Construction desk
Fence Post Calculator
Estimate post count and concrete bags for evenly spaced posts.
Fence Post Calculator: Count posts for one open, straight fence run using a maximum centre-to-centre spacing, then estimate concrete for identical round holes. The run includes a post at each end. Concrete bags are rounded upwards from the combined hole volume using the mixed yield you enter for one bag. Runs 100% locally in your browser with zero server file uploads.
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Posts to set (estimate)
14 posts
- Concrete volume (estimate)
- 19.09 cu ft
- Concrete bags to buy (estimate)
- 32 bags
Posts = ceiling(fence length ÷ spacing) + 1; concrete = posts × cylindrical hole volume- Every result is an estimate; verify surface preparation, measurements, and local practices before ordering.
- Allow for the stated waste and product-coverage assumptions; openings are deducted only where requested.
- Compaction, settling, substrate condition, cuts, and installation method can change actual quantities.
Formula and worked example
Posts = ceiling(fence length ÷ spacing) + 1; concrete = posts × cylindrical hole volume
A worked example with sample values:
| Inputs | |
|---|---|
| Fence length | 100 ft |
| Post spacing | 8 ft |
| Hole depth | 30 in |
| Hole diameter | 10 in |
| Bag coverage | 0.6 cu ft/bag |
| Results | |
|---|---|
| Posts to set (estimate) | 14 posts |
| Concrete volume | 19.09 cu ft |
| Concrete bags to buy (estimate) | 32 bags |
Post count and cylindrical holes
Let F be fence length in feet, s maximum spacing, D hole diameter in inches, H concrete depth in inches and b mixed bag yield in ft³. Posts N = ceiling(F ÷ s) + 1. Each hole volume = π × (D ÷ 24)² × (H ÷ 12). Total concrete V = N × hole volume; bags = ceiling(V ÷ b).
With F = 100 ft, s = 8 ft, D = 10 in, H = 30 in and an illustrative b = 0.6 ft³, N = 14. A hole holds π × (10 ÷ 24)² × (30 ÷ 12) = 1.363538 ft³. Fourteen holes total 19.089539 ft³, displayed as 19.09 ft³; 19.089539 ÷ 0.6 = 31.815898, so buy 32 bags under these assumptions.
Hole sizing is a separate decision
QUIKRETE's post-setting document (https://www.quikrete.com/athome/setting-posts.pdf) distinguishes the drainage base from the concrete-filled part of a hole. Follow the chosen system's installation directions and local site requirements for depth and diameter. This example does not establish suitable foundations for soil, wind loading or frost conditions.
For bulk concrete, divide the total ft³ by 27 to get yd³. NIST's volume table (https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8) converts yd³ to m³ by multiplying by about 0.7645549. Confirm delivery quantity after accounting for the actual hole shapes.
How to use it
- Enter measurements using the units shown beside each field.
- Choose the product coverage and allowances for your measured job.
- Calculate, then compare the rounded quantity with your layout and product label.
Privacy & limitations
Inputs and calculations stay in your browser. No values are uploaded.
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Frequently asked questions
Why does a 100 ft fence at 8 ft spacing need 14 posts?
There are ceiling(100 ÷ 8) = 13 spans and one more post than spans. If all spans are made equal, actual spacing is 100 ÷ 13 = about 7.69 ft. Eight feet is only an illustrative maximum; check the fence system's requirements.
How should I handle corners, gates and a closed fence loop?
Work out the spans on each straight run, then remove duplicate shared corner posts from the total. Gate openings may require separate supports. A closed loop shares its starting and ending post, so the open-run plus-one rule cannot simply be applied to its perimeter.
Does the concrete quantity subtract the post or gravel base?
No. It uses full cylindrical holes and has no waste input. Post displacement and a drainage base can reduce concrete, while overdigging can increase it. Measure the concrete-filled depth and adjust your take-off separately; check the product label for mixed yield rather than using dry bag weight.
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