How to use the post hole concrete calculator
- Enter the number of posts that share the same hole size and choose whether you are measuring in inches or centimeters.
- Enter the hole diameter and hole depth, then pick a square or round post and enter its actual width. Add a gravel base depth if the post sits on gravel.
- Keep the 10% waste or change it. Open More options if your concrete brand lists different bag yields.
- Read the cubic feet of concrete and the number of 80, 60, 50 and 40 lb bags, then copy the shopping list.
Formula
Concrete depth = hole depth - gravel base depth
Hole volume = 3.14159 x (hole diameter / 2)^2 x concrete depth
Post volume = width^2 x concrete depth (square) or 3.14159 x (diameter / 2)^2 x concrete depth (round)
Concrete per hole = (hole volume - post volume) / 1,728 cubic feet
Total with waste = concrete per hole x posts x (1 + waste percent / 100)
Bags = total with waste / yield per bag, rounded up
There are 1,728 cubic inches in a cubic foot. The post is subtracted because the wood or metal takes up room that concrete does not have to fill. A square post must fit across its corners, so the tool checks that its diagonal is smaller than the hole.
Worked example: six 4x4 posts in 10 inch holes
Each hole is 30 inches deep with a 4 inch gravel base, so the concrete is 26 inches deep. The hole holds 3.14159 x 5^2 x 26 = 2,042.04 cubic inches.
A 4x4 post measures about 3.5 inches square, so it takes up 3.5^2 x 26 = 318.5 cubic inches. The concrete per hole is 1,723.54 / 1,728 = 0.997 cubic feet.
Six posts need 5.98 cubic feet. With 10% waste that is 6.58 cubic feet, or 0.244 cubic yards.
With typical yields, that is 6.58 / 0.60 = 10.97, so 11 bags of 80 lb mix. In 60 lb bags (0.45 cubic feet) it is 14.63, so 15 bags. The gravel for six holes comes to about 1.09 cubic feet.
Why subtract the post
A hole filled with concrete before the post goes in holds the full cylinder volume. Once the post is standing in it, the post displaces part of that volume. For a small hole around a thick post the difference is large, and in the example below ignoring it would overstate the concrete by about 16 percent. This tool takes the displaced volume out for the depth that is filled with concrete.
If you pour concrete after setting the post, the volume works the same way. If you plan to leave the top few inches for soil or a crowned cap, reduce the hole depth by that amount.
Bag yields and bag counts
Bagged concrete is sold by weight, but your hole is measured by volume, so the yield per bag links the two. Typical yields are about 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb. Real yields differ by brand, and the number on the label wins over the typical value, so edit it under More options.
Bags are always rounded up to a whole bag. For a single post it is often cheaper to buy the next size down and one extra bag than to carry a large pile of surplus mix.
What this tool does not decide
How deep a post must go, how wide the hole should be and whether concrete is the right method depend on the structure, the soil, the frost depth in your area and your local code. A deck post that carries a roof has different needs than a mailbox post. Check your plans or your building department for those numbers and enter them here to size the order.
Assumptions and limits
- The hole is a straight cylinder with the diameter you enter. Bell-shaped or uneven holes hold more concrete and are not modeled.
- The post is centered, runs the full depth of the concrete and is a perfect square or circle in cross section.
- Bag yields are typical values that you can edit. They vary by brand and product.
- The waste allowance is yours to set. Spills and over-dug holes are the usual reasons for extra mix.
- Footing size, post depth, frost depth and code requirements are not checked.
Frequently asked questions
How many bags of concrete do I need for a fence post?
Work out the volume of the hole minus the post, then divide by the yield of your bag. A 10 inch by 26 inch hole around a 4x4 post needs about 1 cubic foot, which is roughly two 80 lb bags at typical yields.
How wide should a post hole be?
A common rule of thumb is a hole about three times the width of the post, but the right size depends on the structure and your local code. Enter the size from your plans.
What is the gravel base for?
Many builders put a few inches of gravel at the bottom of the hole so water can drain away from the post end. The tool subtracts that depth from the concrete and lists the gravel volume separately.
Does a square post really need to fit corner to corner?
Yes. A 3.5 inch square post measures about 4.95 inches across its corners, so the hole must be wider than that. The tool warns you when the post cannot fit.
How many cubic feet is one 80 lb bag of concrete?
Many brands list about 0.60 cubic feet per 80 lb bag, but check the label. The yield box under More options lets you enter your own figure.