How Many Bags of Concrete per Fence Post?
A standard 4x4 fence post set in a 10-inch hole needs about 2 to 3 bags of 80-lb concrete mix, or 3 to 4 bags of 50-lb fast-setting mix, while a 6x6 corner or gate post in a wider, deeper hole can take 5 bags or more. Below is the formula, a bag chart by post size and hole depth, and a calculator that gives the exact count for your holes.
Key takeaways
- A 4x4 post in a 10-inch x 24-inch hole needs about 2 bags of 80-lb concrete or 3 bags of 50-lb mix.
- A deeper 10-inch x 30-inch hole for a 6-ft fence needs about 3 bags of 80-lb concrete or 4 bags of 50-lb mix.
- 6x6 corner and gate posts in a 14-inch x 36-inch hole need about 5 bags of 80-lb concrete or 8 bags of 50-lb mix.
- One cubic yard of concrete equals about 45 bags of 80 lb mix, 60 bags of 60 lb, or 72 bags of 50 lb.
- Always add 10% for waste and round up, since a hole running short mid-pour cannot wait for another bag.
The formula: how much concrete one post hole needs
To find out how many bags of concrete a fence post needs, first find the volume of the hole (a cylinder), then subtract the volume the post itself displaces. What is left over is what the concrete has to fill.
The hole is a cylinder, so its volume is pi times the radius squared times the depth, all in feet. The post eats into that same depth, so its volume comes back out and only the space around it gets filled with concrete.
Hole volume (ft3) = π x radius2 x depthConcrete needed = Hole volume - Post volume, plus 10% wasteFor example, a 10-inch diameter hole 30 inches deep holds about 1.36 cubic feet before the post goes in. A 4x4 post (actual size 3.5 by 3.5 inches) displaces roughly 0.21 cubic feet over that depth, leaving about 1.15 cubic feet of concrete to buy, plus waste. For any hole size, the sonotube calculator runs this math for you and returns the bag count directly.
Bags for a standard 4x4 fence post
Most fence posts are 4x4s (actual size 3.5 by 3.5 inches), and the rule of thumb is a hole diameter about three times the post width, so 10 to 12 inches across. Depth follows the fence height: bury at least a third of the total post length, deeper if your frost line runs below 24 inches.
| 4x4 post hole | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 10 in x 24 in (4-ft fence) | 3 | 3 | 2 |
| 10 in x 30 in (6-ft fence) | 4 | 3 | 3 |
| 12 in x 36 in (8-ft or frost-line depth) | 7 | 6 | 4 |
These counts already include a 10% waste allowance and round up to the next whole bag, since a hole is never perfectly dug and you cannot buy a fraction of a bag mid-pour.
6x6 corner, end and gate posts need more
Corner, end and gate posts carry more load and pull harder against the fence line, so they typically go in as a 6x6 (actual size 5.5 by 5.5 inches) set in a wider, deeper hole than the line posts next to them.
| 6x6 post hole | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 14 in x 36 in (corner or gate) | 8 | 7 | 5 |
Gate posts specifically should sit a few inches deeper than the line posts on either side, since the swinging weight of the gate adds leverage the concrete has to resist. If your project also involves footings or piers, the concrete and masonry calculators hub has a tool for each shape.
Bags per cubic yard by bag size
Once a project needs more than about 8 to 10 posts, it is worth checking whether ordering by the cubic yard beats buying bags. One cubic yard of concrete is 27 cubic feet, and the number of bags it takes depends on the bag's mixed yield.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft3 | 90 |
| 50 lb | 0.375 ft3 | 72 |
| 60 lb | 0.45 ft3 | 60 |
| 80 lb | 0.60 ft3 | 45 |
For a long fence run with a dozen or more posts, that math starts to favor a delivered mini-mix truck over hauling dozens of bags by hand. For a slab or pad on the same property, the concrete calculator converts the same volume into yards, bags and an estimated cost.
Fast-setting mix vs standard mix for posts
Fast-setting concrete mix, the kind sold specifically for posts, is designed to be poured dry into the hole around the post and then soaked with water, no mixing tub required. It reaches a workable set in about 20 to 40 minutes, which is why it dominates the post aisle at home centers.
Standard concrete mix needs to be mixed with water first, in a wheelbarrow or mixing tub, before it goes into the hole. It costs a little less per bag and cures to full strength the same way as any poured footing, which matters more for structural posts like deck supports than for a backyard fence line.
Either works for a wood or vinyl fence post. Save the fully mixed, hand-poured route for posts carrying real structural load, since it packs tighter around the post base with fewer voids.
Tips to avoid running short on post day
Dig the hole diameter first, then the depth. A hole cut too narrow forces you to buy more bags than the chart shows, because the post takes up a bigger share of a smaller cylinder. A hole cut too shallow risks frost heave pushing the post out of level after the first hard winter.
Buy one or two extra bags per project regardless of the math. Corner posts, gate posts, and the one hole that hit a buried rock always eat more concrete than the line posts next to them.
Round every bag count up, never down. Running two bags short with wet concrete already curing in the other holes is a worse problem than having a half bag left over at the end.
Frequently asked questions
How many 80 lb bags of concrete do I need per fence post?
How many 50 lb bags of Quikrete per fence post?
How deep should a fence post hole be?
What size hole do I need for a 4x4 fence post?
Do I need to mix concrete for fence posts or can I pour it dry?
References
- Concrete (Wikipedia)
- QUIKRETE Fast-Setting Concrete Mix (product data)
- QUIKRETE Concrete Mix (product data)
- ACI 332: Residential Concrete Code