Estimate concrete, depth, and materials for post holes.
Post Hole Sizing Guide
Proper post hole sizing ensures stable, long-lasting fence posts, deck supports, and structural footings.
| Post Size | Hole Diameter | Hole Depth | Concrete per Hole |
|---|---|---|---|
| 4×4 (fence) | 10-12 in | 24-30 in | 1-2 bags (50lb) |
| 4×4 (frost line) | 10-12 in | 36-48 in | 2-3 bags (50lb) |
| 6×6 (deck) | 16-18 in | 36-48 in | 3-4 bags (80lb) |
| Sonotube 8in | 8 in | 48 in | 2 bags (80lb) |
Frequently Asked Questions
How do I calculate concrete for a post hole?
Volume = π × r² × depth. For a 10-inch diameter hole that is 36 inches deep: π × 5² × 36 = 2,827 cubic inches = 1.64 cubic feet. After placing the post, reduce by the post volume.
How deep should a fence post hole be?
The standard rule is 1/3 of total post length underground. For a 6-foot fence with 8-foot posts, dig 24-30 inches deep. For areas with frost, dig below the frost line (36-48 inches in northern regions).
What diameter should a post hole be?
Post holes should be 3 times the post width. For a 4×4 post (3.5 inches actual): dig a 10-12 inch diameter hole. For a 6×6 post: dig a 16-18 inch hole. This allows room for concrete and adjustment.
How many bags of concrete per post hole?
A standard fence post hole (10-inch diameter, 30 inches deep) needs about 1-2 bags of 50-pound fast-setting concrete or 1 bag of 80-pound mix. Larger holes for deck posts need 3-4 bags of 80-pound mix.
Should I use fast-setting or regular concrete for posts?
Fast-setting concrete is ideal for fence posts — pour dry mix into the hole and add water (sets in 20-40 minutes). Regular concrete provides a stronger final strength and is better for structural posts and deck footings.
How far apart should fence posts be?
Standard spacing is 6-8 feet apart for most fence types. Privacy fences typically use 8-foot spacing. Chain link posts are 10 feet apart. Gate posts and corner posts should be set deeper with more concrete.
Can I set fence posts without concrete?
Yes, using compacted gravel (also called ‘dry setting’). Pack gravel tightly around the post in layers. This works well in areas with good drainage. Not recommended for gate posts or areas with soft soil.
How long should I wait before attaching fence panels?
With fast-setting concrete: 4-6 hours for fence panels. With regular concrete: wait 24-48 hours. Allow full 28-day cure before applying heavy loads or hanging gates.
What is a sonotube and when do I need one?
A sonotube is a cardboard form tube used for pouring cylindrical concrete footings. Use them for deck posts, pergola supports, and structural footings. They create cleaner, stronger footings than just pouring into a hole.
How do I keep posts from rotting?
Use pressure-treated lumber rated for ground contact. Apply additional wood preservative to cut ends. Set posts in gravel at the bottom for drainage. Avoid trapping moisture around the base of the post.