How many bags of concrete do I need per fence post?

For a 4 × 4 post in a round hole 10 in across and 24 in deep, about 0.92 cu ft of concrete, which is 2.5 bags of 50 lb fast-setting mix (0.375 cu ft each) or 1.5 bags of 80 lb concrete mix (0.6 cu ft each). Plan on about 2 to 3 bags of 50 lb mix for a typical post. Bigger or deeper holes take more: the table above lists common sizes.

Do I subtract the post from the hole volume?

Yes. The post fills the middle of the hole, so the concrete is only what is left around it. In a hole 10 in across and 24 in deep, the post takes 0.17 cu ft of the 1.09 cu ft hole. A 6 × 6 post (5.5 in) takes about 2.5 times as much as a 4 × 4, so leaving the post out overstates the bags for big posts. This calculator subtracts it for you.

How big should the hole be?

A common rule of thumb, which Quikrete’s setting-posts instructions give, is a hole about 3 times the width of the post: roughly 10 to 12 in for a 4 × 4 and about 16 to 18 in for a 6 × 6. It is guidance, not a code requirement, and a wider hole takes more concrete. Your local code, soil and frost depth may call for something different.

How deep should a fence post hole be?

The usual guidance is about 1/3 to 1/2 of the post height above ground (a 6 ft fence needs at least about 2 ft in the ground), and below the frost line where your code requires it. Quikrete also describes about 1/3 of the overall post length plus 6 in of gravel for drainage. Use the rule-of-thumb table on this page as a starting point, then check local code.

Is fast-setting concrete better for fence posts?

Fast-setting mix is popular for posts because you pour it dry into the hole around a plumb post and add water, with no mixing; Quikrete says it sets hard in about 20 to 40 minutes. A 50 lb bag makes about 0.375 cu ft, so it covers less than an 80 lb bag of regular mix (about 0.60 cu ft) and costs more per cubic foot, so for many posts regular mix is often cheaper if you are willing to mix it. Pick the bag you plan to buy in the calculator and the count follows its yield.

How many 80 lb bags for a 4 × 4 post?

About 1.5 of an 80 lb bag per post for a 10 × 24 in hole, so a bit more than one bag for every two posts: 12 posts take 21 bags with 10% waste. Round up once for the whole job, not for each post, since leftover concrete from one hole fills the next.

Do I need gravel under the post?

Quikrete’s instructions for setting posts put about 6 in of gravel in the bottom of the hole for drainage. The gravel replaces some of the depth, so a hole dug for a base is that much deeper, and the concrete above it is shorter. Enter the depth of the concrete itself as the hole depth, and use the gravel calculator for the base.

How many posts do I need for my fence?

Divide the fence length by the post spacing, round up and add one, plus one per gate. The fence calculator does it for you and also counts pickets, rails and concrete; enter the number of posts it gives here to compare bag sizes and hole sizes.

Fence Post Concrete Calculator: How Many Bags Per Post?

Setting 12 fence posts (4 × 4) in round holes 10 in across and 24 in deep takes 33 bags of 50 lb fast-setting mix, with 10% extra. Each hole holds 0.92 cu ft of concrete once the post's volume is taken out, so 11 cu ft for the job (12.2 cu ft with waste), or about 2.7 bags per post. A 50 lb fast-setting bag makes about 0.375 cu ft; an 80 lb bag of concrete mix makes about 0.6 cu ft.

Figures based on the sources listed belowUpdated 2026-10-07How we calculate

Hole shape

Hole diameter (in)

Rule of thumb: about 3 times the post width, so 10 to 12 in for a 4 × 4 post.

Hole depth (in)

Rule of thumb: about 1/3 to 1/2 of the post height above ground, and below the frost line where your code requires. The concrete is poured to the top of the hole.

Waste allowance

Standard: ragged holes and spills (10%)Tight, neat holes (5%)Exact (0%)Custom

Number of posts

Every post that gets its own hole, gate and corner posts included. The fence calculator counts them from the fence length.

Post sizeThe post fills the middle of the hole, so its volume is subtracted. Nominal lumber sizes are smaller than their names.

Bag sizeFast-setting mix is poured dry into the hole and watered; regular mix is mixed first. The bag count follows the yield of the bag you pick.

Price per bag (optional)

Buy 33 bags

0.41 cu yd + 10% waste = 0.45 cu yd

Concrete per hole (hole less the post): 0.92 cu ft
Concrete for 12 posts (with waste): 12.2 cu ft
Bags per post: 2.7 bags (waste included)

Low

$231.00

Expected

$330.00

High

$495.00

Includes 0.04 cu yd waste

How this number is calculated

0.41 cu yd + 10% waste = 0.45 cu yd ÷ 0.0139 cu yd per bag = 33 bags

0.5 cu yd — ~5% of a standard truck load

Bagged vs. Ready-mix delivery

Bagged (80 lb bags)

21 bags

$94.50–$168.00 (typ. $126.00)

Ready-mix delivery

0.5 cu yd

$123.02–$150.02 (typ. $134.27)

For ~0.5 cu yd, bagged and ready-mix cost about the same — pick whichever is more convenient.

Shopping list

Fast-setting concrete mix (50 lb)
Post hole digger
Post level
Gravel for post bases
Mixing tub
Earth auger for post holes

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Concrete per hole and bags per post, by hole size

Round holes around a 4 × 4 post (3.5 in), one post, no waste, worked by this calculator. Bags are shown to one decimal before rounding up; for a job, add the posts first and round up once.
Hole (diameter × depth)	Concrete per hole (cu ft)	50 lb fast-setting bags	60 lb bags	80 lb bags
8 in × 18 in	0.4	1.1	0.9	0.7
10 in × 24 in	0.92	2.5	2	1.5
12 in × 24 in	1.4	3.7	3.1	2.3
12 in × 30 in	1.75	4.7	3.9	2.9
12 in × 36 in	2.1	5.6	4.7	3.5
14 in × 36 in	2.95	7.9	6.6	4.9

Whole bags for 4 to 100 posts, by bag size

4 × 4 posts in round holes 10 in across and 24 in deep, 10% waste, rounded up once for the job, worked by this calculator.
Posts	50 lb fast-setting bags	80 lb bags	60 lb bags	40 lb bags
4	11	7	10	14
10	28	17	23	34
20	55	34	46	68
50	136	85	113	169
100	271	169	226	338

Rule of thumb for hole depth (guidance)

Quikrete’s guidance is a depth of about 1/3 to 1/2 of the post height above ground. The full post length adds the depth to the height above ground. Treat these as an estimate: frost depth, soil and local code can call for more.
Post height above ground	Hole depth, 1/3 to 1/2	Post length to buy (at least)
3 ft	12 to 18 in	4 to 4.5 ft
4 ft	16 to 24 in	5.3 to 6 ft
5 ft	20 to 30 in	6.7 to 7.5 ft
6 ft	24 to 36 in	8 to 9 ft
8 ft	32 to 48 in	10.7 to 12 ft

Methodology & assumptions

Each hole is a round (or square) prism: its footprint × its depth. The post stands in the middle of it, so the concrete per hole is the hole's volume less the post's volume over the same depth: a 4 × 4 is 3.5 × 3.5 in, a 6 × 6 is 5.5 × 5.5 in, and a round post is π × diameter² ÷ 4. For the default hole, 10 in across and 24 in deep, that is 1.09 cu ft less 0.17 cu ft for the post, or 0.92 cu ft. The job's concrete is that times the number of posts, plus the waste you choose (10% by default, for ragged holes and spills), divided by the bag yield and rounded up once for the whole job. Bag yields are the manufacturer's published figures: 0.375 cu ft for a 50 lb bag of Quikrete Fast-Setting Concrete Mix and 0.6, 0.45 and 0.3 cu ft for 80, 60 and 40 lb bags of Quikrete Concrete Mix; other brands, and high-yield or lightweight mixes, differ, so check your bag. The post is assumed to run the full depth of the hole and the concrete to fill it to the top; a gravel base under the post, and any concrete crowned above the ground to shed water, are not counted. A 6 in gravel base is a common recommendation, so a hole dug for one is that much deeper. The common rule of thumb for the hole is guidance, not a calculation: a diameter of about 3 times the post width, and a depth of about 1/3 to 1/2 of the post height above ground; frost depth and local code can call for deeper holes. The metric toggle shows lengths in m and diameters and depths in cm, with the concrete in m³.

Quikrete, Setting Posts project sheet (SettingPosts.pdf): hole diameter about 3 times the post width; depth about 1/3 to 1/2 of the post height above ground; fast-setting mix poured dry into the hole, then watered
Quikrete Fast-Setting Concrete Mix (No. 1004) data sheet: 50 lb ≈ 0.375 cu ft, 60 lb ≈ 0.45 cu ft; sets hard in about 20 to 40 minutes
Quikrete Concrete Mix (No. 1101) data sheet: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft
Nominal and actual lumber sizes (American Softwood Lumber Standard, PS 20): a 4 × 4 is 3.5 × 3.5 in, a 6 × 6 is 5.5 × 5.5 in
Typical US retail prices for 50 lb fast-setting and 40 to 80 lb concrete mix bags, 2026 (estimate; fast-setting mix seen at about $13 to $15)
Local frost-depth and fence code requirements (check with your building department; they can set deeper holes than the rule of thumb)

How to calculate concrete for fence posts

Count the posts. Count every post that gets its own hole, gate and corner posts included.
Choose the hole size. Enter the hole diameter and depth. As a rule of thumb, dig about 3 times the post width across and about 1/3 to 1/2 of the post height above ground deep, and deeper where frost or code requires.
Pick the post size. Choose a 4 × 4, a 6 × 6 or a round post; its volume is taken out of each hole.
Calculate the concrete per hole. Hole volume (π × radius² × depth for a round hole) less the post (width × width × depth), in cubic feet.
Convert to bags. Multiply by the number of posts, add about 10% for waste, divide by the bag yield (0.375 cu ft for a 50 lb fast-setting bag, 0.60 cu ft for 80 lb of concrete mix) and round up once.

Worked examples

Project	Result
12 posts (4 × 4), round holes 10 in across and 24 in deep, 50 lb fast-setting bags, 10% waste	33 bags of 50 lb fast-setting mix: 0.92 cu ft per hole, 11 cu ft in all
8 posts (6 × 6), round holes 16 in across and 30 in deep, 80 lb bags of concrete mix, 10% waste	44 bags of 80 lb concrete mix: 2.97 cu ft per hole, 23.7 cu ft in all
20 round posts 4 in across, round holes 12 in across and 36 in deep, 60 lb bags, 10% waste	103 bags of 60 lb concrete mix: 2.09 cu ft per hole, 41.9 cu ft in all

Frequently asked questions

How many bags of concrete do I need per fence post?: For a 4 × 4 post in a round hole 10 in across and 24 in deep, about 0.92 cu ft of concrete, which is 2.5 bags of 50 lb fast-setting mix (0.375 cu ft each) or 1.5 bags of 80 lb concrete mix (0.6 cu ft each). Plan on about 2 to 3 bags of 50 lb mix for a typical post. Bigger or deeper holes take more: the table above lists common sizes.
Do I subtract the post from the hole volume?: Yes. The post fills the middle of the hole, so the concrete is only what is left around it. In a hole 10 in across and 24 in deep, the post takes 0.17 cu ft of the 1.09 cu ft hole. A 6 × 6 post (5.5 in) takes about 2.5 times as much as a 4 × 4, so leaving the post out overstates the bags for big posts. This calculator subtracts it for you.
How big should the hole be?: A common rule of thumb, which Quikrete’s setting-posts instructions give, is a hole about 3 times the width of the post: roughly 10 to 12 in for a 4 × 4 and about 16 to 18 in for a 6 × 6. It is guidance, not a code requirement, and a wider hole takes more concrete. Your local code, soil and frost depth may call for something different.
How deep should a fence post hole be?: The usual guidance is about 1/3 to 1/2 of the post height above ground (a 6 ft fence needs at least about 2 ft in the ground), and below the frost line where your code requires it. Quikrete also describes about 1/3 of the overall post length plus 6 in of gravel for drainage. Use the rule-of-thumb table on this page as a starting point, then check local code.
Is fast-setting concrete better for fence posts?: Fast-setting mix is popular for posts because you pour it dry into the hole around a plumb post and add water, with no mixing; Quikrete says it sets hard in about 20 to 40 minutes. A 50 lb bag makes about 0.375 cu ft, so it covers less than an 80 lb bag of regular mix (about 0.60 cu ft) and costs more per cubic foot, so for many posts regular mix is often cheaper if you are willing to mix it. Pick the bag you plan to buy in the calculator and the count follows its yield.
How many 80 lb bags for a 4 × 4 post?: About 1.5 of an 80 lb bag per post for a 10 × 24 in hole, so a bit more than one bag for every two posts: 12 posts take 21 bags with 10% waste. Round up once for the whole job, not for each post, since leftover concrete from one hole fills the next.
Do I need gravel under the post?: Quikrete’s instructions for setting posts put about 6 in of gravel in the bottom of the hole for drainage. The gravel replaces some of the depth, so a hole dug for a base is that much deeper, and the concrete above it is shorter. Enter the depth of the concrete itself as the hole depth, and use the gravel calculator for the base.
How many posts do I need for my fence?: Divide the fence length by the post spacing, round up and add one, plus one per gate. The fence calculator does it for you and also counts pickets, rails and concrete; enter the number of posts it gives here to compare bag sizes and hole sizes.

Handy tools

Feet and Inches Calculator: Turn a hole depth read off a tape, like 2 ft 7 1/2 in, into the decimal inches the fields above take.