Concrete column and post hole calculator
For anything round: fence post holes, deck piers, Sonotube and other tube forms, and columns. Enter the diameter, the depth or height, and how many, and the calculator gives cubic yards, bag counts and cost.
Calculator
Your estimate
Enter your measurements and select Calculate to see how much to order.
- Concrete to order
- cu yd
- Exact volume
- cu yd / cu ft
- Bags of 80 lb mix
- Cost with bags
- Cost with ready-mix
- Best way to buy
How it is calculated: length × width × thickness in feet gives cubic feet. Divide by 27 for cubic yards, then add your extra for waste and round up to the next quarter yard.
How the column calculator works
A round column, pier or hole is a cylinder. Its volume is the area of the circle times the height. The area of a circle is π times the radius squared, and the radius is half the diameter.
Volume (cu ft) = π × (diameter (in) ÷ 24)² × height (ft) × count
Dividing the diameter by 24 converts it to feet and halves it in one step. The result is multiplied by the number of columns, then by the waste allowance, and converted to cubic yards and bags.
Worked example
Six deck piers in 10 in tube forms, each 3 ft deep:
- Radius: 10 ÷ 24 = 0.4167 ft
- Area: 3.1416 × 0.4167² = 0.5454 sq ft
- One pier: 0.5454 × 3 = 1.636 cu ft
- Six piers: 9.82 cu ft = 0.364 cu yd
- With 10% waste: 10.80 cu ft, so buy 18 bags of 80 lb mix (10.80 ÷ 0.60 = 18.0) or order 0.5 cu yd
Quick reference: concrete per foot of tube
| Diameter | Cu ft per ft | 80 lb bags per ft |
|---|---|---|
| 6 in | 0.196 | 0.33 |
| 8 in | 0.349 | 0.58 |
| 10 in | 0.545 | 0.91 |
| 12 in | 0.785 | 1.31 |
| 16 in | 1.396 | 2.33 |
| 18 in | 1.767 | 2.95 |
| 24 in | 3.142 | 5.24 |
Bags per foot are fractional for reference. Round up to whole bags for the full depth.
Fence posts
For a fence post, the post itself takes up some of the hole. A 4 × 4 post (actually 3.5 × 3.5 in) in a 10 in hole displaces about 0.085 cu ft per foot of depth, roughly 15% of the hole. The calculator does not subtract the post, so for post holes the 10% waste allowance roughly cancels out. If you want a closer figure, reduce the waste to 0 for posts.
What to enter
- Diameter. The inside diameter of the tube form, or the diameter of the dug hole.
- Height or depth. The height of the concrete. For a pier that sits on a gravel pad, enter only the concrete part.
- How many. The number of identical columns or holes.
Limitations
- Holes dug by hand or auger are rarely a clean cylinder and often bell out at the bottom. Measure the real diameter and keep the waste allowance.
- Square columns are a slab calculation: use the slab calculator with the side as width.
- Pier depth below the frost line and pier size for the load are set by your building code, not by this calculator.
- A footing pad under the pier is a separate pour. Add it with the footing calculator.
Frequently asked questions
How many bags of concrete for a 12 inch Sonotube?
A 12 in tube holds 0.785 cu ft per foot of depth. At 4 ft deep that is 3.14 cu ft, which is 6 bags of 80 lb mix with 10% waste. Multiply by the number of tubes.
How many bags of concrete per fence post?
A 10 in hole with 2 ft of concrete holds 1.09 cu ft, which is 2 bags of 80 lb mix or 3 bags of 60 lb. A 12 in hole 2.5 ft deep holds 1.96 cu ft, about 4 bags of 80 lb. Enter your hole size and the number of posts for an exact count.
How deep should a post hole be?
A common rule is one third of the post’s above-ground height, and below the frost line in cold climates. Local codes and the fence manufacturer give the final answer.