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Post Hole Concrete Calculator — Bags & Volume

Enter the number of round holes, hole diameter, and concrete depth to estimate total volume and complete 40, 60, or 80 lb bags. Optionally subtract a round or square post.

  • Multiple holes
  • Optional post displacement
  • Metric & imperial

Hole geometry

Enter the post-hole measurements

Measurement system

This device

Saved estimates

Save an estimate to keep it here. Data stays in this browser.

Transparent by design

Post-hole concrete formula

Each round hole is treated as a cylinder. The calculator finds the gross cylindrical volume first, then optionally subtracts the volume occupied by a round or square post before multiplying by the number of holes.

Gross hole volumeV = π × (diameter ÷ 2)² × depthTotal net concreteVnet = (V − post displacement) × hole countOrder volumeVorder = Vnet × (1 + allowance ÷ 100)

This formula estimates material only. It does not determine a safe hole diameter, embedment depth, footing design, reinforcement, or code requirement.

Worked example

One 12-inch hole with 24 inches of concrete depth

Use one round hole, no post displacement, 0% extra allowance, and the published 0.60 ft³ yield for an 80 lb bag. This controlled example verifies the cylinder formula independently of the interface defaults.

  1. 01
    Convert radius12 in diameter = 0.5 ft radius
  2. 02
    Convert depth24 in = 2 ft
  3. 03
    Find volumeπ × 0.5² × 2 = 1.5708 ft³
  4. 04
    Find complete bags1.5708 ÷ 0.60 = 2.618 → 3 bags
Verified result1.571 ft³ and 3 × 80 lb bags

This example does not prescribe a hole size. It only verifies the quantity calculation for the dimensions shown.

Common questions

Post-hole concrete calculator FAQ

Material-estimating context without structural-design assumptions.

How does the post hole concrete calculator work?

It treats each hole as a cylinder, multiplies the per-hole concrete volume by the number of holes, optionally subtracts a round or square post, applies your chosen allowance, and converts the result to cubic feet, cubic yards, cubic meters, liters, and complete bags.

How many 80 lb bags are needed for a 12 inch diameter, 24 inch deep hole?

With no post displacement and no extra allowance, a 12 inch diameter hole with 24 inches of concrete depth is about 1.571 cubic feet. At a published yield of 0.60 cubic feet per 80 lb bag, that rounds up to 3 bags.

Should I subtract the post volume?

Only if the selected post shape and size reasonably represent the volume that will actually occupy the concrete-filled depth. When enabled, this calculator assumes the post occupies the full entered concrete depth. Leave displacement off when that assumption does not match the project.

Does this calculator tell me how deep or wide a post hole should be?

No. It only estimates material from dimensions you enter. Hole diameter, embedment depth, footing details, frost requirements, soil conditions, wind loads, and local code requirements must be determined separately for the project.

Why is the bag count approximate?

Concrete mix yield varies by product. The calculator uses published Sakrete High-Strength Concrete Mix yields of 0.30, 0.45, and 0.60 cubic feet for 40, 60, and 80 lb bags. Confirm the selected product's current yield before purchase.

Continue planning

Use the right concrete geometry for the job.

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