Post Hole Depth & Concrete Calculator

Post Hole Depth & Concrete Calculator

Calculate post hole depth, diameter, and concrete bags for fence posts, decks, and pergolas. Includes frost line depths, gravel drainage, and 1/3 rule sizing.

Lbl presets
Lbl regime

Regime help

Help application

72 Unit inches(6.0 ft)

Help post height

Help post profile

36 Unit inches

Help frost depth

6 Unit inches

Help gravel depth

Help soil type

Wind Exposure & Mix Specs
10 Unit posts

Help post count

Lbl post bury depth
42.0Unit inches
Lbl rule frost
Lbl hole diameter
12Unit inches
3x 4x4 Lbl soil bore
Lbl concrete bags (80lb)
45Unit bags
1.00 cu yd
Lbl frost compliance
Lbl frost safe
+0.0" margin
Lbl diagram title
Lbl concrete collarLbl drainage bed
Lbl ground levelLbl drainage bed (6.0")Lbl concrete collar4x472 inLbl frost line: 36"Lbl frost safeLbl above ground span: 72"Lbl below ground span: 42.0"Lbl total hole depth: 48.0"Lbl hole diameter: 12"
Lbl total hole depth48.0 Unit inches

42.0" bury + 6.0" gravel bed

Lbl post bury depth42.0 Unit inches

Lbl rule frost

Lbl hole diameter12 Unit inches

Help post profile

Lbl min stock length114.0 Unit inches

Lbl recommended purchase: 10ft (Lbl cutoff waste: 6.0")

Lbl total hole depth48.0 Unit inches
Lbl hole diameter12 Unit inches
Lbl concrete bags45 Unit bags
Lbl frost complianceLbl frost safe
Lbl recommended purchase10 Unit feet
Lbl wind resistance1.75x

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Technical Reference & Architecture Guide

Post Hole Depth & Concrete Calculator Guide

Calculate post hole depth, diameter, and concrete bags for fence posts, decks, and pergolas. Includes frost line depths, gravel drainage, and 1/3 rule sizing.

Post Hole Depth & Concrete Calculator Guide — Client-Side In-Memory Architecture & Data Flow
Post Hole Depth & Concrete Calculator Guide — Client-Side In-Memory Architecture & Data Flow

Key Parameters & Controls

Above-Grade Cantilever HeightInches (or Millimeters)

Vertical distance from top of soil to fence crest; determines lateral wind overturning moment and minimum required counter-embedment.

Default: 72 inches (1800 mm)
One-Third Embedment Ratio RuleEngineering Ratio

Universal fencing standard requiring at least one-third (33.3%) of total post length buried underground (or 50% of exposed height).

Default: 1/3 total post (1/2 exposed height)
Frost Line Penetration BoundaryInches (or Millimeters)

Statutory depth below which soil moisture cannot freeze. Hole base must extend at least 6 inches (150 mm) deeper to prevent winter frost heaving.

Default: 36 inches (900 mm)
Three-Times Post Diameter RuleAuger Dimension Rule

Hole diameter must be at least 3 times nominal post width (e.g., 3.5" 4x4 requires a 10" to 12" diameter bore) to provide adequate concrete collar.

Default: 3x post width
Crushed Stone Drainage BedCompacted Layer Depth

Coarse #57 aggregate placed under post end allowing rainwater to percolate into deep subsoil rather than rotting submerged timber fibers.

Default: 4 to 6 inches (100-150 mm)
Net Concrete Yield Per 80-lb BagCubic Feet Per Bag

One standard 80-lb (36.3 kg) bag of pre-mixed dry concrete yields approximately 0.60 cubic feet (0.017 cubic meters) of consolidated wet concrete.

Default: 0.60 cu. ft / 80-lb bag

Pro Tips & Best Practices

Never Encase the Bottom Wood Grain in Concrete

Rest the bottom of the wooden post on a 4 to 6-inch bed of crushed gravel before pouring concrete. Encapsulating the cut bottom end in a concrete 'boot' creates an impervious cup that traps descending rainwater, accelerating fungal dry rot by up to 400% compared to a gravel-drained base.

Crown the Concrete Surface Above Grade

Trowel the wet concrete collar 1 to 2 inches above the surrounding lawn grade with a gentle 15-degree outward bevel. This sheds pooling surface runoff away from the vulnerable wood-concrete joint line, preventing localized freeze-thaw spalling.

Auger Glaze Mitigation in Heavy Clay

Power augers rotating in moist clay produce smooth, polished hole sidewalls that act like slick teflon tubes, drastically reducing soil skin friction. Scrape the hole walls with a manual spade before pouring concrete to create mechanical interlock.

Hole Width vs. Depth Priority for Wind Load

For solid privacy fences subject to high winds, adding 6 inches of vertical embedment depth increases lateral overturning resistance by over 60%, whereas simply widening the concrete diameter only increases resistance by 15% while doubling concrete costs.

Fast-Setting Concrete Water-Pour Protocol

When using commercial fast-setting concrete (such as Quikrete Fast-Setting in the red bag) with the dry-pour method, pour half the required clean water into the hole first, add half the dry mix, add remaining water, then top off to avoid dry unhydrated core pockets.

Common Gotchas & Troubleshooting

⚠️Encountering Solid Bedrock or Impassable Boulders Before Frost Line

Do not leave a shallow floating post. Anchor directly into bedrock by drilling 3/4-inch (20 mm) holes at least 6 inches into the rock using a rotary hammer drill, epoxy-anchoring threaded rebar pins, and pouring a reinforced surface concrete collar around the post base.

⚠️Loose Sandy Soil Causing Sidewall Collapsing During Excavation

Insert an inexpensive 10-inch or 12-inch cardboard builder tube (Sonotube) into the excavation as a temporary casing liner to maintain clean circular geometry and prevent loose sand from contaminating the gravel bed.

⚠️Post Cut-End Positioned Downward Without Copper Preservative

Factory pressure-treated wood treatment concentrates near the outer surface. If you cut a post to length on site, always place the uncut factory end down in the hole, or saturate the freshly cut end in copper naphthenate wood preservative for 24 hours prior to burial.

⚠️Gate Post Sags Immediately After Hanging Heavy Gate

Standard line post holes are insufficient for gate torque. Dig gate post holes 6 inches deeper and 4 inches wider than line posts, increase post dimensions from 4x4 to 6x6, and anchor the post with an internal diagonal tieback cable or concrete foot collar.

Frequently Asked Questions

How deep should a post hole be for a 6 foot fence?▼

A standard 6-foot privacy fence requires post holes dug to a total depth of 36 to 42 inches (900 to 1050 mm). This accommodates 30 to 36 inches of buried post embedment (satisfying the standard 1/3-to-1/2 rule of thumb) plus a mandatory 6-inch compacted gravel drainage bed at the base. In cold regions, the post must also extend at least 6 inches below your municipality's specific frost line depth.

How many bags of concrete do I need per post hole?▼

For a standard 4x4 wooden post buried 36 inches deep inside a 10-inch diameter hole, you will need approximately 3.5 bags of 60-lb concrete, or 2.5 to 3 bags of 80-lb concrete per hole. If using an expanded 12-inch diameter hole (recommended for corner or gate posts), you will need 5 to 5.5 bags of 60-lb concrete, or 4 bags of 80-lb concrete per post.

What is the rule of thumb for post hole depth and diameter?▼

The universal contractor rule of thumb consists of two ratios: (1) Depth: bury at least 1/3 of the total post length into the ground (or 1/2 of the exposed height above ground), ensuring the base extends beneath the local frost depth; (2) Diameter: excavate the hole to 3 times the width of the post (e.g., a 3.5-inch 4x4 requires a 10-to-12 inch hole, while a 5.5-inch 6x6 requires a 16-to-18 inch hole).

Should the post sit on the bottom of the hole or on gravel?▼

The post should always sit on a 4 to 6-inch bed of tamped crushed drainage gravel (#57 stone), never directly on bare soil and never encased in a solid concrete bottom cup. Resting on gravel allows trapped ground moisture and internal post condensation to drain naturally into the subsoil, preventing bottom rot.

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