Post Hole Calculator

How to Use This Post Hole Calculator

This post hole calculator estimates the volume of concrete needed to set a post — for both square/rectangular and round posts. Unlike a simple hole volume calculator, it subtracts the buried portion of the post from the hole volume, giving you an accurate net concrete requirement. Enter your post dimensions, hole dimensions, burial depth, and number of posts to get the total concrete volume plus bags needed. To estimate all your fence post holes at once without subtracting post volume, use the fence post concrete calculator; for all other concrete shapes, see our concrete calculator.

How to Calculate Concrete for a Post Hole

The formula depends on post shape. For both shapes, the concrete fills the space between the post and the hole wall:

Square / Rectangular Post

Concrete volume = (Hole Width × Hole Thickness − Post Width × Post Thickness) × Burial Depth

Round / Cylindrical Post

Concrete volume = π × Depth × ((Hole Radius)² − (Post Radius)²)

Step-by-Step Example

1. Post: 4×4 (actual 3.5×3.5 in = 0.292×0.292 ft)
2. Hole: 12×12 in = 1×1 ft
3. Burial depth: 2.5 ft
4. Post volume: 0.292 × 0.292 × 2.5 = 0.213 ft³
5. Hole volume: 1.0 × 1.0 × 2.5 = 2.5 ft³
6. Net concrete: 2.5 − 0.213 = 2.29 ft³ per post
7. At 0.45 ft³ per 60 lb bag → ~6 bags per post

Post Hole Size Recommendations

The standard industry rule is to make the hole 3 times the post width. This provides enough concrete mass around the post to resist lateral loads from wind, fence panels, or gates:

• 4×4 post (3.5 in actual) — dig a 10–12 inch hole
• 6×6 post (5.5 in actual) — dig a 16–18 inch hole
• 4 in round post — dig a 12 inch round hole
• Gate posts — go one size larger than standard; gate weight and swing create significant lateral force

Fast-Setting vs. Standard Concrete for Post Holes

Choosing between fast-setting and standard concrete affects your workflow, strength, and cost. Here is how they compare for post hole applications:

• Fast-setting concrete (e.g., Quikrete Fast-Setting) — sets in 20–40 minutes and reaches walk-away strength in 4 hours. You can pour the dry mix directly into the hole and add water on top — no mixing required. Ideal for fence posts and mailbox posts where you need to move quickly. Costs roughly $6–$8 per 50 lb bag.
• Standard concrete mix (e.g., Quikrete 5000) — requires pre-mixing in a wheelbarrow or mixer. Sets in 6–8 hours and reaches full 5,000 PSI strength in 28 days. Better for structural posts (deck posts, pergola posts) where maximum strength matters. Costs roughly $5–$7 per 80 lb bag.
• Strength comparison — fast-setting concrete reaches approximately 4,000 PSI at 28 days, while standard high-strength mix reaches 5,000 PSI. For most fence and mailbox posts, 4,000 PSI is more than sufficient.

For a 20-post fence project using 10-inch holes at 30 inches deep, fast-setting concrete saves roughly 2–3 hours of mixing time but costs about 15–20% more per bag. Many DIYers find the time savings well worth the premium, especially on large projects. Use this calculator to determine your total bag count, then choose the mix type that fits your timeline and budget.

What Type of Post Goes in a Hole? Materials and Selection

The most common post materials for holes filled with concrete are pressure-treated lumber, steel pipe, and fiberglass. Each has different performance characteristics:

• Pressure-treated (PT) lumber — 4×4 and 6×6 PT pine posts are the standard for wood fences and pergolas. Modern PT lumber uses alkaline copper quaternary (ACQ) or copper azole (CA-B) preservatives that are safe for residential use. Ground-contact rated PT lumber (UC4B or UC4C) is required for any buried application.
• Cedar and redwood — naturally rot-resistant; acceptable for above-ground post bases that remain dry, but pressure-treated wood is preferred for direct burial.
• Steel pipe or U-channel posts — used for chain-link fences and agricultural applications. Set in concrete the same way as wood posts. Rust-resistant galvanized or powder-coated steel posts last 20–30 years.
• Fiberglass posts — used where corrosion or rot is a concern (coastal or industrial environments). More expensive but essentially maintenance-free.

For deck applications, post base hardware anchored in the concrete footing — rather than direct burial — is increasingly preferred by building codes because it keeps wood above grade and dramatically reduces rot potential.

Gravel vs. Concrete for Post Holes — Which Should You Choose?

Concrete is not the only option for setting posts. For fence posts (but not structural deck or pergola posts), compacted gravel is a legitimate alternative:

• Gravel-set posts — Fill the hole with pea gravel or crushed stone, tamping every 6 inches. Gravel drains freely, preventing moisture accumulation at the post base that causes rot. Posts are also easier to remove if needed. Works well for wood fence posts in stable soils. Not suitable for structural applications or soft soils.
• Concrete-set posts — Provides maximum lateral strength for heavy fence panels, gates, deck posts, and high-wind regions. Concrete is essentially permanent — post replacement requires breaking out the footing. Slope the concrete collar away from the post to shed water.
• Fast-setting concrete (dry pour) — Pour dry fast-setting concrete around the post, then add water. Sets in 20–40 minutes. The fastest installation method and appropriate for most fence posts.

For most residential fence posts, the concrete advantages outweigh the drainage concerns — especially when the concrete is crowned and the post base is kept above the soil grade. For structural applications like decks, concrete is always required.

Post Hole Depth and Frost Line

Depth requirements are determined by two factors — the 1/3 rule and the local frost line:

• 1/3 rule — bury at least 1/3 of the total post length; a 9-foot post needs at least 3 feet underground
• Frost line — in freezing climates, holes must extend below the frost line to prevent frost heaving. In northern US states the frost line is 36–48 inches; in southern states it may be only 12 inches. Check your local building code.
• Use the greater depth — whichever depth is greater between the 1/3 rule and the frost line requirement governs
• Add 3–4 inches of gravel at the bottom of each hole for drainage before setting the post