Quick answer:for a standard 4×4 wood fence post in a 10-inch diameter by 30-inch deep hole, plan on 2 bags of 80lb pre-mixed concrete, or 3 bags of 60lb, or 4 bags of 40lb. Bigger posts or deeper holes need more. The math is always the same: hole volume minus post displacement, divided by the cured yield per bag.
Reference table by hole size
Bags needed per fence post for common hole dimensions, after subtracting post volume. Numbers rounded up; always buy at least one extra bag to cover spillage.
4×4 wood posts
| Hole size | 40lb bags | 60lb bags | 80lb bags |
|---|---|---|---|
| 8″ × 24″ | 2 | 2 | 1 |
| 10″ × 30″ | 4 | 3 | 2 |
| 12″ × 36″ | 6 | 5 | 3 |
| 12″ × 48″ | 8 | 6 | 4 |
6×6 wood posts
| Hole size | 40lb bags | 60lb bags | 80lb bags |
|---|---|---|---|
| 12″ × 30″ | 4 | 3 | 2 |
| 12″ × 36″ | 5 | 4 | 3 |
| 16″ × 42″ | 9 | 6 | 5 |
Round metal posts (3″ OD)
| Hole size | 40lb bags | 60lb bags | 80lb bags |
|---|---|---|---|
| 8″ × 24″ | 2 | 2 | 1 |
| 10″ × 30″ | 4 | 3 | 2 |
The math
Bag count = (hole volume − post volume) ÷ bag yield. All three numbers in cubic feet.
- Hole volume= π × (diameter ÷ 2) ² × depth (use consistent units, then convert to cubic feet)
- Post volume= post width² × depth in concrete (for square posts) or π × radius² × depth (for round)
- Bag yields (cured concrete):40lb ≈ 0.3 ft³, 60lb ≈ 0.45 ft³, 80lb ≈ 0.6 ft³
Example for a 4×4 post in a 10-inch diameter, 30-inch deep hole:
- Hole volume:π × (5/12)² × (30/12) = 1.36 ft³
- Post volume in concrete:(3.5/12)² × (30/12) = 0.21 ft³
- Concrete needed:1.36 − 0.21 = 1.15 ft³
- 80lb bags: 1.15 ÷ 0.6 = 1.9 → 2 bags
The Fence Post Calculator runs that math for the whole fence at once, including 3 to 6 inches of gravel at the bottom of each hole and a waste factor.
Practical tips
- Order one extra bag per 5 posts. Spillage, slightly oversized holes, and the bag at the end of a pallet that turns out partially set add up faster than people expect.
- Use the dry-set method for line posts (pour dry mix, pour water on top, walk away). It works for residential fence pickets and saves time. Wet-mix corner posts, gate posts, and terminal posts.
- Gravel under every post. 3 to 6 inches at the bottom of the hole, every time. The bag count above does not include the gravel volume because the gravel sits below the concrete pour.
- Crown the top. Slope the top of the concrete away from the post about 1/2 inch above grade so water runs off instead of sitting against the post.
Bag size strategy for a whole fence
Most homeowners buy 80lb bags because they cost less per cubic foot delivered and reduce the number of trips. Anyone setting more than 15 to 20 posts should buy in pallets, which most home centers and ready-mix yards will sell at a discount. For a 30-post fence with 10″ × 30″ holes, that is roughly:
- 60 bags of 80lb (2 per post) — one pickup load or a pallet
- 90 bags of 60lb (3 per post) — two pickup loads
- 120 bags of 40lb (4 per post) — three pickup loads
80lb bags are usually the right call unless you specifically need smaller bags for solo lifting.
Frequently asked questions
How many 80lb bags of concrete per 4×4 fence post?
Two bags of 80lb for a standard 10-inch by 30-inch hole. Bigger holes (12 inch by 36 inch) need 3 to 4.
How deep should a fence post hole be?
At least one-third the height of the above-ground post and below the local frost line. Go deeper if frost line depth requires it.
Do I need gravel under the fence post?
Yes — 3 to 6 inches of gravel at the bottom of every fence post hole. The gravel drains water away from the bottom of the post and prevents rot. Skipping it halves wood-post life in wet climates.
Can I dump dry concrete in the hole?
For line posts on residential fences, yes — dry-set works. Wet-mix corner posts, gate posts, and anything carrying load.
Related Calculators
- Fence Post Calculator — Post count + bag count for the whole fence in one shot
- Concrete Calculator — Cubic yards plus bag estimates for slabs and footings
- Gravel Calculator — The 3 to 6 inches of gravel under every post
Related Guide
For the rest of the install — calling 811, setting corner posts first, bracing diagonals, stringing line posts to a string — see how to install fence posts.
Written by Daniel McCarney — AceCalc