Concrete per Fence Post Calculator
This calculator answers how many bags of concrete per fence post you need, in 80-lb, 60-lb, and 50-lb mix, for one post and for your whole fence. Enter your post size, hole diameter, and depth below to get an exact bag count before you buy concrete.
10 in diameter x 24 in deep hole
| Bag Size | Total Bags |
|---|---|
| 80 lb | 2 |
| 60 lb | 3 |
| 50 lb | 3 |
| 40 lb | 4 |
Total concrete: 0.92 cu ft.
A concrete per fence post calculator answers how many bags of concrete per fence post before you're short a bag halfway through a Saturday of setting posts. The math is a hollow-cylinder volume: a round hole minus the square post sitting inside it, multiplied by depth. This tool runs that formula for the whole fence calculator post count at once, and matches the same volume math used by slabcalc's fence post concrete calculator.
How Many Bags of Concrete per Fence Post
- Hole diameter runs about 3 times the post's actual width, rounded to an even inch — 10 inches for a 4×4 post, 12 inches for a 6×6 or vinyl 5×5.
- Hole depth equals the bury depth from your post length calculation — 24 inches (2 feet) for most fences up to 6 feet tall, deeper for taller fences or a deep frost line.
- Concrete volume = area of the round hole minus area of the square post, times depth: π × (diameter ÷ 24)² × depth − (post width ÷ 12)² × depth, in cubic feet.
- Bags per post = volume ÷ bag yield, rounded up — 0.60 cu ft per 80-lb bag, 0.45 for 60-lb, 0.375 for 50-lb.
- Total bags = bags per post × number of posts.
Worked example — 4×4 post, 10-inch hole, 24-inch (2-ft) deep: volume = π × (10 ÷ 24)² × 2 − (3.5 ÷ 12)² × 2 = 0.92 cubic feet, which needs 2 bags of 80-lb mix (0.92 ÷ 0.60 = 1.53, rounded up). A 100-ft fence with 15 posts (13 line, 2 gate) needs 2 bags per post either way — the 6×6 gate posts round to the same 2 bags at this depth despite their larger 12-inch hole — for 30 bags of 80-lb concrete total.
Concrete per Post Chart by Post Size and Height
Bags needed per post at the hole size and depth matched to each post type and fence height.
| Post | Height | Hole | Depth | Volume | 80-lb bags | 60-lb bags | 50-lb bags |
|---|---|---|---|---|---|---|---|
| 4×4 | 4–6 ft | 10 in | 24 in | 0.92 cu ft | 2 | 3 | 3 |
| 4×4 | 8 ft | 10 in | 36 in | 1.38 cu ft | 3 | 4 | 4 |
| 6×6 | 4–6 ft | 12 in | 24 in | 1.15 cu ft | 2 | 3 | 4 |
| 6×6 | 8 ft | 12 in | 36 in | 1.73 cu ft | 3 | 4 | 5 |
| Vinyl 5×5 | 4–6 ft | 12 in | 24 in | 1.22 cu ft | 3 | 3 | 4 |
| Vinyl 5×5 | 8 ft | 12 in | 36 in | 1.84 cu ft | 4 | 5 | 5 |
The 4×4 post at a 36-inch depth (1.38 cubic feet, 3 bags of 80-lb) is the reference check value this calculator is built against — an 8-ft-tall fence needs that 3-ft bury even on a post professionals would usually upsize to 6×6 at that height.
Total Bags for a Whole Fence
Multiply the per-post bag count above by your total post count, pulled from the fence post calculator or fence post spacing calculator if you haven't already worked it out. A 100-ft, 6-ft fence with 15 posts (mixed 4×4 line and 6×6 gate) needs 30 bags of 80-lb mix, 45 bags of 60-lb, or 47 bags of 50-lb. Bagged mix stops making sense somewhere around 15 to 20 posts on a single job — beyond that, a small ready-mix delivery or a rented mixer usually takes less time than hauling and mixing bags one at a time.
Gravel Base
A 4- to 6-inch gravel base under the post improves drainage and keeps the post off standing water, and most installers add it without reducing the concrete volume above it. If you want to subtract the gravel layer from your concrete depth, reduce the depth in the formula above by the gravel thickness before calculating volume.
Dry-Set vs Wet-Set Concrete
Wet-set concrete is mixed with water and poured or tamped into the hole around the post, then allowed to cure for a day or more before rails and pickets are attached — this is the stronger, longer-lasting method and the one this calculator's bag counts assume. Dry-set concrete is poured into the hole dry, then soaked with a hose, letting groundwater moisture slowly cure it in place over several days. Dry-set uses the same bag count as wet-set but skips on-site mixing, trading that convenience for a longer, less predictable cure.
Waste and Ordering
Round up to the next full bag per post — bagged mix doesn't scale down to fractional bags, and running short mid-post is worse than having one extra. Buy 1 or 2 spare bags beyond the calculated total for a hole that comes in larger than planned or a bag lost to a torn sack.
Store unused bags off the ground and away from moisture, since bagged concrete absorbs ambient humidity and can partially cure inside a torn or damp bag before it ever reaches a hole. A pallet or a sheet of plastic under the stack is enough to keep spare bags usable for a follow-up post or a future repair.
Next Steps
Once you know your total bag count, the fence cost calculator folds concrete into a full installed cost estimate alongside posts, rails, and labor.