How much concrete per fence post?

Hole diameter

3x post widthcommon rule

Buried depth

1/3 of height above gradecommon rule

80 lb bag yield

~0.60 ft³manufacturer TDS, varies

60 lb bag yield

~0.45 ft³manufacturer TDS, varies

50 lb bag yield

~0.375 ft³manufacturer TDS, varies

Frost depth

localset by your building department

Short version

• Hole diameter about 3× the post width; buried depth about 1/3 of the height above grade.
• A 6 ft fence uses an 8 ft post with 2 ft in the ground.
• Typical answer is 2 to 4 bags per hole, depending on hole size and bag weight.
• Frost depth beats every rule of thumb, and only your local building department sets it.
• Corner, end, and gate posts carry the load, so they get wider and deeper holes than line posts.

How big should the post hole be?

Size the hole at roughly three times the post width and one-third of the post’s above-ground height. Those two rules produce holes that resist the leverage a fence puts on a post, which is the actual failure mode — posts almost never fail by sinking, they fail by tipping.

Run the numbers on a standard job. A 6 ft privacy fence uses 8 ft posts, so 2 ft goes in the ground and 6 ft stands above. A nominal 4×4 measures 3.5 inches actual, so three times that is about 10.5 inches — which is why 10 and 12 inch augers are the common sizes. A 6×6 measures 5.5 inches actual and wants a proportionally larger hole.

Widen and deepen the ones that work hardest. Corner posts, end posts, and gate posts take lateral load from the fence sections pulling on them, and common practice is to set those deeper and in larger holes than line posts. A sagging gate is almost always a gate post that was set like a line post.

Dig the hole with straight or slightly belled sides, wider at the bottom than the top. A hole that tapers the other way lets frost lift the whole plug.

How many bags of concrete do you need per post?

Bag counts come straight from hole volume. The table below gives gross hole volume and whole bags rounded up, using the yields published on manufacturer technical data sheets — about 0.60 ft³ for 80 lb, 0.45 ft³ for 60 lb, and 0.375 ft³ for 50 lb. Formulations differ, so confirm the yield on the bag you actually buy.

Those figures ignore the volume the post itself takes up, which is deliberate. A 4×4 displaces roughly 15 to 20 percent of a typical hole, so the table carries built-in margin for the gravel base, an irregular hole, and the one post that always ends up deeper than planned.

To compute a size that is not listed: volume in cubic feet equals 0.7854 × (diameter in feet)² × (depth in feet). Divide by the bag yield and round up.

Buy one or two spare bags for a run of any length. Returning an unopened bag is easy; stopping mid-fence is not.

How does frost depth change the hole depth?

Frost depth overrides the one-third rule wherever the ground freezes. In a freezing climate the bottom of the post hole belongs below the local frost line, because water in the soil above that line expands when it freezes and jacks the post upward a little more each winter.

There is no national frost depth number, and any site that gives you one is guessing. Required depth is set by the authority having jurisdiction and varies enormously by county — it is effectively zero across the Gulf Coast and reaches several feet in the northern tier. Your local building department publishes the figure that applies to your address, and the ICC’s model codes are the framework local jurisdictions amend.

Call the building department before you dig, not after. The same call tells you whether a fence permit is required, how close to the property line you can build, and whether there are height limits.

Two details help in frost country regardless of depth. Put 4 to 6 inches of compacted gravel in the bottom of the hole so water drains away from the post base instead of pooling under it. And keep the hole sides from tapering wider at the top, which gives frost something to grab.

What kind of concrete works best for fence posts?

Either a standard concrete mix or a fast-setting mix works, and the choice comes down to how many posts you are setting and how fast you need them plumb. Both are sold as bagged products with the aggregate already proportioned.

Fast-setting mixes are designed to be poured into the hole dry and then watered per the bag directions, with no mixing container involved. That makes them the practical choice for a long fence run — you brace the post, dump the bag, add the stated amount of water, and move on. Set times are short enough that the post holds position within an hour on most products, though the printed set time is not the same as full strength.

Standard concrete mix has to be mixed to a workable consistency before it goes in the hole, which is slower but gives you more working time to plumb and align a run of posts against a string line. For gate posts and anything carrying real load, the extra control is worth it.

Follow the technical data sheet for water quantity either way. Fast-setting products in particular are formulated around a specific water amount, and adding extra to make the mix flow weakens it the same way it weakens any concrete.

How do you place the concrete around the post?

Set the post before the concrete, not after. Drop 4 to 6 inches of compacted gravel in the bottom of the hole, stand the post on it, then brace it plumb in two directions with stakes and scrap lumber before any concrete goes in.

Fill in lifts rather than all at once. Add concrete about halfway, rod it with a stick or rebar to drive out air pockets, check plumb again, then finish filling. Air pockets against the post are voids where water sits.

Crown the top. Bring the concrete an inch or two above grade and slope it away from the post on all sides so rain sheds instead of standing against the wood. A hole filled flush with grade collects water in a ring around the post, which is the single most common reason wood posts rot at the ground line.

Check plumb one more time before the concrete stiffens, and leave the braces on. Most products want the post left undisturbed while it sets, and fence panels or rails should wait until the concrete has cured enough to take the load — check the bag for the manufacturer’s stated wait time.

Common mistakes

• Digging a hole the same width as the post — the plug has no leverage against tipping, and the fence leans within a season.
• Ignoring the frost line — posts set above it heave a little each winter until the fence is visibly out of line and the gates stop latching.
• Filling flush with grade instead of crowning — water pools around the post and rots the wood at the ground line years early.
• Adding extra water to fast-setting mix — the product is formulated for a specific water amount, and more water means a weaker plug.
• Setting gate and corner posts like line posts — those posts take the lateral load, and undersized holes there show up as sagging gates.