Fence Post Concrete Calculator: Bags per Post Hole
A 4×4 fence post in a 12 in hole 2 ft deep needs 1.40 cubic feet of concrete, which is 5 bags of 50 lb mix or 3 bags of 80 lb with 10% extra.
Advanced options: waste, prices
NRMCA advises ordering 4 to 10 percent over the estimate.
Concrete needed (with 10% extra)
0.06 yd³
0.04 m³ · 1.40 ft³ before extra
Post hole: 3.5 in × 12 in × 24 in. Small pour: premix bags make sense.
Premix bags
- 80 lb3
- 60 lb4
- 50 lb5
- 40 lb6
- Ready-mix cost
- $9
- Weight
- 210 lb
Full breakdown
| Volume before extra | 0.05 yd³ · 0.04 m³ |
|---|---|
| With 10% extra | 0.06 yd³ |
| Weight (150 lb/ft³) | 210 lb · 0.11 tons |
| Pallets of 40 lb (80 bags each) | 1 |
| Pallets of 60 lb (56 bags each) | 1 |
| Pallets of 80 lb (42 bags each) | 1 |
| Truckloads at 12 yd³ | 1 |
| Truckloads at 8 yd³ | 1 |
Bag counts round up, as Quikrete's own calculator does. Cost uses your prices; delivery and short-load fees are extra.
Key takeaways
- A 4×4 post in a 12 in hole 2 ft deep takes 3 bags of 80 lb or 5 of 50 lb.
- Hole depth ≈ ⅓ of the post height above ground, 6 in below frost (Sakrete).
- Hole width ≈ 3× the post width (Quikrete).
- Dry-pour only with fast-setting concrete (Sakrete).
How Much Concrete Do I Need for a Fence Post?
You need the hole's volume minus the post's volume. A 4×4 post (3.5 in actual) in a 12 in hole 24 in deep needs 1.40 ft³ of concrete, which is 3 bags of 80 lb or 5 bags of 50 lb with 10% extra.
Fence Post Concrete Formula
ft³ = [3.1416 × (Hole diameter (in) ÷ 24)² − Post area (ft²)] × Depth (ft)
Square post area = (Post width (in) ÷ 12)²Quikrete's own calculator uses the same approach: it subtracts the post from a hole three times the post's width.
How Many Bags of Concrete per Fence Post?
Bags per fence post, with 10% extra and rounded up, are listed for common posts.
| Post and hole | 80 lb | 60 lb | 50 lb | 40 lb |
|---|---|---|---|---|
| 4×4 post, 12 in hole, 2 ft deep | 3 | 4 | 5 | 6 |
| 6×6 post, 18 in hole, 3 ft deep | 9 | 12 | 14 | 18 |
| 2⅜ in metal post, 8 in hole, 2 ft deep | 2 | 2 | 2 | 3 |
How Deep Should a Fence Post Be?
A fence post hole should be about one third of the post's height above ground, and about 6 in below the local frost line, according to Sakrete's post-setting guide. A 6 ft fence therefore needs holes about 2 ft deep, deeper where the frost line is lower.
How Big Should a Fence Post Hole Be?
A fence post hole should be about three times the post's width, the rule built into Quikrete's calculator. A 4×4 post measures 3.5 in actual, so its hole is about 10 to 12 in across.
Fast-Setting vs Regular Concrete for Fence Posts
Fast-setting concrete can go into a post hole dry, and regular concrete must be mixed first. Sakrete's guide allows the no-mix method only with fast-setting concrete: pour water into the hole first, then the dry mix, because water added on top of dry mix hydrates unevenly and lowers strength. Regular concrete mix is mixed, poured around the post and braced until it sets.
Gravel at the Bottom of a Post Hole
Sakrete advises a 4 to 6 in layer of gravel at the bottom of the hole for drainage, compacted with the end of the post, to help prevent wood rot. Reduce the hole depth in the calculator by the gravel depth to get the concrete only.
Concrete for Metal, Vinyl and Mailbox Posts
Metal, vinyl and mailbox posts use the same hole-minus-post formula. Choose "Round" as the post shape for round metal posts; pergola posts that carry more load are sized with the concrete footing calculator.
Can You Dry Pour Concrete for Fence Posts?
Yes, but only with fast-setting concrete, according to Sakrete: add water to the hole first, pour the dry mix evenly around the post, and add water if the top stays dry.
How Much Does Concrete for a Fence Cost?
Fence concrete costs the number of posts × bags per post × your bag price. Set the post count and your bag price in the calculator to get the total.
Worked Examples: Whole Fences
6 ft privacy fence, 20 posts (4×4), 12 in holes 2 ft deep. 20 × 1.40 = 28 ft³; with 10% extra, 52 bags of 80 lb mix, or 83 bags of 50 lb.
Gate posts, 2 posts (6×6), 18 in holes 3 ft deep. 2 × 4.67 ft³; with 10% extra, 18 bags of 80 lb mix.
Chain-link, 15 round posts 2⅜ in, 8 in holes 2 ft deep. 15 × 0.64 ft³; with 10% extra, 18 bags of 80 lb mix.
Is 2 Feet Deep Enough for Fence Posts?
Two feet is enough for a fence about 6 ft tall under Sakrete's rule of one third of the above-ground height, as long as the hole also reaches about 6 in below the local frost line.
How Much Gravel per Fence Post?
A 4 to 6 in gravel layer, as Sakrete advises, takes 0.33 ft³ in a 12 in hole at 5 in deep, about one 0.5 ft³ bag of gravel per post.
Fence Post Concrete Calculator FAQ
How many 50 lb bags of concrete for a 4×4 post?
A 4×4 post in a 12 in hole 2 ft deep takes 5 bags of 50 lb concrete with 10% extra; a 3 ft deep hole takes 7.
Is it cheaper to buy bags of concrete or mix your own?
For small jobs premix bags are simplest; mixing your own from cement, sand and gravel needs about 7 bags of 94 lb cement plus aggregate per yard, and above about 1 yd³ ready-mix is usually cheapest. See bags vs ready-mix calculator.
Next step
Count posts and total the bags with the calculator's post quantity; for heavy gate or pergola posts, size a round footing with the concrete footing calculator.
Official sources
Last reviewed 2026-11-05. Numbers come from our tested engine; see how we calculate and our sources.