Fence Post Depth Calculator: Burial Depth, Hole And Concrete

Fence post depth calculator that finds post burial depth, hole size, post length and concrete bags for your fence height, frost line and soil, then checks it against the wind load.

Fence Height, Post & Frost Line

Burial depth, hole size and post length for your fence height and frost line.

What to Calculate

Fence Height Above Ground (H)

Post Size

Post Role

Frost Line Depth

Post Length You Have

Depth Rule, Soil & BackfillRatio 0.5 · Sand or Silty Sand · Concrete · 12 in Hole

Depth Ratio, Burial ÷ Height

Soil Type, IBC Table 1806.2

Set Posts In

Hole Diameter

Wind Check, IBC 1807.3105 mph · Exposure B · 8 ft Spacing · 100% Solid

Basic Wind Speed

Wind Exposure

Post Spacing (On Center)

Solid Share of Fence Face

Slope & Footing ShapeLevel Ground · Straight Hole

Ground Slope

Fence on a Slope

Footing Shape

Bell Bottom Diameter

Bell Flare Height

Concrete, Gravel & Post Count6 in Gravel Base · 80 lb Bags · 14 Posts

Gravel Base Under Post

Concrete Bag Size

Waste and Overdig

Number of Posts

Fence Post Depth for a 6 ft Fence

36 in Deep Post Burial

914 mm · Set by the Depth Ratio

Buy 10 ft Posts

Cut to LengthTrim 1 ft off the Top

Buried Share of Post33.3% Underground

Post length is the height above ground plus the buried depth and any slope step. The post stands on the gravel base, so the gravel deepens the hole, not the post.

Dig 12 in Wide × 42 in Deep

Concrete per Post2.31 cu ft · 4 × 80 lb

Gravel Base per Hole0.39 cu ft Under the Post

Hole width is about 3 times the post width, rounded up to an auger size. Fill volume is the hole around the post, with your waste allowance added on top.

Wind Check Needs 50.9 in

Wind Load per Post435 lb at 3 ft

Spacing That Passes HerePosts Every 3 ft 4 in

IBC 1807.3.2.1 embedment for a post resisting ASCE 7 wind on the fence face, with the soil bearing from IBC Table 1806.2 doubled for poles per 1806.3.4.

56 × 80 lb Bags, 14 Posts

Concrete for All Posts32.4 cu ft · 1.2 cu yd

Gravel for All Holes5.5 cu ft

Totals multiply one hole by your post count. Bag yields are Quikrete figures: 0.6 cu ft per 80 lb, 0.45 per 60 lb and 0.375 per 50 lb fast-setting bag.

Post Hole Cross-Section42 in Hole · 9 ft Post

Fence Post Depth Chart by HeightHeights 3 to 8 ft, Your Settings

Fence Height	Depth Needed	Hole Depth	Post to Buy	80 lb Bags per Post	Wind-Check Depth
3 ft	24 in	30 in	6 ft	3	34 in
4 ft	24 in	30 in	6 ft	3	40.2 in
5 ft	30 in	36 in	8 ft	4	45.8 in
6 ft	36 in	42 in	10 ft	4	50.9 in
7 ft	42 in	48 in	12 ft	5	55.8 in
8 ft	48 in	54 in	12 ft	6	60.4 in

Is This Depth Deep Enough?300 mm to 4 ft Deep

Burial Depth	In Inches	Tallest Fence by Rule	Tallest Fence by Wind Check
300 mm	11.8 in	Too Shallow	Under 1 ft
450 mm	17.7 in	Too Shallow	Under 1 ft
500 mm	19.7 in	Too Shallow	1 ft 1 in
2 ft	24 in	4 ft	1 ft 7 in
2.5 ft	30 in	5 ft	2 ft 4 in
3 ft	36 in	6 ft	3 ft 3 in
4 ft	48 in	8 ft	5 ft 5 in

Hole and Concrete by Post SizeWood, Vinyl and Steel

Post	Hole Width	Concrete per Post	80 lb Bags	Wind-Check Depth
4×4 Wood	12 in	2.31 cu ft	4	50.9 in
6×6 Wood	18 in	5.14 cu ft	9	43.2 in
4×4 Vinyl	12 in	2.23 cu ft	4	50.9 in
5×5 Vinyl	16 in	4.03 cu ft	7	45.3 in
1-7/8 in Steel	6 in	0.58 cu ft	1	68 in
2-3/8 in Steel	8 in	1.05 cu ft	2	60.3 in
2-7/8 in Steel	10 in	1.65 cu ft	3	54.9 in

Step-by-Step MathDepth = max(Ratio × H, 24 in, Frost + 6 in)

Depth ratio: 0.5 × 72 in = 36 in; minimum 24 in; frost: 10 + 6 = 16 in.
Depth needed = max(36, 24, 16) = 36 in.
Post length = 72 + 36 = 108 in → 10 ft stock; hole = 36 + 6 gravel = 42 in deep, 12 in wide (3 × 3.5 = 10.5 in, next auger size).
Concrete = ((π × 12² ÷ 4 - 12.25 post) × 36) ÷ 1728 × 1.1 = 2.311 cu ft ÷ 0.6 = 3.85 → 4 bags; base gravel 0.393 cu ft.
Wind: qh = 0.00256 × 0.57 × 0.85 × 105² = 13.67 psf; p = 0.6 × qh × 0.85 × 1.3 = 9.07 psf; P = p × 8 × 6 = 435 lb at h = 3 ft; moment 1,306 ft·lb.
IBC Eq. 18-1: d = 0.5A(1 + √(1 + 4.36h ÷ A)), A = 2.34P ÷ (S1 × b), b = 1 ft, S1 = 2 × 150 × d ÷ 3 = 425 psf → d = 4.25 ft = 50.9 in.
Totals: 4 × 14 = 56 bags, 32.36 cu ft fill and 5.5 cu ft base gravel.
Result: 36 in Deep Post Burial.

Shallower Than the Code Wind Check

Your 36 in depth gives 71% of the 50.9 in that IBC 1807.3 asks for in 105 mph wind. A 29 in wide footing at your depth would pass. Fences up to 7 ft are permit-exempt under IRC R105.2, so this is advice unless your town requires it.

How a Fence Post Depth Calculator Sets Burial Depth

A fence post should be buried half as deep as the fence stands above the ground, which puts one-third of the post's total length underground. A fence post depth calculator then raises that depth to a 24 in minimum, to the frost line, or for a gate post, whichever needs the most.

$D = max (\frac{H}{2}, 24, F + 6)$

D is the burial depth in inches, H is the fence height above ground in inches and F is the local frost depth. Gate hinge and latch posts get another 6 in on top. Quikrete's Fast-Setting Concrete Mix data sheet gives the same half-height rule for hole depth.

A 6 ft fence therefore needs posts buried 36 in, so each post is 9 ft long and you buy 10 ft stock and trim 12 in off the top. The hole goes 42 in deep, because 6 in of gravel sits under the post for drainage.

Some calculators default to burying one-third of the height above ground instead. That gives 24 in for a 6 ft fence, which only puts a quarter of the post underground and lands on the 24 in floor rather than on the half-height rule.

Fence Post Depth Chart by Fence Height

The fence post depth calculator chart covers 4×4 posts in 12 in holes with 80 lb concrete bags. The wind-check column is the depth that IBC Section 1807.3 asks for at 105 mph, exposure B, in sandy soil, with a solid fence and posts 8 ft apart.

Fence Height	Post Burial	Hole Depth	Post to Buy	80 lb Bags per Post	Wind-Check Depth
3 ft	24 in	30 in	6 ft	3	34 in
4 ft	24 in	30 in	6 ft	3	40.2 in
5 ft	30 in	36 in	8 ft	4	45.8 in
6 ft	36 in	42 in	10 ft	4	50.9 in
7 ft	42 in	48 in	12 ft	5	55.8 in
8 ft	48 in	54 in	12 ft	6	60.4 in

An 8 ft fence post should be buried 48 in, so the fence post depth calculator picks a 12 ft post with no trim. The standard depth for the common 6 ft privacy fence is 36 in, and a 4 ft fence sits at the 24 in minimum.

The same fence post height calculator works in reverse. Add the burial depth to the fence height to get the post length, then round up to the next stock length of 6, 8, 10, 12, 14 or 16 ft.

Is 2 Feet, 3 Feet or 500 mm Deep Enough for Fence Posts?

Two feet is deep enough for a fence up to 4 ft tall, and 3 ft of burial carries a 6 ft fence by the half-height rule. Metric depths of 300, 450 and 500 mm all fall short of the 24 in floor, so the fence post depth calculator treats them as too shallow for any fence height.

Burial Depth	In Inches	Tallest Fence by Half-Height Rule	Tallest Fence by Wind Check
300 mm	11.8 in	Too shallow	Under 1 ft
450 mm	17.7 in	Too shallow	Under 1 ft
500 mm	19.7 in	Too shallow	1 ft 1 in
2 ft	24 in	4 ft	1 ft 7 in
2.5 ft	30 in	5 ft	2 ft 4 in
3 ft	36 in	6 ft	3 ft 3 in
4 ft	48 in	8 ft	5 ft 5 in

The wind-check column assumes a solid fence on 8 ft spacing in sandy soil at 105 mph, which is a demanding case. Open picket fences, closer posts and firmer soil all raise the height a given depth can carry, as the wind section shows.

Frost Line and Gate Post Depth

In freezing climates, the bottom of each post hole goes 6 in below the local frost line so frost heave can't lift the post. With a 42 in frost line, a 6 ft fence post is buried 48 in instead of 36 in, and the whole 10 ft stock goes into use.

Gate hinge and latch posts carry the swinging weight of the gate, so they go 6 in deeper than line posts, and the fence post depth calculator adds that extra depth on its own. A 6 ft fence's gate post is buried 42 in, takes 5 bags of 80 lb concrete instead of 4, and still fits a 10 ft post with 6 in trimmed.

Your local building department publishes the frost depth used in your area. Enter it in the fence post depth calculator as measured from the finished ground, since a raised bed or regraded yard changes how deep the hole really sits.

Fence Post Depth on a Slope

A racked fence follows the ground, so every post keeps the same length and burial depth as on level ground. Measure the depth from the low side of each hole, since the uphill side of a sloped hole reads deeper than the post is really set.

A stepped fence keeps each panel level, so every post has to stand taller by the drop between panels. The fence post depth calculator adds that step to each post's length.

$s = g \times S \times 12$

s is the step in inches, g is the ground slope as a decimal and S is the post spacing in feet. On a 20% slope with posts 8 ft apart, each panel steps down 19.2 in, so a 6 ft fence post grows from 108 in to 127.2 in.

Stepped Fence, 6 ft Tall, 8 ft Spacing	Step per Panel	Post Length	Post to Buy	Trim off the Top	Share Buried
Level or racked	0 in	108 in	10 ft	12 in	33.3%
10% slope	9.6 in	117.6 in	10 ft	2.4 in	30.6%
20% slope	19.2 in	127.2 in	12 ft	16.8 in	28.3%
30% slope	28.8 in	136.8 in	12 ft	7.2 in	26.3%

Burial depth, hole size and concrete stay at 36 in, 12 in wide and 4 bags per post at every slope. Only the post above the ground grows. Checked in the fence post depth calculator, a 10 ft post on a 20% stepped fence buries just 28.8 in, which is 7.2 in short, and 12 ft posts are the fix.

What Size Hole for a 4x4 Fence Post?

A 4×4 post needs a hole three times its 3-1/2 in width, or 10-1/2 in, which rounds up to a 12 in auger. The Quikrete data sheet sets hole diameter at three times the post width, and the fence post depth calculator picks the next auger size that meets it.

$V = (\frac{π d ^{2}}{4} - A_{p os t}) \times \frac{D}{1728} \times (1 + w)$

V is concrete per post in cubic feet, d is the hole diameter in inches, A_post is the post's cross-section in square inches, D is the burial depth in inches and w is the waste allowance. A 4×4 buried 36 in a 12 in hole takes 2.31 cu ft with 10% waste, or 4 bags of 80 lb mix.

Post, 6 ft Fence	Hole Width	Concrete per Post	80 lb Bags	Wind-Check Depth
4×4 wood	12 in	2.31 cu ft	4	50.9 in
6×6 wood	18 in	5.14 cu ft	9	43.2 in
4×4 vinyl	12 in	2.23 cu ft	4	50.9 in
5×5 vinyl	16 in	4.03 cu ft	7	45.3 in
1-7/8 in steel line post	6 in	0.58 cu ft	1	68 in
2-3/8 in steel terminal	8 in	1.05 cu ft	2	60.3 in
2-7/8 in steel gate post	10 in	1.65 cu ft	3	54.9 in

Each row buries the post 36 in for a 6 ft fence. Across 14 posts, the 4×4 fence takes 56 bags of 80 lb mix, about 1.2 cu yd, so a mixer or a short ready-mix load is worth pricing for a long fence line.

Should Fence Posts Be Set in Concrete or Gravel?

Concrete gives a post far more resistance to wind than tamped gravel, because the footing's full width bears against the soil. IBC Section 1807.3 measures b as the footing diameter for a concrete-set post but as the post's own diagonal for gravel backfill.

For the 6 ft fence in sand at 105 mph, that difference moves the wind-check depth from 50.9 in in a 12 in concrete footing to 73.8 in with tamped gravel. Gravel backfill drains better and is easier to dig out, but a fence post depth calculator shows it needs a much deeper hole to match.

Either way, 6 in of gravel under the post keeps water from sitting against the end grain. That base takes 0.39 cu ft per 12 in hole, or 5.5 cu ft for 14 posts.

Bell Footings for Fence Posts

A bell footing flares out at the bottom of the hole, so the concrete is wider below than at the surface. The wider base gives the footing more bearing area and makes it harder for frost or wind to lift it out of the ground.

The concrete in a belled hole is a straight section down to the flare plus a tapered section, a frustum, at the bottom.

$V = [\frac{π d ^{2}}{4} (D - h_{b}) + \frac{π h _{b}}{12} (d^{2} + d d_{b} + d_{b}^{2}) - A_{p os t} D] \times \frac{1 + w}{1728}$

d is the top hole diameter, d_b is the bell diameter at the bottom and h_b is the flare height, all in inches. For a 4×4 buried 36 in in a 12 in hole that flares to 18 in over the bottom 12 in, the fence post depth calculator finds 2.82 cu ft per post with 10% waste.

Footing, 4×4, 36 in Deep	Concrete per Post	80 lb Bags per Post	Gravel Base per Hole	Bags for 14 Posts
Straight 12 in hole	2.31 cu ft	4	0.39 cu ft	56
Bell to 18 in over 12 in	2.82 cu ft	5	0.88 cu ft	70
Bell to 24 in over 12 in	3.46 cu ft	6	1.57 cu ft	84

In the fence post depth calculator, the 6 in gravel base spreads under the full bell, which is why it more than doubles with an 18 in flare. The wind check keeps using the 12 in top width, so the bell's extra width is left out and the result errs on the safe side.

How to Calculate the Strength of a Fence Against Wind

Fence strength comes down to wind load on each post and how much soil pressure the footing can use to resist it. The wind pressure comes from ASCE 7, and IBC Equation 18-1 turns the load into a required embedment depth.

$d = 0.5 A (1 + 1 + \frac{4.36 h}{A}), A = \frac{2.34 P}{S _{1} b}$

P is the lateral force on one post in pounds, h is its height above ground in feet, b is the footing diameter and S₁ is the allowable lateral soil pressure at one-third of the embedment depth, as defined in Section 1807.3.2.1 of the building code, quoted in the California Building Code text.

At 105 mph in exposure B, a solid 6 ft fence with posts 8 ft apart puts 435 lb on each post, acting 3 ft above the ground. Section 1806.3.4 lets poles that tolerate 1/2 in of movement use twice the tabular soil pressure, and the fence post depth calculator applies that increase.

6 ft Fence, 4×4 in Concrete	Load per Post	Wind-Check Depth
105 mph, exposure B, solid, 8 ft spacing	435 lb	50.9 in
90 mph, exposure B	320 lb	44.9 in
105 mph, exposure C open field	649 lb	60.1 in
Picket fence, 50% solid	281 lb	42.7 in
Posts 6 ft apart	326 lb	45.3 in

A 36 in depth reaches 71% of the wind-check figure for the baseline fence, and the fence post depth calculator finds that a 29 in wide footing at that depth would pass. Fences up to 7 ft are exempt from a building permit under 2021 IRC Section R105.2, as quoted by Grant County, Washington, so the check is a guide unless your town requires engineering.

Fence Post Depth in Clay, Sand or Gravel Soil

Weak soil needs deeper posts because it pushes back less. IBC Table 1806.2 gives allowable lateral bearing per foot of depth by soil class, and the fence post depth calculator feeds that value into the wind check.

Soil, IBC Table 1806.2	Lateral Bearing	Wind-Check Depth, 6 ft Fence	Post Spacing That Passes at 36 in
Clay or silt	100 psf/ft	60.3 in	2 ft 3 in
Sand or silty sand	150 psf/ft	50.9 in	3 ft 4 in
Sandy gravel	200 psf/ft	45.3 in	4 ft 6 in
Sedimentary rock	400 psf/ft	34.3 in	9 ft
Crystalline bedrock	1,200 psf/ft	22.4 in	Over 16 ft

In clay, the fence post depth calculator's wind check asks for 60.3 in on a solid 6 ft fence, about 9 in more than in sand. Clay also holds water against the post, which makes the gravel base under each post more important there.

Solid crystalline bedrock needs only 22.4 in by the wind check, so the half-height rule still sets 36 in for a 6 ft fence. On rock, the fence post depth calculator's wind check passes with posts more than 16 ft apart.

Should Fence Posts Be 4x4 or 6x6?

A 4×4 post is the usual choice for a 6 ft wood fence on 8 ft spacing, and a 6×6 suits heavy gates and tall fences. In the fence post depth calculator, the bigger post's 18 in footing cuts the wind-check depth from 50.9 in to 43.2 in, but each hole takes 9 bags of concrete instead of 4.

The best spacing for fence posts is 8 ft on center for wood and vinyl panels, which matches 8 ft rails. Closer posts share the wind load, and the Fence Calculator counts the posts, rails and concrete for a whole fence line at any spacing.

Checking Fence Posts You Already Have

With posts already bought, the fence post depth calculator works back from their length. An 8 ft post on a 6 ft fence leaves only 24 in underground, 33% shallower than the 36 in the half-height rule asks for. By that rule, an 8 ft post supports a fence no taller than 5 ft 4 in.

A 10 ft post on the same fence buries 48 in, which is 33% deeper than needed and passes the rule for any fence up to 6 ft 8 in. The extra depth also lifts the wind-check allowance to a 5 ft 5 in solid fence on 8 ft spacing in sand.

Pickets, panels and framing go on once the posts are set. The Fence Picket Calculator, Wood Fence Calculator and Vinyl Fence Cost Calculator cover each style, and deck posts follow their own rules in the Deck Footing Calculator.