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Post Hole Concrete Calculator

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Definition

What is Post Hole Concrete Calculator?

A post hole concrete calculator computes the net volume of concrete needed to fill a cylindrical hole around a post. It subtracts the volume of the post (round or square) from the total hole volume to give the exact amount of concrete required — preventing over-ordering and waste.

Setting posts in concrete is the standard method for fence posts, mailbox posts, deck support posts, pergola posts, sign posts, and basketball hoop poles. The concrete anchors the post, prevents shifting from wind loads, and keeps the post plumb (vertical).

A typical fence post hole is 3× the post width in diameter and 1/3 of the total post length in depth. For a 6-foot fence with 4×4 posts, the hole should be 12 inches in diameter and 24 inches deep (leaving 4 feet of post above ground).

Formula

Post Hole Concrete Calculator Formula

Net Concrete Volume = Hole Volume − Post Volume

Hole Volume (cylinder): V = π × (hole diameter ÷ 2)² × depth

Round Post Volume: V = π × (post diameter ÷ 2)² × depth

Square Post Volume: V = post width × post width × depth

Step-by-step for a 4×4 post in a 10-inch diameter hole, 24 inches deep:

  1. Hole volume: π × 5² × 24 = 1,885 in³
  2. Post volume (3.5×3.5 actual): 3.5 × 3.5 × 24 = 294 in³
  3. Net concrete: 1,885 − 294 = 1,591 in³ = 0.92 ft³
  4. Bags needed: 0.92 ÷ 0.6 = 2 bags of 80 lb concrete

Note: Nominal lumber sizes differ from actual sizes. A 4×4 post is actually 3.5×3.5 inches. A 6×6 post is 5.5×5.5 inches.

Examples

Worked Examples

Fence Post (4×4, 24" deep)

10" hole, 24" deep, 3.5×3.5 post. Net concrete = 0.92 ft³. Needs 2 bags of 80 lb concrete per post.

Mailbox Post (4×4, 18" deep)

8" hole, 18" deep. Net concrete = 0.40 ft³. Needs 1 bag of 80 lb concrete.

Deck Post (6×6, 42" deep)

18" hole, 42" deep, 5.5×5.5 post. Net concrete = 5.16 ft³. Needs 9 bags of 80 lb concrete.

50 Fence Posts

50 posts × 0.92 ft³ each = 46 ft³ = 1.70 yd³ total. Needs 100 bags of 80 lb concrete.

FAQ

Frequently Asked Questions

Answers to common questions about the post hole concrete calculator.

1–2 bags of 80 lb concrete per fence post is typical. A 4×4 post in a 10-inch hole at 24 inches deep needs about 0.92 ft³ of concrete = 2 bags. A shallower 18-inch hole needs 1 bag. Multiply by the number of posts for your total.

The hole should be 1/3 of the total post length. For a 6-foot fence with 8-foot posts: 8 ÷ 3 = 2.67 feet (32 inches) deep. In cold climates, holes must be deeper than the frost line (36–48 inches) to prevent frost heave.

The hole diameter should be 3× the post width. A 4×4 post (3.5" actual) needs a 10–12 inch hole. A 6×6 post (5.5" actual) needs a 16–18 inch hole. Larger holes provide more concrete mass for better stability.

Fast-setting concrete is ideal for post holes. It sets in 20–40 minutes, allowing you to continue work immediately. You can pour it dry into the hole, set the post, and add water — no mixing required. Regular concrete takes 24–48 hours to set and requires pre-mixing.

Fill to 3–4 inches below ground level and crown the top. Shape the concrete so it slopes away from the post to shed water. Leave the top few inches unfilled so you can cover with soil and grass. Water pooling at the base of the post causes rot.