A typical 4×4 fence post set in a 10 inch hole, 24 inches deep, needs about two 80 lb bags of concrete. Widen the hole to 12 inches or dig down to 30 inches and you need three bags. One 80 lb bag yields roughly 0.60 cubic feet, and most fence post holes hold 0.9 to 1.8 cubic feet.
The exact count depends on your post size and how big a hole you dig. You can get a number for your specific fence in under a minute with the free concrete calculator, or use the lookup table below. It covers the setups most people actually build.
One reason forum answers disagree so much: half of them quote the full hole volume without subtracting the post, and the rest assume a different hole size without saying so. The math itself is friendly.
Bags per post: the lookup table
Nearly every question about concrete for a 4×4 fence post comes down to hole size. The table assumes a round hole three times the post width, which is the standard rule, and it subtracts the space the post itself takes up, since concrete only fills the gap around it. Yields used: 0.30 cubic feet per 40 lb bag, 0.375 per 50 lb, 0.45 per 60 lb, and 0.60 per 80 lb.
| Post size | Hole (diameter x depth) | Concrete needed | 40 lb bags | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|---|---|
| 4×4 | 10 in x 24 in | 0.92 cu ft | 3.1 | 2.5 | 2.0 | 1.5 |
| 4×4 | 10 in x 30 in | 1.15 cu ft | 3.8 | 3.1 | 2.6 | 1.9 |
| 4×4 | 12 in x 24 in | 1.40 cu ft | 4.7 | 3.7 | 3.1 | 2.3 |
| 4×4 | 12 in x 30 in | 1.75 cu ft | 5.8 | 4.7 | 3.9 | 2.9 |
| 4×4 | 12 in x 36 in | 2.10 cu ft | 7.0 | 5.6 | 4.7 | 3.5 |
| 6×6 | 18 in x 36 in | 4.67 cu ft | 15.6 | 12.5 | 10.4 | 7.8 |
Always round up to whole bags when you buy. If you pour hole by hole with fast-setting bags, round up per post. If you mix batches in a wheelbarrow for several posts at once, total the volume first and round once at the end. On a long fence with 50 lb bags, that difference alone can save five or six bags. Leftover dry mix keeps for months anyway if you store it sealed and off the ground.
Where those numbers come from
Take one row from the table and run it by hand. Say you are building a 6 ft privacy fence with 4×4 posts. The holes are 12 inches wide and 30 inches deep.
- Hole volume. The radius is 6 inches, which is 0.5 ft. A cylinder is pi times radius squared times depth: 3.1416 x 0.5 x 0.5 x 2.5 = 1.96 cubic feet.
- Post volume. A 4×4 actually measures 3.5 x 3.5 inches, or 0.29 ft per side. So 0.29 x 0.29 x 2.5 = 0.21 cubic feet sits inside the hole.
- Concrete needed: 1.96 – 0.21 = 1.75 cubic feet per post.
- Bags: 1.75 / 0.60 = 2.9, so buy three 80 lb bags for each post.
That 0.60 figure is the 80 lb bag of concrete yield printed on the side of most bags. Bagged mix is ordinary concrete, cement pre-blended with sand and gravel, so the yield stays consistent between brands. The same subtract and divide method handles slabs and footings too, and our guide on how to calculate concrete needed covers those shapes.
Working in metric? A 12 inch hole is about 30 cm and 30 inches of depth is 76 cm. Readers in Pakistan and India will mostly find 50 kg bags of plain cement rather than pre-blended mix, which means batching your own with sand and aggregate. The classic 1:2:4 proportions in our concrete mix ratios guide work fine for posts.
How deep to set a fence post
Depth changes your bag count more than any other choice, so settle it before buying. The working rule: bury one third of the above-ground height, with 24 inches as the minimum. A 6 ft privacy fence wants 24 to 36 inches of depth. A 4 ft picket fence can get away with 24. Gate and corner posts carry extra load, so go deeper and wider there.
In cold climates, dig below the local frost line, which runs 36 to 48 inches across much of the northern United States. Freezing soil grips and lifts anything shallower, and a heaved post pulls the whole panel out of line. A fence is basically a sail. Wind load on a solid 6 ft panel is the reason a leaning fence is usually a shallow-post fence.
One money saver: put 4 to 6 inches of gravel in the bottom of the hole for drainage before the post goes in. In a 30 inch hole with 6 inches of gravel, you only pour 24 inches of concrete, so the 24 inch rows in the table apply. Your post lasts longer and you buy fewer bags. (Honestly, the drainage matters more than the savings. Wood posts rot from the bottom up.)
Fast-setting concrete per post, and the dry pour shortcut
Fast-setting mix exists for exactly this job. Brace the post plumb and pour the dry mix straight into the hole. Then add about one gallon of water per 50 lb bag and let it soak down through the mix. No mixing tub, no wheelbarrow. It sets in 20 to 40 minutes, and you can hang panels after about four hours.
Yield per bag is the same as standard mix, so the table above still works for fast-setting concrete per post. Standard mix costs less, usually by a dollar or two per bag, but it has to be fully mixed before it goes in the hole. My take: for anything under ten posts, pay the premium. Hand mixing gets old around bag four. One warning, though. The dry pour method is only approved for fast-setting products. Regular mix poured dry can cure with dry pockets in the middle, and you will not know until the post moves.
How many bags of concrete in a cubic yard
A cubic yard is 27 cubic feet. At 0.60 cubic feet per bag, that is 45 bags of 80 lb concrete per cubic yard. The same yard takes 60 bags at 60 lb or 90 bags at 40 lb.
This matters because bags stop making sense at scale. A 25 post fence with 12 inch holes at 30 inches deep needs about 44 cubic feet of concrete. That is 1.6 cubic yards, or 73 bags of 80 lb mix. Nearly two pallets, and over 5,800 lb of material to haul and lift. Somewhere past the one yard mark, price out ready mix delivery or rent a mixer instead. Bags run roughly $5 to $8 each in the US, so that pile costs somewhere between $370 and $580, plus a truck that can carry it.
Measure the hole, check the table, round up. For odd hole sizes or metric units, the concrete calculator gives you bag counts for every bag weight at once, free, with no signup.
Frequently asked questions
How many bags of concrete do I need for a 4×4 fence post?
A 4×4 fence post typically needs two to three 80 lb bags of concrete. A 10 inch hole at 24 inches deep holds about 0.92 cubic feet, which two 80 lb bags cover. A 12 inch hole at 30 inches deep holds 1.75 cubic feet, which needs three bags.
How much concrete does an 80 lb bag make?
An 80 lb bag of concrete yields about 0.60 cubic feet of mixed concrete. That fills a 10 inch wide hole to a depth of about 13 inches. A 60 lb bag makes 0.45 cubic feet and a 50 lb bag makes 0.375. A 40 lb bag makes 0.30. Check your bag label, since brands vary slightly.
Is one bag of concrete enough for a fence post?
One bag of concrete is rarely enough for a fence post. A single 80 lb bag yields 0.60 cubic feet, while even a small 10 by 24 inch hole holds about 0.92 cubic feet around a 4×4 post. One bag can work for light garden fencing in shallow holes, but plan on two or more for anything structural.
How deep should a fence post be set in the ground?
A fence post should be set at least 24 inches deep, or one third of the above-ground fence height, whichever is greater. A 6 ft privacy fence needs holes 24 to 36 inches deep. In cold climates, dig below the local frost line so freezing soil cannot push the post upward.
Do you have to mix fast-setting concrete before pouring it into the hole?
No, fast-setting concrete does not need mixing for fence posts. Pour the dry mix straight into the hole around the braced post. Add about one gallon of water per 50 lb bag and let it soak in. It sets in 20 to 40 minutes. Standard concrete mix, by contrast, must be fully mixed before pouring.
How many 80 lb bags of concrete are in a cubic yard?
There are 45 bags of 80 lb concrete in one cubic yard. A cubic yard is 27 cubic feet and each 80 lb bag yields 0.60 cubic feet, so 27 divided by 0.60 equals 45. For comparison, a cubic yard takes 60 bags at 60 lb or 90 bags at 40 lb.