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Fence & posts

Post Hole Concrete Calculator

Calculate concrete for round holes after subtracting the space occupied by the posts. Get net volume, an order allowance and a whole-bag purchase estimate.

Imperial + metricFraction-friendly inputsFree · No loginSee the math ↓

Project measurements

01 / INPUTS

Mix units freely: 8 ft, 100 mm, or 5' 7 1/2". Unmarked dimensions use the selected unit.

View live estimate ↓
Square posts use side width. Round posts use diameter.
80 lb standard concrete mix ≈ 0.60 ft³; 60 lb ≈ 0.45. Verify the label.
Applied before rounding purchase quantities.

Your estimate

LIVE RESULT
Concrete with allowance
0.799yd³
Includes your selected allowance
Net volume19.609 ft³
Net volume0.5553 m³
Order volume0.6108 m³

Purchase / material list

Bags to purchase36 bags

No account. Calculations stay in your browser.

Before you order

  • Straight cylindrical holes; the post occupies the full concrete fill depth. Enter actual post dimensions. Exclude drainage gravel from fill depth. Hole sizing and embedment are design inputs, not recommendations.
  • Bag quantity rounds up after applying the allowance to the total volume. Bag yield must match the product label.
  • Ready-mix volume is unrounded internally. Confirm delivery minimums and order increments with your supplier.

UNDERSTAND YOUR ESTIMATE

How to use the post hole concrete calculator

A post displaces concrete. The difference between the hole cross-section and post cross-section, multiplied by concrete fill depth, gives the net volume for one hole. Multiply by the count before adding waste and rounding bags; rounding each hole independently would overstate a pooled batch.

Formula

Hole area = π × (hole diameter ÷ 2)²
Square post area = actual width²
Concrete volume = (hole area − post area) × fill depth × hole count
Bags = ceil(volume with allowance ÷ bag yield)

Assumptions & scope

  • The hole is cylindrical with a uniform diameter. Bell-shaped footings, sloped walls and over-excavation require separate allowances.
  • The post occupies the full concrete fill depth. Enter the depth actually filled with concrete, excluding a drainage layer.
  • A square post must fit diagonally within the hole. Nominal 4×4 dimensions are not assumed.
  • Diameter and embedment presets are quantity examples only. Soil, frost depth, wind, loads and local requirements determine the design.
ON THE JOB / WORKED EXAMPLE

A calculation you can check

One 12 in diameter hole with 24 in of concrete and a 3½ in square post uses about 1.401 ft³ net. With 10% allowance, that is about 1.541 ft³, requiring 3 bags at 0.60 ft³ per bag.

Quick reference

Post Hole Concrete Calculator reference values
12 in hole, 3½ in square postNet concrete / hole80 lb bags with 10%, single hole
18 in fill depth1.051 ft³2
24 in fill depth1.401 ft³3
36 in fill depth2.101 ft³4

Calculation methodology

Cylinder geometry minus square or circular post displacement. Common bag yields are approximate manufacturer values; verify the exact product and revise the yield input.

Length conversions use the NIST SI conversion factors. Concrete-mix yield examples follow the QUIKRETE Concrete Mix technical data sheet. See our calculation and editorial standards for precision, validation and limitations.

Common questions

Do I subtract the gravel at the bottom?

Yes. Enter concrete fill depth only. For a 30-inch hole with a 6-inch gravel base, enter 24 inches if concrete fills the remaining depth.

Can I use the default hole size for any fence?

No. The calculator estimates a volume from dimensions you provide. Confirm the required foundation design for your site, structure and local requirements.

Why is the bag total less than bags per hole times hole count?

Concrete can be batched across holes. The total is rounded once after adding allowance, avoiding an unnecessary partial-bag round-up for every hole.