How Many Bags of Concrete per Fence Post?
Bag counts for wood and steel fence posts by hole diameter and depth, with the volume of the post itself subtracted — and why you should round up the total, not each post.
Quick answer
A 4x4 post in a 10-inch hole, 24 inches deep, needs about 0.92 ft³ of concrete — 1.53 bags of 80 lb mix (buy 2 for one post). A 6x6 in a 16-inch hole at the same depth needs 2.37 ft³, or 4 bags. For a whole fence, multiply the per-post volume by the number of posts and round up once: 20 of those 4x4 posts take 31 bags (34 with 10% waste), not 40.
Formula
Post ft³ (square post) = (post width in ÷ 12)² × depth in ÷ 12
Concrete ft³ per post = hole ft³ − post ft³
80 lb bags = total ft³ ÷ 0.60 · 60 lb ÷ 0.45 · 40 lb ÷ 0.30
The hole volume is the same cylinder math our concrete column calculator uses; the bag yields are the ones in our concrete bag calculator. Most online post charts skip the subtraction step, which is why they often quote more concrete than the hole can hold.
Concrete per fence post: bag chart
| Post | Hole dia. | Depth | Hole ft³ | Post ft³ | Concrete ft³ | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|---|---|---|
| 4x4 wood (3.5 × 3.5 in) | 10 in | 24 in | 1.09 | 0.17 | 0.92 | 2 | 3 | 4 |
| 4x4 wood (3.5 × 3.5 in) | 10 in | 36 in | 1.64 | 0.26 | 1.38 | 3 | 4 | 5 |
| 4x4 wood (3.5 × 3.5 in) | 12 in | 24 in | 1.57 | 0.17 | 1.40 | 3 | 4 | 5 |
| 4x4 wood (3.5 × 3.5 in) | 12 in | 30 in | 1.96 | 0.21 | 1.75 | 3 | 4 | 6 |
| 4x4 wood (3.5 × 3.5 in) | 12 in | 36 in | 2.36 | 0.26 | 2.10 | 4 | 5 | 8 |
| 6x6 wood (5.5 × 5.5 in) | 16 in | 24 in | 2.79 | 0.42 | 2.37 | 4 | 6 | 8 |
| 6x6 wood (5.5 × 5.5 in) | 16 in | 36 in | 4.19 | 0.63 | 3.56 | 6 | 8 | 12 |
| 6x6 wood (5.5 × 5.5 in) | 18 in | 30 in | 4.42 | 0.53 | 3.89 | 7 | 9 | 13 |
| 6x6 wood (5.5 × 5.5 in) | 18 in | 36 in | 5.30 | 0.63 | 4.67 | 8 | 11 | 16 |
| 2-3/8 in round steel | 8 in | 24 in | 0.70 | 0.06 | 0.64 | 2 | 2 | 3 |
| 2-3/8 in round steel | 8 in | 36 in | 1.05 | 0.09 | 0.95 | 2 | 3 | 4 |
Worked example: a 20-post fence
Job: 20 wood 4x4 posts, 10-inch holes, 24 inches deep, 80 lb bags.
- Hole: π × (10 ÷ 24)² × 2 = 1.091 ft³
- Post: (3.5 ÷ 12)² × 2 = 0.170 ft³
- Concrete per post: 1.091 − 0.170 = 0.921 ft³ (1.53 bags)
- Whole fence: 0.921 × 20 = 18.41 ft³ → 18.41 ÷ 0.60 = 30.7 bags
- With 10% waste: 30.7 × 1.10 = 33.8 → 34 bags
Buy 34 bags of 80 lb mix. Rounding up per post (2 bags × 20) would mean 40 bags — six extra bags, about 480 lb you carry home and may not use.
The leftover from one post simply goes into the next hole, so per-post rounding only matters when you set a single post.
Choosing hole size and depth
- Diameter: a common rule of thumb is a hole about three times the post width — roughly 10–12 inches for a 4x4 and 16–18 inches for a 6x6. Wider holes add concrete fast: going from a 10-inch to a 12-inch hole raises a 4x4's concrete from 0.92 to 1.40 ft³ at 24 inches (+52%).
- Depth: a frequently quoted guideline is to bury about one-third of the post's above-ground height, and to go below the frost line where the ground freezes. Your local building code, fence height and wind exposure take precedence; for gate posts and tall privacy fences, ask your building department or fence supplier.
- Gravel at the bottom: many installers put a few inches of gravel under the post for drainage. Subtract that depth from the "Depth" column before using the chart. To price the gravel, use the gravel calculator.
Assumptions
- Round hole of uniform diameter; belled or tapered holes change the volume.
- Wood post sizes are actual dimensions (4x4 = 3.5 × 3.5 in, 6x6 = 5.5 × 5.5 in). Steel post is a 2-3/8 in outside-diameter round tube.
- Bag yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³ of mixed concrete. Check the yield printed on your bag.
- Fast-setting post mixes may yield a different volume per bag than standard concrete mix.
Planning the posts themselves? The fence calculator counts posts, rails and pickets for a run of fence.
FAQ
How many 80 lb bags of concrete per 4x4 fence post?
In a 10-inch-diameter hole 24 inches deep, the concrete around a 4x4 is about 0.92 ft³, so 1.53 bags of 80 lb mix. Buy 2 bags for a single post; for many posts, multiply 1.53 by the post count and round the total up.
Do you subtract the post when calculating fence post concrete?
Yes. The post fills part of the hole. A 4x4 (3.5 × 3.5 in actual) set 24 in deep displaces 0.17 ft³, about 16% of a 10-inch hole, so ignoring it overstates the concrete needed.
How many bags of concrete for 20 fence posts?
For 4x4 posts in 10-inch holes 24 inches deep: 20 × 0.92 ft³ = 18.4 ft³ = 30.7 bags of 80 lb mix, or 34 bags with 10% waste. Rounding up per post first would give 40 bags.
How many bags of concrete for a 6x6 post?
A 6x6 (5.5 in actual) in a 16-inch hole 24 inches deep needs about 2.37 ft³ of concrete: 4 bags of 80 lb mix. At 36 inches deep it is 3.56 ft³, or 6 bags.
Last updated: October 15, 2026.