How to use the post hole concrete calculator calculator
- 1
Measure the hole diameter
Use the diameter of the auger or the hole as dug, not the width of the post. If the hole is wider at the top than the bottom, using the wider figure is the safer choice for ordering.
- 2
Enter the hole depth
Enter the depth of concrete, which is the part of the hole that will actually be filled. If you are placing gravel in the bottom, subtract that from the hole depth before entering it.
- 3
Set the number of holes
Enter how many identical holes you are setting. Volume per hole is multiplied by the count.
- 4
Decide whether to subtract the post
Tick the option and enter the post size to subtract what the post displaces. Leave it off to calculate the full hole volume, which is the more conservative figure.
- 5
Add a bag yield for a bag count
Post holes are commonly filled from bags, so enter the yield printed on the bag to convert the volume into a count, and add a price if you want the cost.
Formula
Concrete per hole = (pi x (hole diameter(ft) / 2)^2 x depth(ft)) - post volume
Leave the post volume out to calculate the full hole. Multiply the per-hole figure by the number of holes, then divide by 27 for cubic yards.
Worked example
Eight fence post holes, each 12 inches across and 36 inches deep, with no post subtraction
- 1Convert the diameter: 12 in = 1 ft, so the radius is 0.5 ft.
- 2Find the hole area: pi x 0.5^2 = 0.785 square feet.
- 3Convert the depth: 36 in = 3 ft.
- 4Volume per hole: 0.785 x 3 = 2.36 cubic feet.
- 5Total for eight holes: 2.36 x 8 = 18.85 cubic feet, which is 0.70 cubic yards.
Eight holes of this size take about 18.85 cubic feet of concrete, or 0.70 cubic yards, if the posts are not subtracted.
Worked examples
The same holes with the post subtracted
The same eight holes, each holding a nominal 4x4 post measuring 3.5 inches across
- 1Post side in feet: 3.5 in / 12 = 0.2917 ft.
- 2Post cross-section: 0.2917^2 = 0.0851 square feet.
- 3Post volume over 3 feet of depth: 0.0851 x 3 = 0.26 cubic feet.
- 4Concrete per hole: 2.36 - 0.26 = 2.10 cubic feet.
- 5Total for eight: 2.10 x 8 = 16.81 cubic feet, or 0.62 cubic yards.
Subtracting the posts saves about 2.04 cubic feet across the eight holes, roughly 11 percent of the total.
Bag count for the same job
The 16.81 cubic feet above, using a bag with a stated 0.6 cubic foot yield
- 1Divide by the yield entered: 16.81 / 0.6 = 28.01 bags.
- 2Round up to whole bags: 29 bags.
- 3That is between 3 and 4 bags per hole across eight holes.
The job takes 29 bags at that example yield. Read the yield off the bag you actually buy, since it differs by product.
Why a post hole is a cylinder problem
A post hole dug with an auger is close to a cylinder, so its volume is the circular area of the hole multiplied by the depth. That is pi times the radius squared, times the depth, with the radius being half the diameter.
The important consequence is that volume grows with the square of the diameter. A 12 inch hole holds four times as much concrete as a 6 inch hole at the same depth, not twice as much. Depth, by contrast, is linear: doubling the depth doubles the volume.
This is why diameter is the number to measure carefully. An auger that cuts slightly wide, or a hole that has been reamed out at the top, adds more concrete than most people expect. Depth errors are more forgiving.
Post displacement, and when it matters
The post occupies part of the hole, so the concrete only fills what is left around it. Subtracting the post volume gives the more precise figure, and the calculator shows the hole volume, the displacement, and the net concrete separately so the arithmetic is visible.
Whether it is worth subtracting depends on the ratio of post to hole. A 3.5 inch post in a 12 inch hole displaces about 11 percent of the volume, which is real but modest. The same post in an 8 inch hole displaces around 25 percent, which is worth accounting for. A 6 inch post in an 8 inch hole displaces well over half.
Leaving the subtraction off gives a conservative figure that assumes the hole is filled completely. On a small job, buying that little extra is often simpler than working to the tighter number, and unused bags are easy to keep. The calculator defaults to the full hole for that reason and lets you switch.
One assumption to note: when the subtraction is on, the calculator treats the post as buried for the full depth of the hole, which is the usual arrangement. If your post sits on a gravel base or stops short of the bottom, calculate that portion separately.
Gravel bases and partial fills
Many post installations put gravel in the bottom of the hole before the concrete. If that is your plan, the depth to enter is the depth of concrete, not the depth of the hole. Entering the full hole depth when several inches are gravel overstates the concrete by that fraction.
The same applies at the top. Some installations stop the concrete below grade and backfill with soil, while others bring it to the surface and crown it. Enter the depth that will actually be concrete.
Whether to use gravel, how deep to set a post, and how the concrete should be finished at the top are all installation decisions that depend on the post, the loads, the drainage, the frost depth, and local requirements. YardCalc does not prescribe any of them: take them from the post manufacturer instructions, your plan, or your building department, then calculate the volume that results.
Common mistakes
Where post hole estimates go wrong:
- Using the post width as the hole diameter, which understates the volume dramatically.
- Measuring the radius but entering it as the diameter, which quadruples or quarters the answer.
- Entering the full hole depth when part of the hole will be gravel.
- Forgetting to multiply by the number of holes.
- Subtracting the post when the post does not run the full depth of the hole.
- Buying exactly the calculated bags when holes are dug in soft ground that may have widened.
- Assuming every hole came out the same size when they were dug by hand.
Post hole volume examples by size
Full hole volume for a range of diameters and depths, before any post is subtracted. The last column shows how many holes of that size one cubic yard would fill. These sizes demonstrate the arithmetic and are not recommended hole dimensions.
| Diameter x depth | Cubic feet each | Cubic yards each | Number that fit in 1 cubic yard |
|---|---|---|---|
| 8 in x 24 in | 0.698 | 0.0259 | 38 |
| 8 in x 30 in | 0.873 | 0.0323 | 30 |
| 10 in x 30 in | 1.36 | 0.0505 | 19 |
| 10 in x 36 in | 1.64 | 0.0606 | 16 |
| 12 in x 36 in | 2.36 | 0.0873 | 11 |
| 12 in x 42 in | 2.75 | 0.102 | 9 |
| 16 in x 48 in | 5.59 | 0.207 | 4 |
Bag-yield math: bags per cubic yard
Mathematical bag-yield examples showing 27 cubic feet divided by each yield. Use the yield printed on the actual concrete bag or supplier documentation.
| Bag volume | Bags per cubic yard | Whole bags for 1 cubic yard |
|---|---|---|
| 0.33 cu ft | 81.82 | 82 |
| 0.45 cu ft | 60 | 60 |
| 0.6 cu ft | 45 | 45 |
| 0.75 cu ft | 36 | 36 |
| 1 cu ft | 27 | 27 |
Frequently asked questions
- How much concrete do I need per post hole?
- It depends on the hole size. A 12 inch hole 36 inches deep holds 2.36 cubic feet if filled completely. A 10 inch hole at the same depth holds 1.64 cubic feet, and an 8 inch hole holds 1.05 cubic feet. Enter your own dimensions above.
- How many bags of concrete per fence post?
- Divide the hole volume by the yield printed on your bag. A 12 inch by 36 inch hole at a 0.6 cubic foot yield is 3.93 bags, so four per hole. At a 0.45 yield it is 5.24 bags, so six per hole. Yields differ by product, so use the figure on your bag.
- Should I subtract the post from the hole volume?
- It depends on how much of the hole the post fills. A 3.5 inch post in a 12 inch hole takes up about 11 percent, while the same post in an 8 inch hole takes up around 25 percent. The calculator can do either, and leaving the post in gives the more conservative figure.
- How deep should a post hole be?
- That is not something this calculator decides. Depth depends on the post, the loads, the soil, the frost depth, and local code, so take it from the manufacturer instructions, your plan, or your building department, then enter it here.
- Does hole diameter matter more than depth?
- Yes, because volume grows with the square of the diameter but only linearly with depth. Doubling the diameter quadruples the concrete, while doubling the depth only doubles it.
- What if I am putting gravel in the bottom of the hole?
- Enter the depth of concrete rather than the depth of the hole. If a 36 inch hole has 6 inches of gravel at the bottom, enter 30 inches.
Hole volume uses the exact cylinder formula, and post displacement is subtracted only when you supply a post size. This page does not recommend hole diameters, depths, or spacing, and no bag yield or price is assumed. Concrete calculations estimate material quantity from geometry and user-entered values. Confirm mix design, thickness, reinforcement, code requirements, bag yield, delivery, and final order quantity with the appropriate supplier or professional.
Results are planning estimates. Last reviewed 2026-07-28.