Post Hole Concrete Calculator

1. Pick the post shape and enter the actual post width or diameter in inches.
2. Enter the hole diameter, or leave it blank to use the 3 x post rule, and the total hole depth.
3. Set the gravel base and any gap left at the top, then the number of posts.
4. Add the overall post length if you want a depth check, choose the mix and bag, and set a waste allowance.

Here is what each result line means:

A post hole concrete calculator estimates the bagged concrete needed to set posts in round holes, such as fence posts, mailbox posts, pergola legs and deck posts. Instead of treating the hole as an empty cylinder, it subtracts the space the post itself takes up, because you only buy concrete for the ring around the post.

It also builds in the hole-size guidance from the QUIKRETE setting-posts instructions. A hole three times the post diameter, about a third of the post buried and 6 inches of gravel underneath are the starting points it checks your entries against. If you plan to use a rapid-set product throughout, the fast-setting concrete calculator is built around that one product, while this tool lets you compare it with standard concrete mix bags.

A dug post hole is close to a cylinder. The concrete fills the cylinder from the top of the gravel base to the top of the pour, minus the column of space the post occupies. Everything is measured in inches, so the volume comes out in cubic inches and is divided by 1,728 to give cubic feet.

1. Find the hole area: pi times the hole diameter squared, divided by 4.
2. Find the post area: width times width for a square post, or pi times the diameter squared over 4 for a round one, and subtract it.
3. Find the concrete height: hole depth minus the gravel base minus any gap left at the top, and multiply.
4. Divide by 1,728 for cubic feet, multiply by the number of posts, add waste and divide by the bag yield, rounding up.

The bag yields are the published figures: 0.60, 0.45 and 0.30 cubic feet for 80, 60 and 40 lb bags of QUIKRETE Concrete Mix, and about 0.375 cubic feet for a 50 lb bag of Fast-Setting Concrete Mix. For bag counts on slabs and other shapes, the concrete bag calculator uses the same yields.

Suppose you are setting 10 fence posts that measure 3.5 inches square and are 8 feet long overall, in 12 inch holes 38 inches deep. Each hole gets 6 inches of gravel, and the Fast-Setting mix stops 4 inches below the top, with 10 percent waste.

Calculation: the hole area is pi x 12 x 12 / 4 = 113.10 sq in and the post area is 3.5 x 3.5 = 12.25 sq in, leaving 100.85 sq in of concrete. The concrete height is 38 – 6 – 4 = 28 in, so each hole holds 100.85 x 28 = 2,823.7 cubic inches, or 2,823.7 / 1,728 = 1.634 cu ft. Ten posts are about 16.342 cubic feet, and 10 percent waste makes 17.98 cu ft, about 0.67 cubic yards. At 0.375 cubic feet per bag that is 47.9, rounded up to 48 bags, or about 4.8 bags per post and about 48 gallons of water.

Both QUIKRETE checks pass: the 3 x post guide calls for a 10.5 inch hole, and one third of 96 inches plus 6 is a 38 inch depth. The lesson: subtracting the post saves about a tenth of the concrete here, and the gap at the top and the gravel base matter just as much as the hole width.

Hole Diameter

The hole area grows with the square of the diameter, so a slightly wider auger adds a lot. Going from a 10 to a 12 inch hole raises the area by about 44 percent before the post is subtracted.

Concrete Height

Only the part of the hole that gets concrete counts. A deeper hole adds concrete in a straight line, while the gravel base and any gap at the top take depth away.

Post Size and Shape

A bigger post fills more of the hole and needs less concrete around it. A square post of a given width takes up more area than a round post of the same diameter, by a factor of 4 divided by pi.

Bag Type

Bag yields differ by product and size, so the same hole takes a different number of bags of Fast-Setting mix than of standard concrete mix.

How the Holes Are Dug

Dug holes may not be perfect cylinders and can end up wider or deeper than planned. A waste allowance covers some of this. For a formed, above-ground pier or column instead of a dug hole, the concrete column calculator is the better fit.

Both come in bags. The QUIKRETE Setting Posts instructions are written for Fast-Setting Concrete Mix; for standard Concrete Mix, follow the directions on the bag. This tool counts bags for either.

The example row uses the 17.98 cubic feet worked out above: 17.98 / 0.375 rounds up to 48 bags, and 17.98 / 0.60 rounds up to 30 bags. Follow the directions printed on whichever product you buy.

Building a Fence

Count the posts, enter one typical hole and buy the bags for the whole run in one trip, with a few spares for the corner and gate posts if they are larger.

Sizing the Holes Before You Dig

Leave the hole diameter blank to see the 3 x post guide, and enter the post length to see the suggested depth before you rent an auger.

Setting Deck, Pergola or Mailbox Posts

Compare bag counts for a few hole sizes. For deck and other structural posts, QUIKRETE says the hole must reach at least 6 inches below the frost line, and local code may set other requirements.

Choosing a Bag Size

Switch between the bag options to see which leaves the fewest part bags.

1. Forgetting to Subtract the Post

Treating the hole as empty over-counts the concrete. The tool removes the post area over the full concrete height.

2. Using the Nominal Post Size

Lumber and tube sizes are names, not always measurements. Measure the post and enter the real width or diameter.

3. Counting the Gravel as Concrete

The gravel base sits under the post and takes up depth. Enter it so the concrete height is not overstated.

4. Digging the Hole Too Narrow

A tight hole uses fewer bags but leaves a thin ring of concrete. The 3 x post guide gives a fuller collar around the post.

5. Skipping the Frost Line for Structural Posts

For decks and other structural posts the hole needs to go at least 6 inches below the frost line. Your code official sets the local frost depth, not this tool.

The geometry is exact for a straight round hole and a straight post; dug holes are rougher.

Find the hole area as pi times the diameter squared over 4, subtract the post area, multiply by the depth of concrete and divide by 1,728 for cubic feet. A 3.5 inch square post in a 12 inch hole with 28 inches of concrete takes about 1.63 cubic feet.

It depends on the hole, the post and the bag. A 3.5 inch square post in a 12 inch hole with 28 inches of concrete takes about 4.8 bags of 50 lb Fast-Setting mix with 10 percent waste, or about 3 bags of 80 lb Concrete Mix.

QUIKRETE says the hole diameter should be 3 times the post diameter. For a 4 inch round post that is a 12 inch hole. This tool applies the same rule to the width of a square post.

QUIKRETE says the hole depth should be one third of the overall post length, plus 6 inches for a gravel base. An 8 foot post gives 32 plus 6, or 38 inches.

For decking and other structural support posts, QUIKRETE says the hole must extend at least 6 inches below the frost line. Your local code official sets the frost depth and any other requirements.

QUIKRETE instructions place 6 inches of gravel or crushed stone in the hole to aid drainage. Enter the gravel depth here so it is not counted as concrete.

For Fast-Setting Concrete Mix, QUIKRETE pours the dry mix until it reaches 3 to 4 inches from the top of the hole. Enter that gap at the top to leave it out of the bag count.

QUIKRETE says about 1 gallon of water for each 50 lb bag of Fast-Setting Concrete Mix placed in the hole, so 48 bags need about 48 gallons.

The post takes up part of the hole, so that space never gets concrete. In the 12 inch hole example, the post fills about a tenth of the hole area, which is a tenth of the bags.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE project instructions: Setting Posts (PDF) (QUIKRETE; hole diameter, hole depth, gravel base, frost line and Fast-Setting fill and water guidance).
• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft).
• QUIKRETE Fast-Setting Concrete Mix product page (QUIKRETE; product information).