Fence Post Concrete Calculator
Find out how many bags of concrete you need to set fence posts, mailbox posts or pergola posts. Enter the hole size, the post size and the number of posts: the calculator subtracts the space the post takes up, so you don't overbuy.
Safety: Post holes must reach below your local frost line where the ground freezes; check your local code for the required depth.
How to use the fence post concrete calculator
- Keep Post holes selected. Enter the hole diameter and depth in inches. QUIKRETE suggests a hole about 3× the post width and about ⅓ of the post's total length deep.
- Enter the post width: 3.5 in for a 4×4, 5.5 in for a 6×6. Enter 0 for a round hole with no post (a pier).
- If you put gravel at the bottom of the hole for drainage, enter its depth; that part isn't filled with concrete.
- Set the number of posts and read the bags. The table below gives bags per post for common post and hole sizes.
How much concrete goes around a post
The concrete fills the hole minus the post. The hole is a cylinder and a square post is a box, so:
Concrete (ft³) = π × (hole diameter ÷ 24)² × depth − (post width ÷ 12)² × depth
with the depth in feet (minus any gravel at the bottom). Multiply by the number of posts, add waste, and divide by the bag yield: about 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb (QUIKRETE, Sakrete).
Worked example: ten 4×4 posts in 10 × 36 in holes
This is the setup loaded above, typical for a 6 ft privacy fence.
- Hole: π × (10 ÷ 24)² × 3 = 1.636 ft³.
- Post: (3.5 ÷ 12)² × 3 = 0.255 ft³.
- Concrete per post: 1.636 − 0.255 = 1.38 ft³, or 2.3 bags of 80 lb mix.
- Ten posts with 10% waste: 15.19 ft³ (0.56 yd³), which is 26 bags of 80 lb mix, 34 of 60 lb or 51 of 40 lb.
Buying for all posts together saves bags: rounded up post by post, ten posts would take 30 bags, but the shared total is 26 because part-bags carry over to the next hole.
How big and deep should the hole be?
QUIKRETE's fence-post method is a good starting point: make the hole about 3 times the width of the post (about 10–12 in for a 4×4) and about one third of the post's total length deep. A 6 ft fence on 9 ft posts needs holes about 3 ft deep.
That rule is the minimum. Where the ground freezes, the bottom of the hole must be below the frost line, or the post can heave out over a few winters. Gate posts carry more load, so many builders use a bigger post and a wider, deeper hole there. Always call 811 to have underground utilities marked before you dig.
Concrete per fence post by post size and hole size
| Post | Hole (diameter × depth) | Concrete per post | 80 lb bags per post | 60 lb bags per post | 80 lb bags for 10 posts (+10%) |
|---|---|---|---|---|---|
| 4×4 | 8 × 24 in | 0.53 ft³ | 0.9 | 1.2 | 10 |
| 4×4 | 10 × 24 in | 0.92 ft³ | 1.5 | 2 | 17 |
| 4×4 | 10 × 30 in | 1.15 ft³ | 1.9 | 2.6 | 22 |
| 4×4 | 10 × 36 in | 1.38 ft³ | 2.3 | 3.1 | 26 |
| 4×4 | 12 × 36 in | 2.1 ft³ | 3.5 | 4.7 | 39 |
| 4×4 | 12 × 42 in | 2.45 ft³ | 4.1 | 5.4 | 45 |
| 6×6 | 12 × 36 in | 1.73 ft³ | 2.9 | 3.8 | 32 |
| 6×6 | 16 × 42 in | 4.15 ft³ | 6.9 | 9.2 | 77 |
| 6×6 | 18 × 48 in | 6.23 ft³ | 10.4 | 13.8 | 115 |
Post widths are actual sizes (4×4 = 3.5 in, 6×6 = 5.5 in). Per-post figures have no waste and are not rounded, so you can see how much of the last bag is used.
Setting posts in concrete
- Bell the bottom, not the top. A hole that is a little wider at the bottom resists frost heave; one that flares at the top acts as a lever for frost to lift.
- Crown the top. Slope the concrete away from the post at ground level so rain runs off instead of pooling against the wood.
- Fast-setting mixes have their own yield. Quick-setting post mixes come in different bag weights and yields; check the bag and adjust the count.
- Brace before the concrete sets. Plumb each post on two faces and brace it, and run a string line along the tops so the fence stays straight.
- Don't skip the waste allowance. Holes dug with a post-hole digger are rarely perfect cylinders and often end up wider than planned.
Frequently asked questions
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10 in × 36 in hole, about 2.3 bags of 80 lb mix (1.38 ft³). In practice, buy 3 bags for a single post, or work out the total for all posts at once: 10 posts need 26 bags with 10% waste.
How many bags of concrete for a 6×6 post?
A 6×6 post in a 12 in × 36 in hole needs about 1.73 ft³ of concrete, or 2.9 bags of 80 lb mix per post before waste. In a 16 in × 42 in hole it is about 6.9 bags.
How deep should fence post holes be?
QUIKRETE suggests about one third of the post's total length, so a 6 ft fence on 9 ft posts needs 3 ft deep holes. Go below your local frost line where it is deeper, and follow local code.
How wide should a fence post hole be?
About 3 times the post width: roughly 10–12 in for a 4×4 (3.5 in) and 16–18 in for a 6×6 (5.5 in).
Should I put gravel at the bottom of the post hole?
Many installers add a few inches of gravel at the bottom for drainage. If you do, enter its depth under Gravel at the bottom; the calculator then leaves that part out of the concrete.
How much concrete do I need for 20 fence posts?
With 4×4 posts in 10 × 36 in holes and 10% waste, about 30.4 ft³ (1.13 yd³), which is 51 bags of 80 lb mix. At that size, compare the bag price with a small ready-mix load.
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