Concrete calculator · post holes v1.0
Post Hole Concrete Calculator
Calculate how much concrete you need for post holes using the actual hole and buried-post dimensions, then see net volume, allowance, whole bags and cost separately.
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✓Multiple hole groups
✓Post displacement
✓Verified bag yields
Fence-post concrete example
How many 80-lb bags are needed per fence post?
Bag count depends on the measured hole, concrete-filled depth, actual post displacement, product yield, number of posts, and allowance. The example below is a material check, not a recommended hole detail.
- Net concrete per post
- 2.10 ft³ Gross hole minus actual post displacement
- With 10% allowance
- 2.31 ft³ Before whole-bag rounding
- Bags per example post
- 4 bags 80-lb QUIKRETE Concrete Mix at 0.60 ft³ per bag
- Ten identical posts
- 39 bags total Combined first, then rounded to whole bags
Assumptions in this example
- Nominal 4 × 4 post, using 3.5 × 3.5 in actual dimensions.
- 12-in round hole and 36-in concrete-filled depth; total excavated depth is a separate field decision.
- 10% example allowance and the verified 0.60-ft³ yield of the selected 80-lb product.
Use dimensions from the applicable plan, product instructions, and local requirements. This example does not recommend a structurally correct diameter or depth.
Step 1
Enter the measured hole groups
Keep matching holes together and add another group when corner, end, gate or other holes use different dimensions. How to measure a post hole for concrete.
Hole and post measurements
Use the concrete-filled depth, which can differ from the total excavated depth. Actual post dimensions are used only to subtract the space occupied within that concrete.
Quantity estimate only. This calculator does not choose the hole diameter, depth, spacing, post embedment, concrete strength or structural detail.
Ordering & pricing
Advanced assumptions: bag product, yield, and supplier increment
How the post-hole concrete calculation works
The calculator finds the gross volume of each hole, subtracts only the part of the post that lies within the concrete-filled depth, and multiplies the net volume by the number of identical holes. Every group is combined before the allowance and whole-bag rounding are applied.
Concrete per hole = hole cross-section × fill depth − post cross-section × post length within concrete
Adjusted project concrete = all group volumes × (1 + allowance ÷ 100)
Whole bags = adjusted volume ÷ published product yield, rounded up
Round and rectangular holes or posts
Hole shape and post shape are independent. The calculator supports round or rectangular holes, with no post, a round post or a rectangular post. It checks physical fit as well as volume, including the diagonal of a rectangular post inside a round hole.
Use actual finished post dimensions. A nominal product description may not equal its measured cross-section.
Concrete bags use product yield, not bag weight alone
The result uses the published cubic-foot yield of the selected concrete product. The same bag weight can represent different products, so the data sheet or current label for the exact mix takes priority. The list includes standard Concrete Mix yields from the QUIKRETE Concrete Mix Data Sheet 1101 plus verified QUIKRETE and Sakrete fast-setting product yields.
Raw volume, allowance, and whole bags stay separate
- Gross round hole
- 2.36 ft³ Before the post is subtracted
- Post displacement
- 0.26 ft³ 3.5 × 3.5 in through the concrete fill
- Raw net concrete
- 2.10 ft³ Material requirement before allowance
- Whole-bag order
- 4 × 80 lb After 10% allowance and whole-bag rounding
Use the existing calculator for the actual job
The live calculator keeps the ordering allowance visible, exposes the verified bag product and published yield, and combines line, corner, end, or gate posts before procurement rounding. Use Add another hole group whenever dimensions differ.
Enter concrete-filled depth—not total excavated depth—because only the filled portion creates concrete volume.
Fence-post concrete bag table by measured size
Raw equivalents are shown before allowance; whole bags use a 10% example allowance. Choose the exact product in the calculator because bag weight alone does not establish yield.
| Measured geometry | Raw bag equivalents | Whole bags with 10% allowance | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Post example | Hole diameter | Concrete depth | Gross hole ft³ | Post displacement ft³ | Net ft³ | 40 lb | 60 lb | 80 lb | 40 lb | 60 lb | 80 lb |
| Nominal 4 × 4 3.5 × 3.5 in actual | 10 in | 24 in | 1.09 | 0.17 | 0.92 | 3.07 | 2.05 | 1.53 | 4 | 3 | 2 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 10 in | 30 in | 1.36 | 0.21 | 1.15 | 3.84 | 2.56 | 1.92 | 5 | 3 | 3 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 10 in | 36 in | 1.64 | 0.26 | 1.38 | 4.60 | 3.07 | 2.30 | 6 | 4 | 3 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 10 in | 48 in | 2.18 | 0.34 | 1.84 | 6.14 | 4.09 | 3.07 | 7 | 5 | 4 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 12 in | 24 in | 1.57 | 0.17 | 1.40 | 4.67 | 3.11 | 2.33 | 6 | 4 | 3 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 12 in | 30 in | 1.96 | 0.21 | 1.75 | 5.84 | 3.89 | 2.92 | 7 | 5 | 4 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 12 in | 36 in | 2.36 | 0.26 | 2.10 | 7.00 | 4.67 | 3.50 | 8 | 6 | 4 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 12 in | 48 in | 3.14 | 0.34 | 2.80 | 9.34 | 6.23 | 4.67 | 11 | 7 | 6 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 14 in | 24 in | 2.14 | 0.17 | 1.97 | 6.56 | 4.37 | 3.28 | 8 | 5 | 4 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 14 in | 30 in | 2.67 | 0.21 | 2.46 | 8.20 | 5.47 | 4.10 | 10 | 7 | 5 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 14 in | 36 in | 3.21 | 0.26 | 2.95 | 9.84 | 6.56 | 4.92 | 11 | 8 | 6 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 14 in | 48 in | 4.28 | 0.34 | 3.94 | 13.12 | 8.75 | 6.56 | 15 | 10 | 8 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 18 in | 24 in | 3.53 | 0.17 | 3.36 | 11.21 | 7.48 | 5.61 | 13 | 9 | 7 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 18 in | 30 in | 4.42 | 0.21 | 4.21 | 14.02 | 9.34 | 7.01 | 16 | 11 | 8 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 18 in | 36 in | 5.30 | 0.26 | 5.05 | 16.82 | 11.21 | 8.41 | 19 | 13 | 10 |
| Nominal 4 × 4 3.5 × 3.5 in actual | 18 in | 48 in | 7.07 | 0.34 | 6.73 | 22.43 | 14.95 | 11.21 | 25 | 17 | 13 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 10 in | 24 in | 1.09 | 0.42 | 0.67 | 2.24 | 1.49 | 1.12 | 3 | 2 | 2 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 10 in | 30 in | 1.36 | 0.53 | 0.84 | 2.79 | 1.86 | 1.40 | 4 | 3 | 2 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 10 in | 36 in | 1.64 | 0.63 | 1.01 | 3.35 | 2.24 | 1.68 | 4 | 3 | 2 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 10 in | 48 in | 2.18 | 0.84 | 1.34 | 4.47 | 2.98 | 2.24 | 5 | 4 | 3 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 12 in | 24 in | 1.57 | 0.42 | 1.15 | 3.84 | 2.56 | 1.92 | 5 | 3 | 3 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 12 in | 30 in | 1.96 | 0.53 | 1.44 | 4.79 | 3.20 | 2.40 | 6 | 4 | 3 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 12 in | 36 in | 2.36 | 0.63 | 1.73 | 5.75 | 3.84 | 2.88 | 7 | 5 | 4 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 12 in | 48 in | 3.14 | 0.84 | 2.30 | 7.67 | 5.11 | 3.84 | 9 | 6 | 5 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 14 in | 24 in | 2.14 | 0.42 | 1.72 | 5.73 | 3.82 | 2.86 | 7 | 5 | 4 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 14 in | 30 in | 2.67 | 0.53 | 2.15 | 7.16 | 4.77 | 3.58 | 8 | 6 | 4 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 14 in | 36 in | 3.21 | 0.63 | 2.58 | 8.59 | 5.73 | 4.29 | 10 | 7 | 5 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 14 in | 48 in | 4.28 | 0.84 | 3.44 | 11.45 | 7.64 | 5.73 | 13 | 9 | 7 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 18 in | 24 in | 3.53 | 0.42 | 3.11 | 10.38 | 6.92 | 5.19 | 12 | 8 | 6 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 18 in | 30 in | 4.42 | 0.53 | 3.89 | 12.98 | 8.65 | 6.49 | 15 | 10 | 8 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 18 in | 36 in | 5.30 | 0.63 | 4.67 | 15.57 | 10.38 | 7.79 | 18 | 12 | 9 |
| Nominal 6 × 6 5.5 × 5.5 in actual | 18 in | 48 in | 7.07 | 0.84 | 6.23 | 20.76 | 13.84 | 10.38 | 23 | 16 | 12 |
A nominal 6 × 6 example is labelled as 5.5 × 5.5 in actual. Measure the post that will occupy the concrete rather than relying on the nominal description.
Safety and project scope
This calculator applies only where concrete or post-setting mix is already specified. It does not decide whether concrete, driven posts, ground screws, sleeves or surface-mounted bases are appropriate.
Before digging in the United States, use the applicable underground-service locating process; 811 Before You Dig explains the national 811 process. Follow the exact product label and safety data sheet: cement dust and wet concrete can injure skin and eyes. Hole sizing, embedment, frost, soil, wind, gate, deck and other loading decisions require project-specific information.
Post hole concrete calculator FAQs
How many 80-lb bags are needed per fence post?
Enter the measured hole, concrete-filled depth, actual post dimensions, quantity, allowance, and exact bag product. The calculator uses the product’s published cubic-foot yield and rounds the combined job up to whole bags.
Should the post’s volume be subtracted from the hole?
Yes when the post occupies space within the concrete. The calculator subtracts only the post volume overlapping the concrete-filled depth.
Can I calculate a square post inside a round hole?
Yes. Hole and post shapes are separate inputs. The calculator also checks that the rectangular post diagonal is smaller than the round hole diameter.
Is a nominal 4×4 actually 4 inches square?
Often it is not. The labelled example uses 3.5 × 3.5 in actual dimensions; measure the installed post because actual products can differ.
How do I calculate concrete for 10 or more posts?
Set the quantity for identical holes. The engine combines their net volume, then applies the allowance and rounds the project total to whole bags.
Can gate, corner, and line posts use different hole sizes?
Yes. Add a separate hole group for each distinct set of dimensions so larger gate or corner holes are not hidden inside an average.
Do fast-setting and standard concrete bags have the same yield?
Not necessarily. Yield is product-specific even at the same bag weight. Select the exact product or enter its current published yield.
Does this calculator tell me the correct hole depth or diameter?
No. It is a quantity calculator. Obtain structural, frost, soil, wind, load, product, and code requirements separately.
Sources and calculation basis
- QUIKRETE Concrete Mix Data Sheet 1101 — verified 40/60/80-lb product yields used in the table.
- JobsiteMeasure methodology and source register — calculation, allowance, and whole-bag-rounding basis.
Values generated September 3, 2026.