Fence Post Concrete Calculator

Work out exactly how much concrete your fence-post holes need — as a precise volume, an “equivalent bags” figure, and a whole-bag shopping list that won’t leave you short mid-pour. This is a material quantity calculator: it does not price concrete, size your holes, or check structural adequacy.

How to use this calculator

1. Measurement units — pick feet / inches or metres / millimetres. You can switch at any time and your numbers convert automatically.
2. Post count — either enter the number of posts directly, or choose Calculate from fence length and spacing and give the run length and target spacing; the tool shows the resulting post count live.
3. Hole shape and size — round (enter diameter) or square (enter width), plus the total hole depth and an optional gravel base depth.
4. Post shape and size — round (diameter) or square (width). The calculator checks the post physically fits the hole, including the diagonal for a square post in a round hole.
5. Bag size — defaults to an 80 lb bag; pick another size to refill the yield, or choose Custom and type your product’s stated yield.

Press Calculate for the full breakdown.

The formula

For each hole, the concrete you need is the hole volume minus the part of that volume the buried post occupies, minus any gravel base:

A_hole

cross-sectional area of the hole

A_post

cross-sectional area of the buried post

d_fill

depth actually filled with concrete

d_hole

total hole depth

d_gravel

gravel base depth (0 if none)

The cross-sectional areas depend on shape:

w

diameter (round) or side width (square)

Then the whole project is simply the per-hole figure times the number of posts:

Turning volume into bags

The exact (“equivalent”) number of bags is the total volume divided by one bag’s yield:

The calculator’s Recommended purchase uses the first method. If you’re pouring a bag at a time per hole with no shared mixing, use the second:

• Mixed and poured together: round the project total up once — ceil(bagsequivalent). Leftover mix from one hole covers the start of the next, so this is the smaller, realistic shopping number.
• Whole bags per hole: round each hole up first, then multiply — ceil(Vconcrete ÷ Ybag) × nposts. Safer if you’re not sharing a mix between holes, at the cost of some extra material.

Fence length to post count

L_fence

total run length

s_target

maximum spacing between posts

Rounding the division up guarantees the last gap is never wider than your target, and the + 1 puts a post at each end of the run.

Worked example — round hole, 4x4 post

A 120 ft fence with posts every 8 ft. Holes are 10 in. diameter, 30 in. deep, with a 4 in. gravel base; posts are 4x4 (3.5 in. actual). Using 80 lb bags at ~0.6 ft³ yield:

Worked example — square hole, round post

A 60 ft run, posts every 6 ft, 12 in. square holes 36 in. deep, no gravel, 4 in. diameter round steel posts. Using 60 lb bags at ~0.45 ft³:

Common US bag sizes and yields

Yields are approximate cured-concrete volume. The common 40–80 lb presets follow QUIKRETE’s Concrete Mix technical data sheet, checked September 1, 2026. Product and mix design still matter, so check the figure printed on your own bag.

Typical hole and post dimensions

These are common starting points, not a structural specification. Frost depth, soil, fence height, and wind exposure all change what a safe hole actually needs — check your local building department.

A widely used starting rule is a hole diameter around three times the post width and an embedment around one-third to one-half of the above-ground height. It is not a structural specification; frost, soil, wind, and the fence system can require something different.

Unit conversions used

Why subtract the post?

A fence post doesn’t need a solid plug of concrete — it needs the annular gap between the post and the sides of the hole filled. Ignoring the post’s own volume overstates the concrete by the size of the post: for a 4x4 in a 10 in. hole that’s about 15% too much; for a 6x6 in a 12 in. hole, closer to 30%.

Why a gravel base helps

A few inches of compacted gravel or crushed stone under the post:

• keeps the end grain of a wood post out of standing water, slowing rot from below;
• gives water a path away from the post in freeze-thaw climates, reducing frost heave;
• slightly reduces the concrete volume, since it fills depth that would otherwise be concrete.

It’s optional, and some installers skip it in well-drained soil. Set the field to 0 if you’re not using one.

Common mistakes this calculator avoids

• Pricing the full hole volume instead of the concrete that actually goes in around the post.
• Assuming yield from bag weight. An 80 lb bag of one product and a 60 lb bag of another can yield nearly the same volume.
• Rounding equivalent bags down. 1.4 bags is not 1 bag — the calculator shows the true figure and a separate whole-bag purchase count.
• Forgetting the end posts. posts = spans + 1, not spans.

Worked planning scenario

Use the 120 ft example above: 16 posts, 10 in. round holes, 30 in. deep, with a 4 in. gravel base. The calculated total is about 16 ft³, or 26.6 equivalent 80 lb bags. That becomes 27 bags when the project volume is rounded up once, compared with 32 bags when every hole is rounded to two whole bags. These figures assume the dug holes match the dimensions entered. If roots, loose soil, or an uneven auger make some holes wider, measure the actual holes and recalculate rather than treating the fraction in the 27th bag as a general waste allowance.

Methodology

This tool computes material quantity only — the geometric volume of concrete implied by your dimensions, shown as an exact figure and a whole-bag purchase count rounded up so rounding can’t leave you short. It does not evaluate structural adequacy, frost-safe embedment, soil bearing, or code compliance; those depend on your site and local regulations. Choose (or confirm) your hole dimensions first, then use this calculator for materials.

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