Fence post depth calculator

Height gives a starting point; frost and soil can change it

The calculator uses the greater of one-third of the above-ground height or the frost depth you enter. It adds a separate 6 in gravel base below the concrete column. This is a planning baseline, not an engineered footing design. Local requirements, wind, soil and gate loads can require deeper or wider footings.

Check the hole diameter against the post

The default 12 in hole is a planning example for a 3.5 in square post. The QUIKRETE post-setting guide discusses holes roughly three times the post width and an embedment range from one-third to one-half of the above-ground height. Follow the fence manufacturer's footing instructions for your product.

Concrete per hole

Volume equals the circular hole area minus the post cross-section, multiplied by the concrete depth, then converted from cubic inches to cubic feet. Bags per hole round that volume up to the next whole bag. The QUIKRETE 50 lb mix specification gives an approximate yield of 0.375 cu ft; change the yield for the bag you buy.

Confirm underground utility locations before digging. A depth estimate does not identify buried lines or determine your property boundary.

Keep concrete depth and total hole depth separate

The concrete column surrounds the buried post. The gravel base sits below that column, so it increases the excavation without increasing the computed concrete volume. Do not add the gravel depth again after reading the total hole-depth result. If your approved footing detail uses a different base or top finish, use that detail for the actual excavation and material order.

Worked concrete example

The illustrative default is a 6 ft fence, a 12 in diameter hole and a 3.5 in square post. With no frost-depth adjustment, the calculator returns a 24 in concrete column and a 30 in total hole. The net concrete volume is about 1.40 cu ft per hole after subtracting the space occupied by the post.

At 0.375 cu ft per bag, that volume divided by the bag yield rounds up to 4 bags per hole. The dimensions are worked inputs, not a universal footing specification. The post-setting guide gives a starting embedment range, not a design for every fence. The mix specification supplies the approximate bag yield; follow the product instructions for use.

What happens when the entered frost depth is deeper?

Half the example's above-ground height is 36 in, the upper end of the cited post-setting guide's embedment range. Enter that dimension in the frost-depth field to compare a deeper column; this is a sensitivity example, not a frost-depth claim for any location. In the same example hole, the model returns 42 in total excavation and about 2.10 cu ft of concrete, or 6 bags per hole. Ask the local building department what depth applies to your project and how it must be measured.

How to measure the inputs

Measure fence height above the finished ground. Measure the actual post cross-section instead of entering its nominal lumber name. Choose square for a square post and round for a round post; a round-post input is the outside diameter. Use consistent units: the height field takes feet, while post width, hole diameter and frost depth take inches.

Get bag yield from the concrete you will buy. Bag weight alone does not establish the yield of every product. A diameter entered into the tool assumes a circular hole of that size throughout the concrete column; irregular or widened excavations need an adjusted volume takeoff.

Check bag rounding across the project

This tool rounds each hole up separately. That makes a per-hole bag count easy to plan, but multiplying it by every post is not the same as adding all the exact volumes and rounding the entire job once. Count the actual holes by footing type, then check the purchase and mixing plan with your supplier. Gate supports may use a different detail from ordinary line posts.

When this result does not establish a footing design

A volume result does not prove that the fence can resist wind or support a gate. Use the fence-system instructions and the approved local detail for the final depth, width and post length. Retaining structures and other load-bearing work need their own design. A frost entry only changes a dimension in this model; it does not check the design against local requirements.

Before excavation, follow the utility-locate instructions for your state and confirm that the locate request is complete. A marked boundary or a previous fence line is not a utility clearance.