Fence Post Concrete Calculator
Hole and post size to bags per hole.
Also known as post concrete · post hole concrete · fence post footing
With the default inputs: Hole diameter 10 in · Hole depth 30 in · Post size 4 in etc.Bags per post 4 bags
Use your own dimensions ↓Inputs
Result
| Hole volume | 1.36ft³ |
| Less post | 0.28ft³ |
| Net per post | 1.19ft³ |
| Total bags | 40bags |
| Total volume | 0.44yd³ |
| Total weight | 1732lb |
| Post embedment | 2.5ft |
How this is worked out
Net concrete is the hole volume less the buried part of the post. Post section is taken as a square; entering a round post diameter gives close enough. Fence holes commonly run 8-12 in diameter and 24-36 in deep, going to the top of both in soft soil or high wind. In practice two 50 lb bags per post is the standard configuration for a 10 in hole, 30 in deep and a 4x4 post, and that is the order this tool lands on - if yours differs wildly, check whether depth was entered in feet. Put 2-3 in of gravel in the bottom of the hole for drainage; it is not counted here.
Next up: Concrete Volume Calculator · Concrete Mix Design Calculator
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Hole = π × (D/24)² × depth/12 | D in inches |
| Post = (side/12)² × depth/12 | the buried length only |
| Net = (hole − post) × (1 + waste) | |
| Bags = net ÷ bag yield | 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 ft³ |
How to work it out
- Measure the holes you will actually dig - diameter and depth, not a standard size you assume.
- Hole volume = π × (diameter/24)² × depth/12, where diameter in inches over 24 gives the radius in feet directly.
- Deduct the buried volume of the post (side squared for a square post).
- Add 10% waste and divide by bag yield; round up per post, then multiply by the count.
- Put 2-3 in of gravel in the bottom for drainage - that is not counted here.
Where it goes wrong
- Entering depth in feet - typing 30 for a 30 in hole makes it twelve times too big, and ten posts becomes hundreds of bags.
- Mixing fast-set concrete with water first - it is poured dry and wetted in the hole; mixing makes it lumpy and weaker.
- Leaving a flat undrained hole bottom - standing water sits against the concrete and frost heave lifts the post.
Material and cost
50 lb fast-set is about 6-9 a bag and a post takes 2-4, so 15-35 per post. Across a whole fence, add hole digging (labour or an auger rental at 50-100 a day), posts and concrete - a timber fence complete commonly runs USD 20-45 per linear foot. Digging is the slow part in hard or stony ground.
On site
- Set at least three posts before backfilling - one post alone has no reference and plumb is hard to judge. A string line saves the day.
- Dome the concrete away from the post so water runs off instead of pooling against it.
- Ask the building department for frost depth rather than working from experience - it varies with soil, not just region.
Basis
- IRC International Residential Code, Chapter R403 (footing and post embedment, frost depth)
- ACI 332 Residential Code Requirements for Structural Concrete
- ASTM C387 Standard Specification for Packaged, Dry, Combined Materials for Concrete and High Strength Mortar
FAQ
Why does this say 4 bags when two per post is the usual rule?
How deep should the post hole be?
Fast-setting or regular concrete?
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Fence Post Concrete Calculator. 2026-10-25. https://jianzhujisuan.com/en/fence-post-concrete