How Much Concrete for a Fence Post?
Set a fence post in too little concrete and the fence leans within a year. Here is how deep and wide to dig, how much concrete each post takes, and how many bags to buy for the whole run.
By the Calculate My Reno Team / Published
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A fence is only as solid as its posts, and the posts are only as solid as the concrete around them. Getting the hole size and concrete amount right is what keeps the fence standing straight for decades.
Sizing the hole
Two numbers decide both the strength and the concrete you need:
- Depth: bury about one-third of the post’s above-ground height, with a minimum of roughly 2 ft (600 mm). Add a 4–6 inch (100–150 mm) gravel base at the bottom for drainage.
- Diameter: about three times the post width - so a 10–12 inch (250–300 mm) hole for a standard 4×4 post.
The diameter is the big lever on concrete volume - a wider hole is sturdier but uses noticeably more concrete.
How much concrete per post
The concrete fills the hole around the post, so the volume is the hole minus the post:
concrete per post = hole volume − post volume
The easiest way is the concrete calculator: choose the circle/column shape, enter the hole diameter and depth, and read off the volume and bags. The post displaces a little concrete, so round down slightly. As a rough guide, a 4×4 in a 10-inch hole, 2 ft deep, is about one to two 50–60 lb bags per post.
Multiply by the number of posts
For a fence run, work out one post then multiply by the post count - and add a spare bag or two. A 20-post fence at ~1.5 bags each is around 30 bags, comfortably in bagged territory rather than ready-mix (see Quikrete vs Sakrete vs ready-mix).
Use fast-setting mix
For posts, fast-setting concrete is the easy choice: you can often pour the dry mix straight into the hole around a braced, level post and add water, with no pre-mixing - it sets in 20–40 minutes. That lets you plumb each post and move on quickly.
Tips
- Gravel base first for drainage - water pooling at the post base is what rots timber posts.
- Brace and check for plumb before the concrete sets; fast-setting mix gives you little time to adjust.
- Crown the top of the concrete so water runs away from the post rather than pooling against it.
- Setting lots of posts? Get the full bag count with the concrete calculator, then read how many bags of concrete do I need for the method.
Frequently asked questions
How much concrete do I need for one fence post?
For a typical 4×4 post in a 10-inch-wide hole dug about 2 feet deep, you need roughly one to two 50–60 lb bags of concrete per post, depending on hole size and the gravel base. Set the calculator to the circle/column shape, enter the hole diameter and depth, and it gives the volume and bag count - the post itself displaces a little, so round down slightly.
How deep should a fence post hole be?
A common rule is to bury about one-third of the post's above-ground height, with a minimum of around 2 feet (600 mm), plus a 4–6 inch (100–150 mm) gravel layer at the bottom for drainage. Taller fences and gate posts need to go deeper. Local frost depth can also dictate a deeper hole.
How wide should the post hole be?
About three times the width of the post - so roughly a 10–12 inch (250–300 mm) diameter hole for a standard 4×4 post. A wider hole holds the post more firmly but uses more concrete, so it is the main thing that changes your bag count.