How Much Concrete for Fence Posts? Bags Per Post Guide

Quick Answer

These numbers assume a 10–12 inch diameter hole at 24–36 inches deep. Your actual needs depend on hole size, depth, and post dimensions — see the full table below. For a quick total based on your project dimensions, use our fence calculator which calculates concrete bags per post automatically.

Hole Size Matters

The amount of concrete you need is determined by the volume of the hole minus the volume of the post. Both hole diameter and depth have a large effect on concrete quantity.

Hole diameter: The standard recommendation is 3 times the post width. For a 4x4 post (actual dimension 3.5"), that means a 10.5-inch hole — in practice, a 10–12 inch hole works fine. For a 6x6 post (actual 5.5"), dig a 12-inch hole. Most post hole diggers produce 6–8 inch holes, while power augers come in 8, 10, and 12-inch bits.

Hole depth: The general rule is to bury one-third of the total post length. For a standard 6-foot privacy fence, you use 8-foot posts and bury 24–30 inches. In cold climates, you need to reach below the frost line to prevent heaving — that can mean 36–48 inches deep in northern states. Check your local frost line depth at your county building department or see our fence post depth guide for a state-by-state reference.

Concrete Bags Per Post Table

This table shows the number of 80-lb bags of premixed concrete needed per post. Values are rounded up because you always buy whole bags. A 10% waste factor is included.

50-lb bags: If you prefer the lighter bags, multiply the 80-lb count by 1.6. So 2 bags of 80-lb equals approximately 3 bags of 50-lb concrete.

To figure out how many fence posts you need in total, multiply these per-post figures by your post count.

Fast-Setting vs Standard Concrete

There are two main types of bagged concrete used for fence posts. Each has a legitimate use case:

Fast-setting concrete (Quikrete Fast-Setting, Sakrete Fast-Set) is the go-to for most DIY fence builders. You pour the dry mix directly into the hole around the post, add water, and it sets firm enough to hold the post plumb within 20–40 minutes. No wheelbarrow, no mixing — just pour and wet. The final cured strength is equivalent to standard premix.

Standard premix makes more sense on large projects (20+ posts) where the per-bag savings add up and you have a helper to manage the mixing workflow. You mix it in a wheelbarrow, pour it into the hole, and wait a full day before attaching rails and boards.

Concrete vs Gravel vs Post Foam

Concrete is the default, but it is not the only option. Here is an honest comparison:

Gravel backfill

Crushed gravel (not round pea gravel) compacted in 6-inch layers provides decent hold and excellent drainage. Some builders prefer it in cold climates because gravel does not heave with frost the way concrete can. However, gravel alone does not provide the lateral rigidity of concrete — in loose or sandy soil, posts set in gravel will eventually lean. Best for: short fences (4 ft or less), temporary fences, and well-drained firm clay soil.

Expanding post foam

Products like Sika PostFix and Secure Set use expanding polyurethane foam that you pour into the hole. It expands to fill the void and sets hard in 3–5 minutes. One bag replaces 1.5–2 bags of concrete and weighs under 3 lbs. The foam grips the post and the soil walls, providing surprisingly strong hold. It also sheds water rather than absorbing it, which can extend post life in wet climates.

The downsides: post foam costs $12–$18 per post (vs $8–$12 for concrete), it cannot be dug out easily if you ever need to move the post, and it has no track record beyond 10–15 years. For gate posts and corner posts that bear significant loads, concrete remains the safer choice.

When to always use concrete

• All gate posts (the lateral forces demand it) — see our gate sizing guide for post size and depth requirements by gate type
• Corner and end posts (they resist tension from the fence line)
• Any post in sandy or loose soil
• Privacy fences over 5 feet tall (wind load is significant)

How to Pour Concrete Around a Post

Whether you use fast-setting or standard mix, the process is similar. Here is the step-by-step:

1. Dig the hole to the correct depth and diameter. Use a post hole digger or power auger. Make the sides of the hole rough (not smooth) so the concrete bonds to the soil.
2. Add a gravel base. Pour 3–4 inches of crushed gravel into the bottom. This provides drainage so water does not pool at the base of the post and accelerate rot. Tamp it level.
3. Set the post. Place the post on the gravel base and check it for plumb on two adjacent sides with a level. Have a helper hold it or use temporary braces (two 1x4s screwed to the post and staked to the ground).
4. Add concrete. For fast-setting: pour the dry mix directly into the hole around the post, filling to within 3–4 inches of ground level. Then add water per the bag directions (typically 1 gallon per bag). For standard premix: mix in a wheelbarrow to a thick oatmeal consistency and shovel it into the hole in lifts, poking with a stick to remove air pockets.
5. Check plumb again. The post will shift as you add concrete. Check and adjust plumb on both sides before the concrete firms up.
6. Crown the top. Mound the concrete slightly above ground level and slope it away from the post. This sheds rainwater instead of letting it pool against the wood.
7. Wait before loading. Fast-setting concrete can hold braces within 40 minutes, but wait 4 hours before hanging heavy rails or gates. Standard concrete needs 24–48 hours.

For the full post installation process including layout, string lines, and spacing, see our complete fence post installation guide.