Fence post concrete calculator: bags per post

Quick answer

A 10 in round hole, 30 in deep, with 6 in of gravel and a 4×4 post takes about 0.92 cu ft of concrete. With 10 percent extra that is 2.7 bags of 50 lb fast-setting mix (0.375 cu ft each, as stated by Quikrete), so plan on 3 bags for a single post. Sakrete states 3 bags of its 50 lb fast-setting mix for a 10 in hole 30 in deep.

What this calculator does

It calculates the concrete for a row of identical post holes. The hole can be round or square. The post is subtracted, because the concrete only fills the space around it, and so is a layer of gravel at the bottom of the hole. The result is the number of bags for one post, for all posts, and the volume of concrete and gravel.

Calculators disagree about bags per post mostly because some subtract the post and some do not. This one shows the volume it subtracted, so you can see the difference. Choose "Do not subtract the post" to get the volume of the empty hole.

It does not tell you how deep or wide a hole must be for your fence. That depends on the height of the fence, the wind, the soil and the frost depth where you live.

How to use it

1. Choose metric or US units. In US units you can type fractions, such as 10 1/2, and feet and inches, such as 2' 6".
2. Choose a round or square hole and enter its diameter or width and its full depth.
3. Enter the depth of gravel you put at the bottom, or 0 if the post stands on the soil.
4. Enter the number of posts.
5. Choose the post. A US 4×4 is 3 1/2 in square and a 6×6 is 5 1/2 in square. For other posts, correct the size.
6. Choose the bag. This fills in the yield stated by a manufacturer for that bag; check the yield printed on the bag you buy.
7. Set the allowance and select Calculate.

How the result is calculated

1. Depth of concrete = hole depth − gravel.
2. Area of a round hole = π × (diameter ÷ 2)²; of a square hole = width × width. The same for a round or square post.
3. Concrete per post = (area of the hole − area of the post) × depth of concrete × (1 + allowance ÷ 100).
4. Bags per post = concrete per post ÷ yield of one bag, rounded up to one decimal.
5. Bags for all posts = concrete per post × number of posts ÷ yield of one bag, rounded up to a whole bag.
6. Gravel = area of the hole × depth of gravel × number of posts, without allowance.

One litre is 1,000,000 mm³. One cubic foot is 1728 cubic inches or 28.32 litres. The post is subtracted over the depth of concrete only, because it stands on the gravel.

A square post turned in a round hole needs a hole wider than its diagonal, which is 1.41 times its width. The calculator refuses a post that does not fit.

Worked example

Metric: 10 posts of 100 mm square in round holes of 250 mm diameter and 700 mm depth, with 100 mm of gravel, and 20 kg bags that make 9.3 L each. Depth of concrete: 700 − 100 = 600 mm. Hole: 3.14159 × 125² = 49,087 mm². Post: 100 × 100 = 10,000 mm². Concrete per post: (49,087 − 10,000) × 600 = 23.45 L, with 10 percent extra 25.8 L.

Bags per post: 25.8 ÷ 9.3 = 2.77, shown as 2.8. For 10 posts: 258 L of concrete, 258 ÷ 9.3 = 27.7, rounded up to 28 bags. The posts save 10 × 10,000 × 600 mm³ = 60 L. Gravel: 10 × 49,087 × 100 mm³ = 49.1 L.

US units: 10 posts, 4×4 (3 1/2 in square), in round holes of 10 in diameter and 30 in depth, with 6 in of gravel, and 50 lb fast-setting bags of 0.375 cu ft. Depth of concrete: 24 in. Hole: 3.14159 × 5² = 78.54 sq in; post: 3.5 × 3.5 = 12.25 sq in. Concrete per post: 66.29 × 24 = 1591 cubic inches = 0.921 cu ft, with 10 percent extra 1.013 cu ft.

Bags per post: 1.013 ÷ 0.375 = 2.70, shown rounded up as 2.8. For 10 posts: 10.13 cu ft, 10.13 ÷ 0.375 = 27.01, rounded up to 28 bags. The posts save 1.7 cu ft, and the gravel is 2.73 cu ft.

Bag sizes by country

Bags are sold by weight, but the calculator needs the yield: the volume of concrete one bag makes. The presets use the yields that these manufacturers state. Fast-setting mixes are listed separately, because their yield and their instructions differ from standard concrete.

Other 20 kg bags in Australia state between 9 L and 11 L. UK post mixes such as Blue Circle Postcrete and Hanson Postfix are sold by the post rather than by volume: Hanson states one bag per post, and the Postcrete data sheet has a table of bags by hole size. For those products, follow the table on the bag.

Hole size rules of thumb

Manufacturers give simple rules for the hole, and they do not agree exactly. Quikrete, Sakrete and the Blue Circle Postcrete data sheet say the hole should be about three times the width of the post. Easy Mix in Australia says twice the diameter of the post.

For depth, the Quikrete Fast-Setting data sheet says one third of the overall post length. Its project guide adds 6 in for a gravel base, Sakrete recommends 4 to 6 in of compacted gravel, and the Postcrete sheet says one third of the fence height above ground. Where the ground freezes, local practice or the building department may require deeper holes.

The examples use a 10 in hole for a 4×4 and a 250 mm hole for a 100 mm post. They show the calculation and are not a recommendation for your fence.

Common mistakes

• Using the nominal size of a US post. A 4×4 is 3 1/2 in square, not 4 in.
• Forgetting the gravel. Six inches of gravel in a 30 in hole is one fifth of the depth that does not need concrete.
• Rounding the bags for each post before multiplying. Buy for the total: 10 posts at 2.70 bags each need 28 bags (27.01 rounded up), not 10 × 3 = 30.
• Mixing up bag weight and yield. A 50 lb bag and a 20 kg bag make different volumes, and fast-setting and standard mixes of the same weight may differ too.
• Digging holes wider at the bottom than at the top without measuring. The calculator assumes straight sides; measure the diameter at mid-depth if the hole is uneven.