Fence Post Concrete Calculator
Work out how much concrete and how many bags your fence posts need, with post displacement, a gravel base, and waste already accounted for.
- Concrete depth (per hole)
- —
- Hole volume (per hole)
- —
- Post displacement (per hole)
- —
- Net concrete (per hole)
- —
- Total concrete (with waste)
- —
- Bags needed
- —
- Estimated materials cost
- —
Not a structural specification
Planning the full fence project?
This estimate covers concrete for the holes you entered. Add your location, fence type, terrain, and installation preference for a full project-specific cost range.
Plan My Full Fence ProjectHow much concrete goes around a fence post?
A fence post doesn’t need a solid cylinder of concrete — it needs enough to fill the gap between the post and the sides of the hole, from the bottom of the hole up to grade (or to where you stop the pour). That gap is smaller than the full hole volume, because the post itself displaces some of it.
How the calculator accounts for post displacement
The calculator starts with the gross volume of the hole (a cylinder for a round hole, a rectangular prism for a square one), then subtracts the volume of the buried post over the same depth. What’s left is the actual concrete you need to buy — not the full hole volume, which would overstate the true amount of concrete for fence posts by the size of the post itself.
How bag yield affects the answer
Bag yield — how much cured concrete volume one bag produces — varies by product, not just by weight. An 80 lb bag of one brand and a 60 lb bag of another can yield surprisingly close volumes depending on the mix design. That’s why this calculator lets you pick a common bag size as a starting point, but always lets you enter your product’s actual stated yield.
Bags are rounded up per hole, not just once for the whole project. That costs a little extra material on paper, but it means you won’t come up short mixing one hole at a time.
Fence length to post count
If you know your total run but not your post count, tell the calculator your fence length and target post spacing (commonly 6–8 ft for wood fence panels). It places a post at both ends and at every full span in between, rounding up so the last gap is never longer than your target spacing.
Worked example
A 120 ft fence run with posts every 8 ft needs 16 posts. For 10 in. diameter, 30 in. deep round holes with a 4 in. gravel base and 3.5 in. (4x4 nominal) square posts:
- Concrete depth per hole: 26 in. (30 in. total depth minus the 4 in. gravel base)
- Concrete needed per hole, with 10% waste: about 1.1 ft³
- Bags needed per hole (80 lb bag, ~0.6 ft³ yield): 2
- Total for the project: about 17.6 ft³, or 32 bags
Why a gravel base
A few inches of compacted gravel under the post keeps the post’s end off standing water, which is one of the more common causes of a wood post rotting from the bottom up. It also reduces the concrete volume slightly, since it takes up part of the hole depth that would otherwise need concrete.
Common bag sizes in the US
Ready-mix concrete for fence posts is typically sold in 40 lb, 60 lb, and 80 lb bags, yielding roughly 0.3, 0.45, and 0.6 cubic feet respectively — these are approximate, and vary by brand and mix design, so check the yield printed on your specific bag.
Methodology
This tool calculates material quantity: the volume of concrete implied by your hole and post dimensions, converted to bags using the yield you enter, with a waste allowance applied on top. It does not calculate structural adequacy — hole depth, diameter, frost line, soil type, fence height, and wind exposure all affect what a safe design actually needs, and those depend on your local building code and site conditions. Choose your hole dimensions first (or confirm them with a local contractor or code office), then use this calculator to plan materials.
FAQs
Is this the same as a post hole concrete calculator?
Yes. Whether you search fence post concrete calculator or post hole concrete calculator, you're asking the same question — how many 40, 60, or 80 lb bags a hole needs. This tool answers both.
How many bags of concrete per fence post?
It depends on hole size, post size, and bag yield. For a common 10 in. diameter, 30 in. deep hole with a 4x4 post, most 80 lb bags land around 2 bags per hole once waste is included — but always check your own numbers, since hole size varies a lot by fence height and soil.
Do I need a gravel base under a fence post?
It's common practice in the US to add 2–4 in. of gravel at the bottom of the hole for drainage, which helps reduce wood rot and frost-related movement. It's optional in the calculator — set it to 0 if you're not using one.
What size hole do I need for a fence post?
That depends on your post size, fence height, wind exposure, and local frost depth — this calculator doesn't set that for you. A common rule of thumb is a hole diameter about 3 times the post width and a depth of roughly one-third to one-half the post's above-ground height, but confirm against your local building code first.