How many bags of concrete do I need per fence post?

For a standard 4×4 post with a 10-inch diameter hole at 30 inches deep, you need 2–3 bags of 80lb concrete per post (or 3 bags of 60lb). This includes a 10% waste factor.

How deep should a fence post hole be?

Follow the rule of thirds: bury one-third of the total post length. For a standard 8-foot post, dig the hole about 32 inches (2.6 feet) deep. The hole diameter should be 3 times the post width — 10–12 inches for a 4×4 post.

Should I use fast-setting or regular concrete for fence posts?

Fast-setting concrete (like Quikrete Fast-Setting) is ideal for fence posts because it sets in 20–40 minutes, letting you brace and move on to the next post quickly. Regular concrete works too but requires bracing for 24–48 hours.

How many bags of concrete for 20 fence posts?

For 20 standard 4×4 fence posts with 10-inch holes at 30 inches deep, you need approximately 50 bags of 80lb concrete or 67 bags of 60lb concrete (with 10% waste factor included).

Do I fill the hole all the way to the top with concrete?

No. Leave 2–3 inches below ground level and crown (dome) the concrete surface away from the post. This allows water to drain away from the wood rather than pooling against it, which prevents rot.

How Much Concrete for Fence Posts: The Complete Guide

How Much Concrete Do You Need Per Fence Post?

A standard 4×4 fence post with a 10-inch diameter hole at 30 inches deep needs approximately 2–3 bags of 80lb concrete (or 3 bags of 60lb). For a full 100-foot fence with posts every 8 feet, that’s about 28–39 bags of 80lb mix total.

Standard Fence Post Hole Sizes

Most fence posts require a hole that’s 3 times the width of the post and about 1/3 the total post length deep:

Post Size	Actual Width	Hole Diameter	Hole Depth	Concrete per Post
4×4	3.5”	10–12 inches	24–36 inches	2–3 bags (80lb)
6×6	5.5”	16–18 inches	30–42 inches	6–8 bags (80lb)

How Many Bags Per Fence Post? (The Math)

For a standard 4×4 fence post with a 10-inch diameter hole, 30 inches deep:

Calculate hole volume: π × (5/12)² × (30/12) = 1.36 cu ft
Subtract post volume: A 3.5” × 3.5” post displaces ≈ 0.2 cu ft
Net concrete needed: 1.36 − 0.2 = 1.16 cu ft per post
With 10% waste: 1.16 × 1.10 = 1.28 cu ft
Bags: 1.28 ÷ 0.60 = 2.1 → 3 bags of 80lb (rounded up)

Bag Counts for Common Fence Projects

Project	Posts	80lb Bags	60lb Bags
Single fence post	1	2–3	3
Gate section (2 posts)	2	5–6	7
50-foot fence (8ft spacing)	7	18	24
100-foot fence (8ft spacing)	13	28–39	37–52
150-foot fence (8ft spacing)	20	50	67

All counts include 10% waste factor. Based on 10-inch diameter holes at 30 inches deep.

Pro Tips for Fence Post Concrete

Don’t fill to the top — leave 2–3 inches below ground level and crown the concrete away from the post for drainage
Set posts before concrete cures — you have about 20–30 minutes working time with standard mix
Brace posts plumb — use 2×4 braces angled to stakes to keep posts perfectly vertical while concrete sets
Fast-setting mix — consider Quikrete Fast-Setting for fence posts (sets in 20–40 minutes, no bracing overnight)
Add gravel first — place 4–6 inches of crushed gravel at the bottom of each hole for drainage before inserting the post
Multiple posts? — use our calculator’s quantity multiplier to calculate all posts in one step

When to Use Bags vs. Ready-Mix for Fence Posts

Project Size	Recommendation
Under 10 posts	Bags (mix-and-pour at each hole)
10–25 posts	Bags (buy a pallet for bulk discount)
25+ posts	Consider ready-mix if holes are accessible to a truck/wheelbarrow

For most residential fences, bags are the practical choice because you pour each hole individually and can work at your own pace.

Simple Volume Calculator for Fence Post Holes — detailed cylinder math for post holes
Why You Need a Waste Factor — why post holes need 15–20% waste
How Many 80lb Bags Do You Need? — general bag calculation guide

Use Our Calculator

Enter your hole diameter, depth, and number of posts in our fence post concrete calculator to get the exact bag count for your fence project.