Estimate bags for identical cylindrical concrete zones around square or round posts. The default 10 posts, 10-inch diameter, 30-inch concrete height, 3.5-inch square posts and 10% allowance needs 22 bags at 0.6 cubic feet per bag. This calculates material only; it does not choose hole depth, frost protection or structural sizing.
Keep post concrete separate in the fence budget
Price the whole-bag count using the yield and price of the actual mix you plan to buy. These bags cover only the identical concrete zones entered above. They do not include the posts, rails, boards, gates, hardware, gravel bedding or other materials for the fence, and the calculated quantity does not approve the footing dimensions.
Use our home-improvement planning guide to compare quotes for the same fence and site scope. Keep delivery, digging, removal, labor and permit costs visible. If an installed fence quote already includes post concrete, do not add the bag cost again. For different gate-post or corner-post details, calculate the separate groups using their own approved dimensions.
Concrete volume around a post
Cylinder volume (cu in) = π × (diameter / 2)² × concrete fill height.
Square-post displacement = actual width² × fill height.
Round-post displacement = π × (post diameter / 2)² × fill height.
Net concrete (cu ft) = (cylinder volume - post displacement) / 1,728.
Bags = net volume × post count × (1 + allowance / 100) / bag yield, rounded up. Our calculator uses full π and rounds the final batch bag count, not intermediate volumes.
Measure actual post dimensions. The square options assume 3.5-inch and 5.5-inch widths as labeled; verify your material. The 3-inch option is now explicitly round and subtracts a cylinder, not the square prism previously used. Square posts need a hole wider than their diagonal to fit geometrically; actual concrete clearance needs the design, not just the fit test.
Concrete height is not always the full hole depth
The cited QUIKRETE Fast-Setting sheet describes stopping mix about 3 to 4 inches below the top and filling the remainder with soil or sod.1 If that exact product/detail applies, exclude that soil zone from the concrete height. Exclude any gravel bedding as well. The post must occupy the whole entered concrete zone under this model; a pad below the post requires a separate volume.
Example only: a 30-inch excavation with 4 inches reserved for soil and 6 inches of bedding would leave 20 inches of concrete height. That arithmetic is not an instruction to use those layers. Confirm the chosen assembly and actual post penetration before estimating.
Bag yields: use the exact mix
| Example product/bag | Approximate mixed yield | Source |
|---|---|---|
| QUIKRETE Concrete Mix, 80 lb | 0.60 cu ft | Concrete Mix sheet |
| QUIKRETE Fast-Setting, 60 lb | 0.45 cu ft | Fast-Setting sheet |
| QUIKRETE Fast-Setting, 50 lb | 0.375 cu ft | Fast-Setting sheet |
These cited products support the preset yields, not every bag of the same weight.1,2 Read the exact package. Other yields cannot be selected in this form; use net volume divided by the actual yield separately. Do not mix dry-pour directions for a fast-setting product with a mix that requires premixing.
Bags for ten square 3.5-inch posts
Our calculated table uses a 30-inch concrete height and 10% example allowance. Diameter values are quantity scenarios, not approved post-hole sizing. Bags use total batch rounding and the stated yield.
| Diameter | Net per post | 80 lb / 0.60 yield | 50 lb / 0.375 yield |
|---|---|---|---|
| 10 in | 1.151 cu ft | 22 | 34 |
| 12 in | 1.751 cu ft | 33 | 52 |
| 14 in | 2.460 cu ft | 46 | 73 |
| 16 in | 3.278 cu ft | 61 | 97 |
Three checked examples
1. 10 posts, 10-inch cylinder, 30-inch concrete height
Post model: 3.5-inch square width. Hole volume: 2356.194490 cu in; post displacement: 367.500000 cu in. Net concrete per post: 1.150865 cu ft. Across 10 posts with 10% example allowance: 12.659514 cu ft. Divide by 0.6 cu ft per bag: 21.099189, rounded up to 22 bags. These are material examples, not design specifications.
2. 10 posts, 10-inch cylinder, 26-inch concrete height
Post model: 3.5-inch square width. Hole volume: 2042.035225 cu in; post displacement: 318.500000 cu in. Net concrete per post: 0.997416 cu ft. Across 10 posts with 10% example allowance: 10.971578 cu ft. Divide by 0.375 cu ft per bag: 29.257542, rounded up to 30 bags. These are material examples, not design specifications.
3. 5 posts, 9-inch cylinder, 24-inch concrete height
Post model: 3-inch round diameter. Hole volume: 1526.814030 cu in; post displacement: 169.646003 cu in. Net concrete per post: 0.785398 cu ft. Across 5 posts with 0% example allowance: 3.926991 cu ft. Divide by 0.375 cu ft per bag: 10.471976, rounded up to 11 bags. These are material examples, not design specifications.
Batch rounding versus per-post rounding
For ten identical posts, the form adds unrounded volume and rounds the final bag total. Rounding each post up first can overstate the batch need. If you cannot share mixed material between holes within the product working time, plan staging separately. Do not mix an entire batch simply because the calculator combines volumes.
Depth and diameter are design decisions
The Fast-Setting sheet gives product-scoped post-setting proportions, including approximately three times post diameter and one third of total post height.1 Those instructions do not establish local frost depth, wind/load requirements, soil bearing, gate-post sizing or your fence system. Do not treat the default 10-inch diameter and 30-inch concrete height as approval of a footing. Confirm the actual hole and cover detail with the local requirements and fence designer/installer.
Contact 811 and follow state locate, response and safe-excavation rules before digging; private lines may need separate locating. Check property boundaries and utilities. Gate and corner loads can differ, so estimate their checked dimensions as separate groups rather than assigning every post the same default.
Metric mode
The toggle converts cylinder diameter and concrete height to centimeters. Post-size and bag-yield options remain in their labeled US product units. Ten posts, 25.4 cm diameter and 76.2 cm concrete height preserve the default 22-bag estimate with the square 3.5-inch post and 0.6 cu ft yield. Count must stay a whole number.
FAQ
Can I enter a 4×6 post?
No. This form has two square-post options and one 3-inch round option. For a rectangular post, subtract actual width × actual thickness × the concrete height. Use the post concrete guide for related quantity examples, not to choose footing dimensions.
Why are fractional post counts rejected?
The count represents whole posts. The old form silently rounded it; the revised form asks for an integer so it does not change your input unnoticed.
Can I use the entire hole depth?
Only when concrete occupies that entire height around the post. Soil cover, bedding, a pad beneath the post or a flared/noncylindrical hole need separate treatment.
Related tools
Concrete calculator | Fence picket calculator | Post concrete guide
Sources
- QUIKRETE: Fast-Setting Concrete Mix sheet (50/60-lb yields and product-specific post-setting directions)
- QUIKRETE: Concrete Mix sheet (80-lb mixed yield and product directions)
What you'll need
A plain checklist. Needs vary by project, so follow the instructions on your materials.
- Post hole digger or auger - to dig holes.
- Level - keeps posts plumb.
- Gravel for the hole bottom, optional - some installers use it for drainage.
- Fast-setting concrete or mix - check bag yield and set time.
- Wheelbarrow and water - for mixing.
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Estimates only. Confirm quantities with your supplier before you order.