purchase quantity

0.55 m³

Net measurement: 0.5 m³ · Waste 10 %

Bags of dry concrete mix47 bag
Concrete per hole0.05 m³
Drainage gravel at bottom (4 in)0.07 m³

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How it's calculated (visible formula)

hole = π/4 × (D² − d²) × depth; bags = volume × (1 + waste) ÷ yield per bag

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Enter the number of posts.
Set hole diameter and depth.
Subtract the post volume.
Multiply by holes and add waste.
Convert to bags with the product's yield.

Worked example: 10 posts of 4 in in 12 × 32 in holes

≈ 0.050 m³ (1.8 cu ft) per hole
× 10 × 1.1 = 0.55 m³

→ About 47 bags of 55 lb (or 33 of 80 lb).

Common mistakes

Not subtracting the post volume.
No gravel at the bottom — posts rot.
Pouring without plumbing and bracing.

Pro tips

Put 4 in of gravel in the bottom for drainage.
Plumb and brace the post before pouring.
Crown the top so water runs away.
Set wood posts on the gravel, not encased at the bottom.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

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FAQs

How many bags per post?: For a 12 × 32 in hole, about 3 bags of 80 lb.
How deep should posts be?: One third of the above-ground height, at least 24 in.
Dry-pour concrete in the hole?: Works for light posts; mix it for loaded ones.
What hole size for a 4×4 post?: About 10–12 in — three times the post width.
How deep?: One third of the post's exposed height, at least 24 in.
Concrete or mortar for posts?: Concrete — mortar lacks coarse aggregate.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.