Fence Post Concrete Calculator
A fence post concrete calculator subtracts the post from the hole: π × r² × depth − post² × depth. A 10 in hole 2 ft deep around a 4×4 needs 0.92 ft³.
How much concrete a fence post needs
A fence post concrete calculator finds the concrete that fills the gap between a round hole and a square post. Work out the hole as a cylinder, then take away the post. Concrete = π × (hole diameter ÷ 2)² × depth − post width² × depth, with everything in feet. A 10-inch hole 2 ft deep around a 4×4 post needs 0.92 ft³ of concrete.
The result is per post. Multiply by your post count, then divide by the yield of the bag you are buying.
The formula in plain words
- The hole. A round hole is a short cylinder. Its volume is the area of the circle (π × radius²) times the depth. The calculator converts the hole diameter from inches to feet for you.
- The post. A square post is a square column. Its volume is width × width × depth. A “4×4” is 3.5 in square, and a “6×6” is 5.5 in.
- The difference is the ring of concrete you pour.
The depth you enter should be the depth of concrete. If you put 4 to 6 inches of gravel at the bottom of the hole for drainage, subtract that from the hole depth.
Worked examples
- Standard privacy fence: 10 in hole, 2 ft deep, 4×4 post (3.5 in)
- Hole: π × (5 ÷ 12)² × 2 = 1.091 ft³
- Post: (3.5 ÷ 12)² × 2 = 0.170 ft³
- Concrete: 1.091 − 0.170 = 0.92 ft³ per post
- For 20 posts: 0.921 × 20 = 18.42 ft³ ÷ 0.6 = 30.7, so 31 bags of 80 lb
- Gate post: 12 in hole, 3 ft deep, 6×6 post (5.5 in)
- Hole: π × (6 ÷ 12)² × 3 = 2.356 ft³
- Post: (5.5 ÷ 12)² × 3 = 0.630 ft³
- Concrete: 1.73 ft³, or 2.88 bags of 80 lb, so 3 bags
Gate posts carry the swinging weight of the gate, so a bigger, deeper hole is normal there even when line posts are smaller.
Concrete per post and bags
| Hole | Depth | Post | ft³ per post | 80-lb bags | 60-lb bags |
|---|---|---|---|---|---|
| 8 in | 2 ft | 4×4 | 0.53 | 1 | 2 |
| 10 in | 2 ft | 4×4 | 0.92 | 2 | 3 |
| 10 in | 3 ft | 4×4 | 1.38 | 3 | 4 |
| 12 in | 3 ft | 4×4 | 2.10 | 4 | 5 |
| 12 in | 3 ft | 6×6 | 1.73 | 3 | 4 |
| 16 in | 4 ft | 6×6 | 4.75 | 8 | 11 |
Bag counts are per single post, rounded up, using 0.6 ft³ per 80-lb bag and 0.45 ft³ per 60-lb bag. For many posts, total the cubic feet first.
Sizing the hole
Common guidelines, which your fence plans or local code may override:
- Diameter: about three times the post width. That is roughly 10 to 12 inches for a 4×4.
- Depth: about one-third to one-half of the post’s above-ground height, and below the frost line in cold climates.
- Gravel base: a few inches of gravel under the post helps water drain away from the end grain.
The fence post depth calculator works through the depth for your fence height.
How to measure
- Measure the auger or digger width rather than guessing. That is your hole diameter.
- Measure the actual post, not its name. Rough-sawn and metal posts differ from dimensional lumber.
- Measure concrete depth from the top of any gravel to just below grade, where you will crown the top to shed water.
Common mistakes
- Using nominal post size. Entering 4 instead of 3.5 understates the concrete slightly.
- Bell-shaped holes. Holes that widen at the bottom take more concrete than a straight cylinder.
- Rounding per post instead of per job. Twenty posts at 1.54 bags each is 31 bags, not 40.
- Leaving a flat top. Slope the concrete away from the post so water does not pool against it.
Related calculators
Count posts first with the fence post calculator. The post hole concrete calculator handles holes without a post subtraction, and the concrete bag calculator converts totals to other bag sizes.
A general construction guideline. Local frost depth and code requirements vary. Confirm post depth requirements with your local building department.
Frequently asked questions
How much concrete do I need per fence post?
Work out the hole as a cylinder and subtract the post. A 10 in hole 2 ft deep holds 1.09 ft³; a 4×4 post (3.5 in actual) takes up 0.17 ft³; the concrete needed is 0.92 ft³. That is 1.54 bags of 80-lb mix, so buy 2 bags per post.
Why does the formula subtract the post's volume?
The post fills the middle of the hole, so concrete only goes in the ring around it. Leaving the post in the math would overstate the order by about 18% for a 4×4 in a 10 in hole.
How many bags of concrete per fence post?
Divide the cubic feet per post by the bag yield: about 0.6 ft³ for an 80-lb bag and 0.45 ft³ for a 60-lb bag. At 0.92 ft³ per post, that is 1.54 bags of 80 lb or 2.05 bags of 60 lb. Total the cubic feet for all posts before dividing, so partial bags are shared across holes.
Why does a wider hole need so much more concrete?
Hole volume grows with the square of the radius. Going from a 10 in to a 12 in hole at 3 ft deep raises the hole volume from 1.64 to 2.36 ft³, so the concrete around a 4×4 jumps from 1.38 to 2.10 ft³, about 52% more for 2 extra inches.
Does this work for round or square post holes?
It assumes a round hole, as dug by a post-hole digger or auger, and a square post. For a square hole, multiply hole width × hole width × depth instead of using π × r², then subtract the post.
Estimates only. Verify quantities with your supplier before purchasing.