Fence Post Concrete Calculator

HomeMath takes the gravel off the bottom of the hole first: the concrete fills the rest, the hole's diameter × its depth above the gravel, π × radius² × depth. The post stands in that concrete, so HomeMath takes out its cross-section × the same depth, at the post's actual size (a 4x4 measures 3.5 × 3.5 in, a 6x6 5.5 × 5.5 in). The waste allowance (10% to start) covers rough hole sides. That is the concrete per post; × the number of posts gives the fence. Bags are the volume ÷ each bag's yield: 0.30, 0.45 and 0.60 cu ft for the 40, 60 and 80 lb mix bags and 0.375 cu ft for a 50 lb bag of fast-setting concrete, rounded up for the whole fence and shown to one decimal per post. The gravel is π × radius² × its depth in each hole. In metric, HomeMath converts meters and centimeters exactly (1 in = 2.54 cm) and shows the result in cubic meters, liters and square meters.

The published rules it uses: US customary units; metric conversion; premixed bag yields; post holes, fast-setting bags and tube forms. Their sources, and how HomeMath tests every result, are on the about page.

Quikrete's setting-posts sheet sizes the hole from the post: 3 times its width, and 1/3 of its overall length deep plus 6 in for the gravel. The table follows that rule for 4x4 and 6x6 posts of 6, 8 and 10 ft, with 10% waste. Lumber is sold by its nominal size, so a 4x4 measures 3.5 × 3.5 in and a 6x6 5.5 × 5.5 in; the calculator takes those sizes out of the hole.

Many fences use a narrower hole than the rule, such as 10 in for a 4x4, which takes less concrete. Your local code, the fence maker's instructions and the frost depth where you live set the real depth.

Per post, it comes down to the hole. The table gives one 4x4 post by the hole's diameter and the depth of concrete above the gravel, before waste. Each bag size yields its own volume: 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.375 for a 50 lb bag of fast-setting concrete and 0.30 for 40 lb.

Count the bags for the whole fence, not post by post: what is left of a bag after one hole goes into the next. 20 posts of 4x4 in 10 in holes 3 ft deep take 0.94 cu yd of concrete, 43 bags of 80 lb or 68 bags of 50 lb fast-setting mix. 20 posts at 2.1 bags of 80 lb each are 43 bags, not 60.

The hole matters far more than the post: the concrete grows with the square of the hole's diameter, so a 12 in hole holds 1.44 times as much as a 10 in one.

Quikrete's Fast-Setting Concrete data sheet says two 50 lb bags set a 4 in post in a 10 in hole 2 ft deep. HomeMath's math for that hole gives 2.4 bags filled to the top and 2.0 filled to 4 in below the top, where Quikrete's setting-posts sheet stops the mix and finishes with soil.

• Width: 3 times the post's width, by Quikrete, and larger, dish-shaped holes in loose or sandy soil.
• Depth: 1/3 of the post's overall length, plus 6 in for the gravel. A post that carries a structure, such as a deck, goes at least 6 in below the frost line.
• Gravel: tamp the bottom and sides firm, then put 6 in of gravel or crushed stone in the hole for drainage.
• The top: Quikrete stops the mix 3 to 4 in below the top and fills the rest with soil. The calculator fills to the top; to match, enter the depth to where the concrete stops.

These are the product maker's rules of thumb for a fence; the calculator measures the concrete for whatever hole you dig.

Two kinds of bag set a post. The calculator counts both, so you can compare them before you buy.

• Fast-setting concrete goes into the hole dry, around the braced post, and is soaked with about 1 gallon of water per 50 lb bag. Quikrete's data sheet says it sets in 20 to 40 minutes and takes heavy loads after 4 hours, and that holes deeper than 2 1/2 ft are filled in lifts so the water soaks all the way through. A 50 lb bag yields about 0.375 cu ft.
• Regular concrete mix in 40, 60 or 80 lb bags is mixed with water first, in a wheelbarrow or a mixer, then shoveled in. It yields more per pound (0.60 cu ft per 80 lb bag), so a long fence needs fewer pounds of it, at the cost of mixing.

For 20 posts in 10 in holes 3 ft deep, that is 68 bags of 50 lb fast-setting (3,400 lb) or 43 bags of 80 lb mix (3,440 lb).

Take a 4x4 post in a 10 in hole 3 ft deep with 6 in of gravel. Work in feet.

1. Depth of concrete: 3 − 6/12 = 2.50 ft.
2. Radius: 10 ÷ 2 = 5 in, or 0.417 ft.
3. Hole: 3.1416 × 0.417² × 2.50 = 1.36 cu ft.
4. Post: 3.5 × 3.5 = 12.25 sq in, ÷ 144 × 2.50 = 0.21 cu ft.
5. Concrete: 1.36 cu ft − 0.21 cu ft = 1.15 cu ft, or 1.27 cu ft with 10% waste.
6. Bags: 1.27 cu ft ÷ 0.60 = 2.1 bags of 80 lb, or ÷ 0.375 = 3.4 bags of 50 lb fast-setting.

For a fence, multiply the concrete by the number of posts before you divide by the bag's yield, then round up once. The concrete calculator's column tab gives the same hole without the post.

Yes: Quikrete puts 6 in of gravel or crushed stone in the tamped hole to drain water away from the post's end. The post stands on the gravel and the concrete fills around it, so 6 in under a 10 in hole is 0.27 cu ft.

That density is the middle of Inch Calculator's range. For a large job, the gravel calculator prices it by the ton or the yard.

Switch the units to metric and the hole and gravel take centimeters, the depth meters or centimeters, and a round post its diameter in centimeters; a 4x4 shows as 89 mm and a 6x6 as 140 mm. A 25 cm hole 90 cm deep on 15 cm of gravel takes 34 L per 4x4 post with 10% waste. Bags sold by the kilogram differ by brand, so type your bag's weight and its yield in liters from the label, and the calculator counts them too.

A mailbox post, a lamp post or a swing set takes the same math. A deck post or pier poured in a cardboard tube form is a column, sold by the tube's diameter: use the sonotube calculator, which adds a footing. To price bags against a ready-mix truck for a long fence, use the concrete cost calculator.