Quick answer
Concrete per hole = π ÷ 4 × hole diameter² × depth − post cross-section × depth. A 12 inch hole 36 inches deep around a 4 inch round post needs 2.094 ft³, or 5 bags of 60 lb mix.
Enter the hole diameter and depth, the post shape and size and the bag size. The calculator returns concrete per hole and bags.
Related tools: Concrete Steps Calculator: Concrete volume for solid stacked steps. Concrete Volume Calculator: Cubic yards of concrete and bags needed for a slab, footing or pad. Concrete Reinforcing Mesh Calculator: Mesh sheets for a slab, allowing for overlaps.
How to use the Post Hole Concrete Calculator
- Enter hole diameter (in) (above 0).
- Enter hole depth (in) (above 0).
- Choose post shape: Round and Square.
- Enter post diameter or side (in) (above 0).
- Choose bag size: 80 lb bag (0.60 ft³), 60 lb bag (0.45 ft³) and 40 lb bag (0.30 ft³).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Hole diameter | A number, in in | > 0 |
| Hole depth | A number, in in | > 0 |
| Post shape | Choose one: Round; Square | Starts at Square |
| Post diameter or side | A number, in in | > 0 |
| Bag size | Choose one: 80 lb bag (0.60 ft³); 60 lb bag (0.45 ft³); 40 lb bag (0.30 ft³) | Starts at 80 lb bag (0.60 ft³) |
Formula and method
Formula
concrete = π/4 × hole diameter² × depth − post cross-section × depth
IEEE-754 double precision; shown to the decimals indicated.
Per hole; bag counts use typical label yields. Plan geometry only; confirm quantities with your supplier and local code before ordering.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Hole diameter = 12; Hole depth = 36; Post shape = Round; Post diameter or side = 4; Bag size = 60 lb bag (0.45 ft³).
| Result | Value |
|---|---|
| Concrete per hole | 2.094 ft³ |
| Bags per hole | 5 |
Example 2
Inputs: Hole diameter = 10; Hole depth = 30; Post shape = Square; Post diameter or side = 4; Bag size = 80 lb bag (0.60 ft³).
| Result | Value |
|---|---|
| Concrete per hole | 1.086 ft³ |
| Bags per hole | 2 |
Example 3
Inputs: Hole diameter = 12; Hole depth = 42; Post shape = Square; Post diameter or side = 6; Bag size = 40 lb bag (0.30 ft³).
| Result | Value |
|---|---|
| Concrete per hole | 1.874 ft³ |
| Bags per hole | 7 |
The first line is the headline result (concrete per hole). Below it the post Hole Concrete Calculator lists bags per hole, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Subtract the post
The hole holds 2.356 ft³, but the post takes up 0.262 ft³, leaving 2.094 ft³ of concrete. Dividing by 0.45 ft³ per 60 lb bag gives 4.65, rounded up to 5.
Footing guidance
Hole diameter is commonly about three times the post width, and depth about one-third of the post's total length, below the frost line in cold areas. A gravel base under the post reduces the concrete needed and helps drainage.
How the result changes with hole diameter
Holding every other input at the first worked example, this table changes only hole diameter. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Hole diameter | Concrete per hole | Bags per hole |
|---|---|---|
| 6 | 0.327 ft³ | 1 |
| 12 | 2.094 ft³ | 5 |
| 24 | 9.163 ft³ | 21 |
| 48 | 37.437 ft³ | 84 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Hole diameter: entering "0" shows "Hole diameter must be greater than 0." beside that field and no result.
- Post diameter or side: entering "8" shows "The post is larger than the hole." beside that field and no result.
Choosing between the Post Hole Concrete Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Concrete Steps Calculator](/construction/concrete-steps-calculator/) | number of steps, riser height and tread depth | Concrete volume |
| [Concrete Volume Calculator](/construction/concrete-volume-calculator/) | length, width and thickness | Concrete needed |
| [Concrete Reinforcing Mesh Calculator](/construction/concrete-reinforcing-mesh-calculator/) | slab area, sheet width and sheet length | Sheets needed |
| [Concrete Weight Calculator](/construction/concrete-weight-calculator/) | volume, volume unit and density | Weight |
| [Concrete Mix Calculator](/construction/concrete-mix-calculator/) | volume of concrete, volume unit and nominal mix | Cement |
Background: construction estimates
Material quantities are geometry plus a margin. Calculate the exact volume or area, add an allowance for waste, spillage, cuts and uneven ground (calculator pages commonly mention 5 to 10 percent, a rule of thumb that varies by material and job) and round up to whole units. One cubic yard is exactly 27 cubic feet.
Suppliers often have minimum order sizes and may charge for small loads, so confirm quantities, product yields and local code with your supplier or a qualified estimator before ordering.
Assumptions and limitations
- Gravel base under the post reduces concrete
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for hole diameter and post diameter or side to confirm this.
Frequently asked questions
How many bags per post?
About 4 to 6 for a typical fence post.
Do I need gravel?
A few inches helps drainage.
Can I use fast-setting mix?
Yes, it reduces waiting time.
What does the Post Hole Concrete Calculator calculate?
Concrete per post hole after the post's volume. It returns Concrete per hole and Bags per hole.
What formula does the Post Hole Concrete Calculator use?
concrete = π/4 × hole diameter² × depth − post cross-section × depth. Per hole; bag counts use typical label yields.
What do I need to enter in the post Hole Concrete Calculator?
You enter Hole diameter (in), Hole depth (in), Post shape, Post diameter or side (in) and Bag size.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Gravel base under the post reduces concrete.
What does the “Post shape” option change?
It sets how the post Hole Concrete is worked out. The choices are Round and Square.
What is an example of a post Hole Concrete result?
With hole diameter 12, hole depth 36, post shape Round, post diameter or side 4 and bag size 60 lb bag (0.45 ft³), the concrete per hole is 2.094 ft³.
Where does the method come from?
The method follows: OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem; NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international foot 0.3048 m, inch 25.4 mm, US gallon 3.785411784 L); Typical pre-mixed bag yields as printed on manufacturers' bags (varies by brand and product; check your bag).
Can I rely on this calculator for a decision?
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work. For this calculator in particular: Gravel base under the post reduces concrete.
Related calculators
- Concrete Steps Calculator — Concrete volume for solid stacked steps.
- Concrete Volume Calculator — Cubic yards of concrete and bags needed for a slab, footing or pad.
- Concrete Reinforcing Mesh Calculator — Mesh sheets for a slab, allowing for overlaps.
- Concrete Weight Calculator — Weight of a volume of concrete from its density.
- Concrete Mix Calculator — Cement, sand and aggregate for a nominal volume mix such as 1:2:4.
- Concrete Block Fill Calculator — Grout or concrete needed to fill block cores.
Sources
- OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international foot 0.3048 m, inch 25.4 mm, US gallon 3.785411784 L)
- Typical pre-mixed bag yields as printed on manufacturers' bags (varies by brand and product; check your bag)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.