Fence Post Concrete Calculator

You are standing at the box store deciding how many bags of concrete mix to put on the cart for a fence line. Guess low and you make a second trip mid-pour with concrete already setting in the mixer. Guess high and you store forty pounds of mix that will absorb humidity and go hard in the shed.

This calculator treats each hole as a cylinder, subtracts the volume the post itself takes up, multiplies by your post count, adds a waste allowance, and converts the result into bag counts for the three common bag sizes. It is a materials number, not a structural design — post depth, diameter, and whether concrete belongs in the hole at all are covered further down.

Hole diameter — the diameter your auger or digging bar actually produces, not the nominal bit size. A 9 in auger in loose sandy soil often leaves a 10–11 in hole; in tight clay it may cut close to size. Measure one finished hole before you commit to a bag count.

Hole depth — the depth of the concrete, measured from the bottom of the hole up to where you stop the pour. If you are putting 4 in of drainage gravel in the bottom and stopping the concrete 2 in below grade, the concrete depth is shorter than the hole depth.

Post size — the cross-section of the post, entered as a square dimension. A nominal 4×4 measures about 3.5 in actual; entering 4 slightly overstates displacement and gives you a small cushion. For round posts, enter roughly 0.9 × the post diameter to approximate equivalent area.

Number of posts — how many holes get concrete. If only your corners, ends, and gate posts are being set in concrete and line posts are being driven, enter only the concrete ones.

Waste % — covers spillage, belled hole bottoms, over-dug holes, and the bag you mix slightly wet and lose. 10% is a reasonable default on clean augered holes; 15–20% is realistic in rocky or root-filled ground where holes end up irregular.

Bag yields are the common published values for 50, 60, and 80 lb bags of standard concrete mix. Fast-setting and high-strength formulas yield differently — the number on your bag label overrides these.

• Hole volume: π × (10 ÷ 12 ÷ 2)² × (30 ÷ 12) = π × 0.4167² × 2.5 ≈ 1.364 cu ft
• Post displacement (4 in square): (4 ÷ 12)² × 2.5 ≈ 0.278 cu ft
• Concrete per hole: 1.364 − 0.278 ≈ 1.09 cu ft
• 20 posts: 1.09 × 20 ≈ 21.7 cu ft; with 10% waste ≈ 23.9 cu ft (0.88 cu yd)
• Bags: 64 × 50 lb, 54 × 60 lb, or 40 × 80 lb

That is roughly 3.2 bags of 80 lb mix per hole — a useful sanity check to carry in your head. If your own result lands far from 2–4 bags per hole for a typical 4×4 line post, re-check the diameter and depth inputs before you buy a pallet.

Above about 1 cubic yard total, price a small ready-mix delivery or a trailer mixer against bag mix. Mixing 60-plus bags by hand across a fence line is a full day of work and the pour quality drifts as you tire.

Concrete only, 4 in square post displaced, before waste. Multiply by your post count and add your waste allowance.

Notice how fast diameter drives cost: going from a 10 in to a 12 in hole at the same depth adds roughly 85% more concrete per post. Depth is the cheaper way to buy stability.

One hole size does not fit a fence line. The posts that carry tension and swing weight need materially more than the posts that just hold wire off the ground.

If you are mixing roles, run this calculator twice — once for the line posts, once for the corner, end, and gate posts — and add the results. Averaging them under-buys for the posts that matter most.

• Frost depth is local. A post footing that stops above the frost line can be lifted by freezing ground over successive winters. Frost depth varies enormously by region, and it is published by your local building department or extension service — not derivable from a calculator.
• Depth beats diameter for lateral resistance. When a post is being pushed sideways, embedment depth is the dominant variable. Widening a shallow hole is a poor substitute for digging deeper.
• Soft or saturated soil needs more embedment. Sandy, peaty, or chronically wet ground provides less lateral support than firm loam or clay for the same hole geometry.
• Rock changes the plan. If you hit ledge before target depth, an epoxied pin, a drilled-and-grouted socket, or a surface-mounted base plate is usually a better answer than a shallow concrete plug.
• Permits and setbacks exist. Height limits, setback distances, corner sight triangles, and utility easements are jurisdiction and deed matters. Confirm before you dig, and get utilities located every time.

Concrete around a post is not automatically the right answer, and for wood posts it is sometimes the thing that shortens the post's life.

• A concrete collar can trap water. Encase a wood post to grade with a flat or dished top and you have built a small basin that holds moisture against the post at the exact point where rot starts. Crown the top of the collar so water runs off and away.
• Gravel-set posts are common in agricultural fencing. Compacted crushed stone in lifts drains freely, still resists lateral movement well, and lets you pull and reset a post without jackhammering a footing out of the ground.
• Driven posts often outperform set posts in firm soil. Driving preserves the undisturbed soil around the post instead of replacing it with backfill. It is why so much livestock fence uses driven T-posts and driven wood line posts.
• Broken concrete is worse than no concrete. A cracked footing that moves as a unit wallows out a bigger hole each season. If a set post fails, address the footing, not just the post.
• Follow the manufacturer's method for engineered systems. Vinyl, composite, and steel post systems publish their own footing details, and those instructions govern over any general rule.

Where concrete clearly earns its place: gate hinge posts, corner and end assemblies, anything carrying real tension or swing weight, and posts in loose soil that will not hold a driven post.

• Dig and set all your posts to string line and depth before mixing anything. Concrete does not wait for you to fix alignment.
• Brace each post plumb in two directions and leave the braces until the mix has set — a post nudged at hour two stays crooked for twenty years.
• Mix stiff rather than soupy. Extra water is the easiest way to lose strength.
• A few inches of compacted gravel in the bottom of the hole gives water somewhere to go beneath a wood post.
• Slope the top of every collar away from the post, and keep the collar an inch or two above finished grade rather than below it.
• Cold and hot weather both change set behavior. Read the bag's temperature guidance instead of improvising.

• Sizing the whole fence off the line-post hole, then running out of mix at the gate.
• Using nominal 4 in for a post that actually measures 3.5 in and being surprised by leftover bags — a small, harmless error, but it compounds across 200 posts.
• Entering the full hole depth when the bottom 4 in is gravel and the top 2 in is soil backfill.
• Finishing the collar flat or dished so it holds water against a wood post.
• Assuming concrete substitutes for a proper brace assembly at ends and corners. It does not.
• Buying bags for 20 holes and digging 20 holes that turned out oval and belled in loose sand.
• Setting posts before locating buried utilities along the fence line.

Gate posts are a common failure point on rural property — if the gate is powered, the post also has to hold an opener's operating loads. Planning that side of the job is covered in the gate opener solar power hub.