Fence Post Depth Calculator

About the Fence Post Depth Calculator

The posts are the backbone of a fence. Boards, rails, wire and gates all hang from them, and wind pushes on the whole fence as if it were a sail. A post that is set too shallow leans, wobbles and eventually falls over, taking a run of fence with it. A post set too shallow in cold country can also be lifted out of the ground by frost a little each winter. Getting the depth right at the start is far cheaper than resetting posts later.

This fence post depth calculator works out how deep to set each post from the height of the fence above ground, your local frost depth, the type of post and your soil. It also gives the depth to dig including a gravel base, the hole diameter, the total post length and the standard length to buy.

How to Use the Fence Post Depth Calculator

Enter the fence height above ground in feet.

Enter your frost depth in inches, from the local building department, or 0 where the ground does not freeze.

Enter the post width and choose the post type and the soil.

The Formulas

  height rule  = height above ground ÷ 2   (a third of the post's total length)
  depth        = max(height rule, 24 in, frost depth)
               + 6 in for a corner or gate post
               + 6 in in soft or sandy soil
  hole depth   = depth + 6 in of gravel
  hole width   ≈ 3 × post width
  post length  = height + depth, rounded up to 6, 8, 10, 12, 14 or 16 ft

Step-by-Step Examples

A 6 ft fence, line post, firm soil, 30 in frost depth, 4×4 posts.

  height rule  72 ÷ 2 = 36 in
  depth        max(36, 24, 30) = 36 in = 3 ft
  hole         42 in deep, about 10½ in across
  post         72 + 36 = 108 in = 9 ft → buy 10 ft posts

The same fence with a gate post where the frost depth is 48 in.

  depth        max(36, 24, 48) + 6 = 54 in = 4 ft 6 in
  post         72 + 54 = 126 in → buy 12 ft posts

The One-Third Rule

The traditional rule is that a third of a post's total length should be in the ground. If the post shows 6 feet above ground, a third of its length buried means 3 feet below, a 9-foot post. That is the same as burying half the above-ground height. The rule gives enough soil around the post to resist the leverage of wind on the fence, which acts like a long lever trying to tip the post over. Taller fences catch more wind, so they need proportionally deeper posts.

Frost Depth

In places where the ground freezes, water in the soil expands as ice and can lift anything that is anchored only in the frozen layer. Posts set in concrete are especially vulnerable, because the frozen soil grips the rough concrete and heaves it upward. Setting the bottom of the post, or its concrete, below the frost line prevents this. Frost depths range from almost nothing in the southern United States to 48 inches or more in the northern states and Canada. Your building department can tell you the figure for your area.

Corner, End and Gate Posts

Not all posts carry the same load. Line posts in the middle of a run share the wind with their neighbours. Corner and end posts take the pull of the rails or wire from one or two directions, and gate posts carry the weight and swing of the gate, which levers the post sideways every time it opens. Setting these posts deeper and in wider holes, often with a 6×6 instead of a 4×4, keeps gates from sagging and corners from leaning. For heavy or wide gates, a concrete footing and a brace or cable on the gate itself help further.

Soil Type

Firm clay and compacted soils grip a post well. Loose sand, gravel, fill and wet ground grip less, so posts need to go deeper or have larger holes. In very loose soil, setting posts in concrete, or using steel post anchors driven deep into the ground, may be the only way to keep them firm. Rocky ground can stop a hole short; a rock bar, auger with a rock bit or a hired post-hole driller usually gets through.

Setting the Post

Put about 6 inches of gravel in the bottom of the hole and tamp it, so water can drain away from the end of the post. Stand the post, brace it plumb in two directions, and backfill with concrete or with soil and gravel tamped in thin layers. Slope the top of any concrete away from the post so water runs off rather than collecting around the wood. Pressure-treated posts rated for ground contact last much longer than untreated wood.

Before You Dig

Always call your local utility locating service a few days before digging, so buried power, gas, water and communication lines can be marked. Many fence projects also need a permit, and some neighbourhoods have rules on fence height and which side of the fence faces out.

Understanding Your Result

The headline gives the depth to set the post.

The why line says which rule set the depth and any extra added.

The hole line gives the depth to dig and the hole diameter.

The post length line gives the total length and the stock size to buy.

When Should You Use This Calculator?

Use it when planning a new fence or gate.

Use it to work out what length of post to buy.

Use it to check whether existing posts are deep enough when a fence leans.

Common Mistakes

Setting every post the same depth. Gate and corner posts need more.

Ignoring the frost line. Shallow posts heave in cold climates.

Skipping the gravel. Standing water rots post ends.

Digging without calling. Locate buried utilities first.