How Much Quikrete for a Fence Post

Buy the right bag: Fast-Setting, not the standard mix

The product you want is Quikrete Fast-Setting Concrete Mix (No. 1004), the red bag. It is a blend of cement, sand, and gravel with an accelerator that makes it grab quickly, and it is built for exactly this job. You do not mix it in a wheelbarrow. You pour the dry powder straight into the hole around the post, then add water on top. Quikrete calls it the "no mixing" method, and it is the reason a fence goes up in an afternoon instead of a weekend (Quikrete, Setting Posts in Concrete).

Set time is the selling point. Fast-Setting reaches final set in 20 to 40 minutes, and under normal conditions you can hang weight on the post after about 4 hours (Quikrete data sheet). The standard gray-bag mix works too, but you have to mix it wet and it stays soupy for hours, so you will babysit braces most of the day. For posts, the red bag is the right tool.

Each 50 lb bag of Fast-Setting yields about 0.375 cubic feet of concrete. Hold that number, because every bag count below comes from it.

Size the hole first, then count bags

You cannot know how much Quikrete you need until you know how big the hole is, and hole size is not a guess. There are two rules the whole trade uses.

Hole depth = 1/3 to 1/2 of the above-ground post height, plus 6 inches.

That is Quikrete's wording, and the other published versions disagree with it. Quikrete's own data sheet says one third of the overall post height, and Sakrete says one third of the height above ground, which on a 6 foot fence give 32 and 24 inches against this rule's 30 to 42. Nobody publishes a derivation for any of them and no building code specifies a fence post depth at all. If the depth is what you are actually trying to settle, how deep should a fence post be works through the four conflicting rules, the one published standard in fencing, your frost line and your soil. The rest of this page assumes you have a hole and want to know how many red bags fill it.

A 6 foot fence uses posts around 8 feet long, so 6 feet stick up. One-third of 6 feet is 2 feet, plus 6 inches of gravel puts your hole around 30 inches deep. Go to half the height in loose soil, high wind, or on a gate post. The post below the frost line is what keeps a fence from heaving in winter. The fence post depth calculator works out the depth and hole width for your fence height, gate posts and frost line included.

Hole diameter = about 3 times the post width.

A 4x4 post is actually 3.5 inches across, so three times that is a little over 10 inches. Most people dig a 10 or 12 inch hole and call it right (HGTV). Wider is not wasted, it just means a bigger concrete collar and a few more bags.

Bags per post: the reference table

This is the table you came for. It is built for a 4x4 wood post using 50 lb Fast-Setting bags at 0.375 cubic feet each. It assumes 6 inches of gravel in the bottom of the hole and the concrete brought up to roughly 3 inches below grade, which is how these are actually set. Bag counts are rounded up to whole bags, because you cannot buy two-thirds of a bag.

The pattern matters, not just your row. Depth adds bags in a straight line, but diameter adds them fast, because concrete volume grows with the square of the radius. A 12 inch hole is not a little more than an 8 inch hole, it holds more than twice the concrete. So to keep the bag count down, dig deep and narrow rather than wide and shallow. For a 6x6 post the numbers roughly double, landing most people in the four to six bag range per hole.

A worked example

Say you are setting 4x4 posts for a 6 foot privacy fence in 10 inch holes, 30 inches deep, with 6 inches of gravel in the bottom.

• Concrete depth after gravel and a 3 inch soil cap: about 21 inches, or 1.75 feet
• Hole volume at that depth: 3.14 x (5/12 ft)² x 1.75 = 0.95 cubic feet
• Subtract the post itself (3.5 in square over that depth): about 0.15 cubic feet
• Concrete needed: about 0.81 cubic feet per hole
• Bags: 0.81 ÷ 0.375 = 2.15, so 3 bags rounded up

Now multiply by your post count. A 100 foot fence with posts every 8 feet is about 13 posts, so 13 x 3 = 39 bags. Buy a couple extra, since a split bag or a hole that caves a little wider will eat your cushion. If you would rather not run this by hand for every hole size, the Quikrete bag calculator does the volume and the bag count in one shot.

The dry-pour method, step by step

This is how Fast-Setting is meant to go in. No mixer, no wheelbarrow.

1. Dig the hole to your depth and diameter using a clamshell digger or an auger.
2. Add about 6 inches of gravel or crushed stone to the bottom and tamp it flat. Six inches is Quikrete's own figure, and Quikrete frames gravel as one of the "popular methods" of protecting posts rather than as a requirement. In free-draining soil it is cheap insurance. In clay it can do the opposite, because drainage only works if the water has somewhere to go, and the only authoritative comment on it is a Forest Products Laboratory warning that crushed rock can trap a pool of water around the base of a post in clay soils.
3. Set the post on the gravel, plumb it with a level on two adjacent faces, and brace it with scrap wood and stakes. Do this before any concrete goes in.
4. Pour the dry mix straight from the bag into the hole around the post, filling to about 3 to 4 inches below grade so soil can cap it later.
5. Add water on top, roughly a gallon per 50 lb bag, and let it soak down through the powder. Quikrete's instruction is to pour the water over the dry mix until it is saturated (Quikrete, Setting Posts in Concrete). Poke it with a stick or the end of a level to work water down and pop air pockets.
6. Check plumb one more time in the first few minutes while there is still a hair of movement, then leave it alone.

The mix sets hard in 20 to 40 minutes. Leave the braces on. Do not wet the concrete again or lean on the post while it takes its initial set. If you are shaky on how much water the powder wants or how the accelerator changes the timing, our Quikrete set, cure, and dry time guide breaks down what "set" versus "cured" actually means for the red bag.

Do you have to add water? The dry-set debate

There is a long-running argument online about skipping the water entirely and letting ground moisture cure the concrete. On the DIY Chatroom forum, one poster put the case for it plainly: "The procedure works fine and in fact it is an acceptable government standard. In fact you don't have to add any water and everything will still set up and harden over time" (DIY Chatroom).

The practical consensus in that thread is more careful. Bone-dry powder does eventually harden by pulling moisture from the soil, but it cures unevenly, stays crumbly near the top, and takes far longer to reach strength. The same poster who tried the fully dry method switched to adding water after poor results. The safe move is Quikrete's move: pour dry, then wet it, instead of gambling on how damp your dirt is.

How long before you can hang the gate or panels

Give it 4 hours before you put real load on a post under normal conditions, which is enough to hang a rail or a light panel (Quikrete data sheet). The concrete hits about 1,000 psi at 24 hours, so for anything that pulls hard, a heavy gate, tensioned wire, or a full run of privacy panels catching wind, wait a full day. Cold weather slows everything down, so add time below 50 degrees.

Do not confuse fast set with full cure. The post feels locked in an hour, but the concrete keeps gaining strength for days, and rushing a gate onto a green post is how you end up with a sag you cannot adjust out later.

Which post goes in the concrete

The bag is the easy half. What the post is treated with decides whether the fence outlives the concrete, and it is the one variable in this subject with real published evidence behind it.

A sawn fence post in the ground wants AWPA Use Category 4A, ground contact, general use. A picket or rail above the ground is UC3B, above ground, exposed. The Western Wood Preservers Institute publishes that mapping and USDA NRCS specifies exactly those two categories for exactly those two parts of a fence. Buying a UC3B post because it was on the same rack is the commonest expensive mistake in a fence build.

The service life figures make the case. Four independent datasets spanning 1960 to 2013 agree on the magnitude: untreated pine lasts about 3 years in the South and 3 to 6 in the Midwest and Northeast, while pressure-treated pine is reported at 30 to 40 years anywhere in the United States. Naturally durable species sit in between. Oregon State's long-running post farm recorded western juniper at 27 years, black locust at 24, redwood at 23 and western red cedar at 22 to 23, against Douglas-fir at 4 to 7.

On cedar specifically, since people ask whether cedar belongs in concrete: cedar heartwood is one of the durable species in that list, and nothing published says concrete treats it worse than treated pine. What is published is narrower and more interesting. The one peer-reviewed study on concrete and post decay found that cement contact raises the pH of the outer few millimeters of wood from roughly 5 or 6 up to 7.3 to 9.3, and that the effect on decay depends entirely on the preservative: better with one copper treatment, worse with another, no difference with a third. That study tested chemistry, not moisture, and no study has ever compared the service life of posts set in concrete against posts set in soil or gravel. The claim that concrete rots posts is, in the form everyone states it, unmeasured. Does concrete rot a wood post goes through what the research actually says.

Deeper posts and footings

Fence posts are the simple end of this. If you are setting a structural post for a deck or a pergola that has to carry weight, you are into footing territory, where hole size and concrete volume matter to the load. Our concrete footing calculator sizes those properly. For everything else about the red and gray bags, the Quikrete guide hub collects it, and the how to mix Quikrete guide covers the wet-mix method for the jobs where dry-pour will not do.

Two pages carry the parts this one does not. To size the concrete for your own holes and for the whole fence, including the post count, use the fence post concrete calculator. For how deep and how wide the hole has to be in the first place, what your frost line does to it, whether gravel helps and whether concrete really rots a post, see how deep should a fence post be.