How Much Concrete Do You Need for Fence Posts?
Learn the exact amount of concrete needed per fence post based on post size, hole depth, and spacing. Includes calculator and pro tips.
Setting fence posts in concrete is the gold standard for stability—but how much do you actually need? This guide breaks down the math so you buy exactly what you need.
The Quick Answer
For a standard 4×4 fence post set in a 10-inch diameter hole that’s 24 inches deep:
- 1 bag (50lb) of fast-setting concrete per post
- Or about 0.4 cubic feet of standard concrete mix
For 6×6 posts or deeper holes, plan for 1.5–2 bags per post.
The Formula
Fence post holes are cylinders, so we use:
Volume = π × r² × depth
Where:
- r = hole radius (half the diameter)
- depth = hole depth minus the post depth
Example Calculation
For a 10” diameter hole, 24” deep, with a 4×4 post:
- Hole radius = 10” ÷ 2 = 5” = 0.417 feet
- Post takes up about 16 sq inches of the hole
- Remaining area = π × (5²) - 16 = 62.5 sq inches
- Volume = 62.5 × 24 = 1,500 cubic inches = 0.43 cubic feet
One 50lb bag of Quikrete yields 0.375 cubic feet, so one bag per post with a little extra.
Post Size Reference Chart
| Post Size | Hole Diameter | Hole Depth | Bags per Post |
|---|---|---|---|
| 4×4 | 10” | 24” | 1 bag |
| 4×4 | 12” | 30” | 1.5 bags |
| 6×6 | 12” | 30” | 1.5 bags |
| 6×6 | 14” | 36” | 2 bags |
Pro Tips
1. Don’t Fill to the Top
Leave 2–3 inches at the top for soil. This hides the concrete and allows water to drain away from the post.
2. Crown the Concrete
Slope the concrete away from the post so water runs off instead of pooling. Water pooling = rot.
3. Use Fast-Setting for DIY
Fast-setting concrete (like Quikrete Fast-Setting) lets you pour dry mix directly into the hole and add water. No mixing required. The post is set in 20–40 minutes.
4. The One-Third Rule
Your hole depth should be one-third of the total post length. A 6-foot fence with posts that extend 6 inches above the fence needs 8-foot posts buried 32 inches.
Total Project Calculation
For a 100-foot fence with posts every 8 feet:
- Posts needed: 100 ÷ 8 + 1 = 14 posts (don’t forget the end post!)
- Bags needed: 14 × 1 = 14 bags minimum
- With 10% buffer: 16 bags
At roughly $5 per 50lb bag, that’s $80 in concrete for the entire fence.
When to Skip Concrete
Not every post needs concrete:
- Dry, stable soil: Tamped gravel can work for interior fence posts
- Temporary fencing: Just tamp soil firmly
- Extremely rocky ground: Rocks can provide natural stability
But for gate posts, corner posts, and end posts—always use concrete. These carry the most stress.
Common Mistakes
- Holes too shallow: Posts will lean within a year
- No gravel base: Add 3” of gravel before concrete for drainage
- Touching the concrete: Wood-to-concrete contact accelerates rot; use a post base bracket for longevity
- Mixing too wet: Soupy concrete is weak concrete; follow package directions
Use our concrete calculator to dial in the exact amount for your specific hole dimensions.