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Redmoon Converters
🔨 Construction & DIY

Fence Post Concrete & Spacing Calculator

Calculate how many fence posts you need, the concrete volume per hole, and total bags of concrete mix for any fence length and post spacing.

Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on

Subtracted from the hole — the post displaces concrete.
Concrete bags needed
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Posts needed: — · Total concrete cost: —
MetricValue
Quick reference

Number of posts = ceil(fence length ÷ post spacing) + 1 — a fence always has one more post than the number of spans. Each hole is a cylinder, so its volume is π × (diameter/2)² × depth ÷ 1728 (cubic inches to cubic feet). The post itself sits in that hole and displaces concrete, so its cross-section is subtracted before the depth is applied — a 4×4 (3.5 in actual) in a 10 in hole takes up about 16% of it, and ignoring that overstates every bag count by the same margin. Multiply by the post count for total concrete volume, then divide by the bag yield and round up.

Hole depth should be roughly 1/3 to 1/2 of the post's total length, or below the local frost line in cold climates — whichever is deeper. This calculator only handles the volume and bag math; check your post height and local frost depth separately before setting the depth here.

How it works

Two calculations sit behind the result. Post count is ceil(fence length ÷ post spacing) + 1, because a fence always carries one more post than it has spans. Each hole is treated as a cylinder: π × (diameter ÷ 2)² × depth in cubic inches, divided by 1,728 to reach cubic feet. That per-hole volume is multiplied by the post count, divided by your chosen bag yield and rounded up to whole bags, then multiplied by the price per bag.

Hole diameter is the strongest lever because volume scales with its square: widening a hole from 10 to 12 inches raises the concrete needed by 44 percent, while depth only scales linearly. The bag select carries real yields — 40 lb at 0.30 ft³, 50 lb at 0.375, 60 lb at 0.45, 80 lb at 0.60 — and defaults to the 50 lb bag. On the defaults, a 100 foot fence at 8 foot spacing needs 14 posts and 41 bags.

The cylinder volume is the whole hole; the post standing in it is not subtracted. For a 4×4 in a 10 inch hole that is about 16 percent of the volume, which conveniently absorbs spillage but does mean you are over-ordering slightly on wide posts. There is no bell or flare at the base, no gravel drainage layer, no extra allowance for gate and corner posts that usually want deeper holes, and no metric mode — lengths are feet and hole dimensions inches.

Frequently asked questions

How many bags of concrete does one fence post take?

For the default 10 inch wide, 24 inch deep hole, the volume is 1.09 cubic feet. That is about three 50 lb bags per hole, or two 80 lb bags. Wider or deeper holes climb quickly, since diameter enters the formula squared.

How deep should a fence post hole be?

The usual rule is a third to a half of the post's total length, or below the local frost line in cold climates, whichever is deeper. This tool only handles the volume and bag arithmetic, so decide the depth from your post height and frost depth first, then enter it.

Why does the calculator add one extra post?

Because posts and spans are not the same count. A 100 foot fence at 8 foot spacing has 13 spans but needs 14 posts, one at each end of every span with the ends shared. The formula rounds the span count up, then adds one.

Does it subtract the volume the post itself occupies?

No, it costs the full cylinder. A 4x4 post in a 10 inch hole displaces roughly 16 percent of the concrete, so the figure runs a little high. Most people treat that as useful slack for spillage rather than deducting it.

Can I enter the fence length in metres?

Not currently. The fields are fixed units: total fence length and post spacing in feet, hole diameter and depth in inches, and bag yields in cubic feet. Convert before entering, using 1 metre equals 3.281 feet and 1 centimetre equals 0.394 inches.

The hole, minus the post

Concrete per post is the hole volume less the volume the post occupies, which on a narrow hole is a meaningful deduction rather than a rounding detail. The conventional sizing is a hole about three times the post width and a depth of roughly a third of the post's above-ground height.

That depth rule is about leverage, not about holding weight. A fence post fails by rotating in the ground under wind load on the panel, and depth is what resists that.

Drainage, frost and the detail that decides lifespan

A concrete collar that fully encases the post base creates a bowl that holds water against the timber, which is the most common reason posts rot at ground level. Gravel beneath the post for drainage, and a collar sloped away from the post at the top so water runs off, both matter more to lifespan than the concrete volume does.

In climates with frost, the hole should extend below the frost line or freeze-thaw will lift the post over successive winters regardless of how much concrete is in it.

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