How many bags of concrete for a fence post?
For a common post hole size, 10 inches in diameter and 3 feet deep, you need about 3 bags of 80 lb concrete mix, 4 bags of 60 lb mix, or 6 bags of 40 lb mix, including a 10 percent waste factor and using the hole's true cylindrical volume.
If you instead use this site's concrete calculator, which treats a round hole as a square for simplicity, you will get a slightly higher, more conservative count of 4, 6, or 8 bags. Both methods are shown in full below.
How to measure a post hole
Measure the diameter of the hole in inches, straight across at its widest, and the depth in feet from the bottom of the hole to ground level. A common size for deck and fence posts is about 10 inches across and 3 feet deep, though your local building code and post size may call for something different.
A round hole is a cylinder, so its volume is pi × radius² × depth. Radius is half the diameter. This calculation treats the hole as empty and solid concrete; if you are setting a post in the middle of it, see the note on subtracting post volume below.
The full arithmetic (cylinder method)
| Hole diameter | 10 in |
| Radius | 10 ÷ 2 = 5 in = 0.4167 ft |
| Hole depth | 3 ft |
| Volume | π × 0.4167² × 3 = π × 0.1736 × 3 = 1.636 cu ft |
| Plus 10 percent waste | 1.636 × 1.10 = 1.800 cu ft |
| 40 lb bags (0.30 ft³ each) | 1.800 ÷ 0.30 = 6.00, round up to 6 bags |
| 60 lb bags (0.45 ft³ each) | 1.800 ÷ 0.45 = 4.00, round up to 4 bags |
| 80 lb bags (0.60 ft³ each) | 1.800 ÷ 0.60 = 3.00, round up to 3 bags |
A quicker, more conservative shortcut
This site's concrete calculator handles round footings and post holes by entering the diameter as both the length and width, treating the circle as its bounding square. That is faster to enter and always gives you at least as much material as the true cylinder, never less, which is why the site documents it as the safe default for the tool.
| Square side (the diameter) | 10 in = 0.8333 ft |
| Depth | 3 ft |
| Volume | 0.8333 × 0.8333 × 3 = 2.083 cu ft |
| Plus 10 percent waste | 2.083 × 1.10 = 2.292 cu ft |
| 40 lb bags | 2.292 ÷ 0.30 = 7.64, round up to 8 bags |
| 60 lb bags | 2.292 ÷ 0.45 = 5.09, round up to 6 bags |
| 80 lb bags | 2.292 ÷ 0.60 = 3.82, round up to 4 bags |
The bounding-square volume is exactly 4 ÷ π, or about 1.273 times the true cylinder volume, which is the geometric ratio between a square and the circle inscribed in it. That is about 27 percent extra material built into the shortcut on purpose, as a margin rather than a mistake.
Multiple posts
For several identical holes, multiply the volume before rounding to bags, not after, so you do not over-round on every hole individually. Three holes at 10 inches by 3 feet, cylinder method, with waste:
| Holes | Volume with waste | 40 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 1 | 1.800 cu ft | 6 | 4 | 3 |
| 3 | 5.400 cu ft | 18 | 12 | 9 |
The concrete calculator's "number of identical sections" field does exactly this, multiplying the per-hole volume by your count before converting to bags.
Should you subtract the post itself?
Strictly, the post occupies some of the hole, so the concrete you pour fills the gap around it, not the full cylinder. A nominal 4x4 post is actually about 3.5 by 3.5 inches, a cross-sectional area of about 12.25 square inches against a 10 inch hole's roughly 78.5 square inches, under 16 percent of the hole. Combined with a 10 percent waste factor already built in, most people round up and skip this step for a single post. For a large post, a cluster of posts, or a precise bulk order, measure the post's footprint and subtract its volume from the hole before dividing by bag yield.
Frequently asked questions
How many bags of concrete for one fence post hole?
For a common 10 inch diameter, 3 foot deep round post hole, with a 10 percent waste factor, you need 3 bags of 80 lb mix, 4 bags of 60 lb mix, or 6 bags of 40 lb mix, using the actual cylindrical volume of the hole.
How many bags of concrete do I need for 3 fence posts?
For three holes of the same 10 inch by 3 foot size, with 10 percent waste, you need 9 bags of 80 lb mix, 12 bags of 60 lb mix, or 18 bags of 40 lb mix, which is each single-hole count multiplied by three.
Why does the concrete calculator give a higher number than the cylinder math?
The concrete calculator's footing and column mode treats a round hole as a square with sides equal to the diameter, which is simpler to enter and deliberately overstates volume rather than understating it. For a 10 inch, 3 foot deep hole, that shortcut gives 4 bags of 80 lb mix instead of 3, about 27 percent more material, which is the known ratio between a circle and its bounding square.
Do I need to subtract the post's own volume from the hole?
For a single wood or metal post, the post itself usually takes up a small enough share of the hole that most people round up and ignore it, especially once a waste factor is already added. For a large post, several posts, or a precise order, measure the post's cross-sectional area and subtract that volume from the hole before converting to bags.
How deep should a fence post hole be?
A common rule of thumb is one third of the post's above-ground height, with a minimum of about 24 inches, and deeper still if your local frost line requires it. Check your local building code, since frost depth and fence height rules vary significantly by region.
Should I use 40, 60, or 80 lb bags for a post hole?
Any of the three works; they are the same mix in different package sizes. For a handful of post holes, 80 lb bags mean fewer bags to carry and mix. For many holes spread across a day, or if you are working alone, 40 or 60 lb bags are easier to lift and pour one at a time.