BuildCalc Guides · Concrete
How Many Bags of Concrete for a 4×4 Post?
The short answer is one bag of fast-setting concrete per typical line post — but corner posts, gate posts and frost-line posts all change the count. Here is the chart and the cylinder math behind it.
By hole size
Hole diameter and depth set the concrete volume — the post itself displaces a small amount (3.5 × 3.5 in) which is subtracted in the counts below.
| Hole | ft³ | 50 lb fast-set | 60 lb std | 80 lb std | Use |
|---|---|---|---|---|---|
| 9″ dia × 24″ deep | 0.71 | 1 | 2 | 2 | Standard line post, 6 ft fence |
| 10″ dia × 30″ deep | 1.09 | 2 | 3 | 2 | Heavier line post or 8 ft fence |
| 12″ dia × 30″ deep | 1.79 | 3 | 4 | 3 | Gate post or corner post |
| 12″ dia × 42″ deep | 2.42 | 4 | 6 | 5 | Deep-frost-line post |
Counts assume a 4×4 nominal post (3.5 × 3.5 in actual) and include normal waste. Round up.
The math
For a 9 inch diameter hole 24 inches deep with a 4×4 post inside:
- Hole volume = π × r² × h = π × 4.5² × 24 = 1,527 cubic inches = 0.88 ft³.
- Subtract the post = 3.5 × 3.5 × 24 = 294 cubic inches = 0.17 ft³.
- Net concrete = 0.88 − 0.17 = 0.71 ft³.
- Bag count. A 50 lb fast-set bag yields about 0.375 ft³, so 0.71 ÷ 0.375 = 1.9 → round to 2 if the hole runs full, but 1 bag covers the typical hole well because contractors leave a slight crown above grade.
Deck posts vs. fence posts
Deck post footings are bigger because they carry live load from above. A typical 6×6 deck post sits in a 12 inch diameter, 4 ft deep sonotube — that is 3.14 ft³ of concrete per footing, or about 6 bags of 80 lb. For a 4×4 deck post in a 10 inch sonotube at 4 ft, figure 4 bags of 80 lb.
Fence posts are pure bracing — they carry their own weight, the fence panels and wind load. Most local codes do not even require a permit, so the diameter and depth come from rule of thumb (3× post width, below frost line) not engineering tables.
Calculate for your own post
The chart above covers standard hole sizes. For any other diameter or depth — including sonotubes and bigger 6×6 deck-post footings — BuildCalc's round-column mode handles the cylinder math and subtracts the post displacement automatically.
Open the concrete calculator →FAQ
How deep should a 4x4 fence post be set?+
At least one-third of the above-grade post height, with 24 inches as a common minimum for a 6 ft fence. In freezing climates, set the bottom of the hole below the frost line — usually 36 to 48 inches in the upper US and Canada. The post needs to brace against frost heave, not just gravity.
How wide should the post hole be?+
About 3 times the post width. For a 4×4 post, that is 9 to 12 inches in diameter. Wider holes use more concrete but give better lateral resistance, which matters more for gate posts and corner posts than for line posts.
How many bags of concrete per fence post?+
For a 9 inch hole 24 inches deep: 1 bag of 50 lb fast-setting concrete (Quikrete or Sakrete), or 2 bags of 60 lb standard mix. For a 12 inch hole 30 inches deep (typical gate or corner post): 2 bags of fast-set, or 3 bags of 60 lb standard.
Should I use fast-setting or regular concrete?+
Fast-setting wins for fence posts — it sets in 20 to 40 minutes versus 24 hours, you can move on to the next post without bracing, and the bag is sized to one post hole. Use regular mix only if you are setting dozens of posts and want to pour them all in one batch from a mixer.
How much concrete do I need for 20 fence posts?+
Roughly 20 bags of 50 lb fast-setting concrete for a 6 ft fence (1 bag per post in a 9 in × 24 in hole), or about 40 bags of 60 lb standard concrete. Add 2 to 4 extra bags for corner and gate posts that take a wider, deeper hole.
Do you wet the mix or pour the bag in dry?+
Quikrete Fast-Setting Concrete is designed to pour in dry, then add water on top — that is the right method. Standard mix needs to be wet-mixed in a wheelbarrow or mixer before going in the hole. Read the bag label; dry-pour standard concrete is a common mistake that produces weak, crumbly footings.
Related guides
- How many bags of concrete for a fence post? — general fence-post overview, all post sizes.
- How many bags of concrete for a sonotube? — deck footings and column piers.
- How many bags of concrete for a 10×10 slab? — flat-slab math for a small patio.