How to use the fence post concrete calculator
- Choose the post. Pick a square post, a round post, or a hole without a post if you only want the volume of the hole.
- Enter the hole and the post. Type the diameter of the hole, the depth of the concrete in the hole, and the real width or diameter of the post. Then type the number of posts.
- Read the bags. The result shows the bags to buy in the chosen size for all the posts, the bags in the other sizes and the volume of concrete.
What is a fence post concrete calculator?
A fence post concrete calculator is a tool that finds how much concrete it takes to set posts in the ground. It works out the volume of a round hole, takes away the space the post fills, and multiplies by the number of posts. The answer is in bags, because posts are almost always set with bagged concrete.
Use it for fence posts, gate posts, mailbox posts, deck posts set in the ground and sign posts.
Features
- Bags of concrete for one post hole or for a whole fence
- Square posts and round posts, with the post taken out of the volume
- A hole without a post, for the full volume of the hole
- Bag counts for 40 lb, 50 lb, 60 lb and 80 lb bags
- Volume in cubic feet, cubic yards and cubic metres
- An adjustable waste allowance and a cost from your own price per bag
- A live 3D preview, a PDF of the estimate and a WhatsApp share button
What you can use it for
- A new wood or metal fence, post by post
- Gate posts, which often sit in a wider hole
- Mailbox posts, basketball hoop posts and clothesline posts
- Deck and pergola posts set straight in the ground
How post hole concrete is calculated
| Post | Concrete for one post |
|---|---|
| Square post | (π × (hole diameter ÷ 2)² − post width²) × depth of concrete |
| Round post | π × ((hole diameter ÷ 2)² − (post diameter ÷ 2)²) × depth of concrete |
| No post | π × (hole diameter ÷ 2)² × depth of concrete |
Convert every dimension to feet first. The answer is in cubic feet. Multiply by the number of posts, add waste, and divide by the yield of one bag.
Worked example: a 4×4 post in a 12 inch hole
- Hole: 12 in across, so the radius is 0.5 ft. Area: 3.1416 × 0.5² = 0.785 square feet.
- Post: a 4×4 post is 3.5 in wide, which is 0.292 ft. Area: 0.292² = 0.085 square feet.
- Concrete depth: 24 in = 2 ft. Volume: (0.785 − 0.085) × 2 = 1.40 cubic feet.
- With 10% waste: 1.54 cubic feet.
- 80 lb bags at 0.60 cubic feet each: 1.54 ÷ 0.60 = 2.6, so 3 bags for one post.
- For 10 posts: 15.41 cubic feet, so 26 bags of 80 lb mix.
Ten posts need 26 bags, not 30, because the bags are counted for the whole job and only the last bag is rounded up.
How wide and how deep to dig the hole
These sizes come from Quikrete's published instructions for setting posts in concrete. Where the ground freezes, check the post depth your local code requires.
- Hole diameter: three times the width of the post.
- Hole depth: one third to one half of the above-ground length of the post, plus 6 inches.
- Put 6 inches of gravel or crushed stone in the bottom of the hole.
- Fill with concrete up to 3 to 4 inches below ground level.
For a post that stands 6 feet above the ground, one third to one half is 24 to 36 inches, so the hole is 30 to 42 inches deep. Take away the 6 inches of gravel and the 3 to 4 inches left empty at the top, and the depth of concrete to enter is about 20 to 33 inches.
Enter the depth of the concrete, not the depth of the hole. Your local building department can require a deeper hole, for example below the frost line. This site has no figure for that.
What the calculator includes
- The hole is treated as a round cylinder with straight sides. A hole that is wider at the bottom takes more concrete.
- Use the real size of the post. The American Softwood Lumber Standard (PS 20-25) gives 3 1/2 inches as the dry dressed size of a nominal 4 inch piece, so a 4×4 post is 3.5 inches wide.
- The post must be smaller than the hole. A square post must fit corner to corner.
- Bag counts use the yields of Quikrete Concrete Mix (product 1101 data sheet, revised October 2022). Quikrete's Fast-Setting Concrete Mix (product 1004) data sheet gives the same yield for a 50 lb bag (0.375 cubic feet) and a 60 lb bag (0.45 cubic feet).
- Each dimension can be up to 10,000 in its unit, the number of posts up to 1,000 and waste 0 to 50%.
Is it private?
Yes. The calculation and the PDF are made in your browser. The numbers you type are not sent to our server and are not saved.
Tips for an accurate estimate
- Dig every hole to the same depth so the bag count per hole is the same.
- Put a gravel base in the bottom of the hole and enter only the depth the concrete will fill.
- Enter the real post size: a nominal 4 × 4 post is smaller than 4 inches.
- Fast-setting mix saves time on many posts; check the yield printed on the bag.
- Check local rules for post depth, especially where the ground freezes.
Related calculators
Frequently asked questions
- How much concrete do I need for a fence post?
- A 4×4 post in a hole 12 inches across with 24 inches of concrete needs 1.40 cubic feet. With 10% waste that is 3 bags of 80 lb concrete mix, or 5 bags of 50 lb mix.
- How many bags of concrete do I need per fence post?
- It depends on the hole. For a 12 inch hole with 24 inches of concrete around a 4×4 post, one post needs 3 bags of 80 lb mix, 4 bags of 60 lb mix or 5 bags of 50 lb mix, with 10% waste.
- How many bags of concrete do I need for 10 fence posts?
- With a 12 inch hole, 24 inches of concrete and 4×4 posts, 10 posts need 15.41 cubic feet with 10% waste. That is 26 bags of 80 lb mix or 42 bags of 50 lb mix.
- How wide should a fence post hole be?
- Quikrete's instructions for setting posts say to make the hole three times the width of the post. For a nominal 4 inch post that is about 12 inches.
- How deep should a fence post hole be?
- Quikrete's instructions say one third to one half of the above-ground length of the post, plus 6 inches for a gravel base. For a post 6 feet above the ground that is 30 to 42 inches. Local rules can require more.
- Does the calculator subtract the post?
- Yes. Choose square post or round post and the volume of the post inside the concrete is taken out. Choose hole without a post to get the full volume of the hole.
Sources
Every figure this calculator uses comes from the published sources below. The formulas are on this page, so any result can be checked by hand.
- Quikrete, How to set posts in concrete: instructions. Hole width of three times the post and hole depth of one third to one half of the post above ground, plus 6 in of gravel.
- Quikrete, Concrete Mix No. 1101: data sheet and product page. Bag yields: 0.30 ft³ (40 lb), 0.375 ft³ (50 lb), 0.45 ft³ (60 lb) and 0.60 ft³ (80 lb); bags per pallet: 80, 56 and 42.
- Quikrete, Fast-Setting Concrete Mix No. 1004: data sheet. Yields of the 50 lb and 60 lb fast-setting bags.
- National Institute of Standards and Technology (NIST), PS 20-25, American Softwood Lumber Standard. A nominal 4 × 4 post measures 3 1/2 in.
- Federal Highway Administration (FHWA), Report FHWA-HRT-05-057. Normal-weight concrete: 145 lb per cubic foot (1.96 tons per cubic yard).