Calculators · Post Hole Concrete Calculator

Concrete & masonry

Post Hole Concrete Calculator

Enter the hole and the post to get the concrete for each hole, and the bags of Fast-Setting and Concrete Mix.

Hole diameter

QUIKRETE says about 3 times the width of the post.

Hole depth

QUIKRETE says 1/3 of the overall post height.

Number of holes

Empty counts as 1.

Post

The post takes up room, so its volume is subtracted.

Gravel at the bottom

Optional. Taken off the concrete depth. Empty counts as 0.

Open space at the top

Optional. The Fast-Setting sheet leaves 3 to 4 in for soil or sod. Empty counts as 0.

Waste to add (%)

Optional. Empty counts as 0%.

How to calculate concrete for a post hole

The concrete in a post hole is the volume of the hole minus the volume of the post. The hole is a cylinder, so its volume is π × (diameter ÷ 2)² × depth. The post is a square or a circle times the same depth. The calculator works in inches and turns them into cubic feet, and it takes off the depth of any gravel at the bottom or any open space at the top before it multiplies.

Step	Formula
1. Hole	π × (hole diameter ÷ 2)² × filled depth
2. Post	width × width × filled depth, or π × (post diameter ÷ 2)² × filled depth
3. Concrete per hole	hole − post
4. Bags	concrete per hole × number of holes ÷ yield of the bag, rounded up

Example: one 4x4 post in a 12 inch hole, 30 inches deep

A 4x4 wood post measures 3.5 inches across in real size, according to SPIB (checked October 2026). It goes in a 12 inch hole that is 30 inches deep, which is 2.5 feet.

Step	Operation	Result
Hole	π × 0.5² × 2.5	1.9635 ft³
Post	(3.5 ÷ 12)² × 2.5	0.2127 ft³
Concrete	1.9635 − 0.2127	1.7508 ft³ = 0.0648 yd³
Fast-Setting, 50 lb	1.7508 ÷ 0.375 = 4.67, rounded up	5 bags
Fast-Setting, 60 lb	1.7508 ÷ 0.45 = 3.89, rounded up	4 bags
Concrete Mix, 80 lb	1.7508 ÷ 0.60 = 2.92, rounded up	3 bags
Concrete Mix, 60 lb	1.7508 ÷ 0.45 = 3.89, rounded up	4 bags

The yields are the manufacturer’s figures: 0.375 cubic feet for a 50 lb bag and 0.45 for a 60 lb bag of Fast-Setting Concrete Mix (data sheet revised November 2025), and 0.30, 0.375, 0.45 and 0.60 cubic feet for the 40, 50, 60 and 80 lb bags of Concrete Mix (revised October 2022). All of them are checked October 2026 and are approximate.

Hole size: the QUIKRETE rules

Both QUIKRETE data sheets say to dig the hole about 3 times the diameter of the post, with a depth of 1/3 of the overall post height. The calculator on QUIKRETE’s site says the same thing in other words, with the hole depth at one half of the height above ground.

Overall post length	Hole depth, 1/3 of the length	Above ground
6 ft	24 in (2 ft)	4 ft
8 ft	32 in (2 ft 8 in)	5 ft 4 in
9 ft	36 in (3 ft)	6 ft
10 ft	40 in (3 ft 4 in)	6 ft 8 in
12 ft	48 in (4 ft)	8 ft

Your local code or the fence plans can ask for more, so use them if they do. For a post that carries a load, QUIKRETE’s Concrete Mix sheet says to follow your local building code for the footing specifications.

Bags per hole with the QUIKRETE method

This table is for one hole, a round post and a hole 3 times as wide, filled to the top with 50 lb bags of Fast-Setting Concrete Mix. It is the result of the formula in the script of QUIKRETE’s own calculator, which subtracts the post and divides by 0.375 cubic feet (checked October 2026). The calculator on this page gives the same bags when you choose a round post and leave the gravel and the open space empty.

Round post / hole	24 in deep	30 in deep	36 in deep
2 in post / 6 in hole	1	2	2
3 in post / 9 in hole	3	3	4
4 in post / 12 in hole	4	5	6
6 in post / 18 in hole	9	11	13

4x4 posts in 10 and 12 inch holes

A 4x4 is a square post, so its cross section is 3.5 × 3.5 inches. These figures are for one hole, no gravel and no open space at the top.

Hole	Concrete	Fast-Setting 50 lb	Concrete Mix 80 lb
10 in wide, 24 in deep	0.92 ft³	3 bags	2 bags
10 in wide, 30 in deep	1.15 ft³	4 bags	2 bags
10 in wide, 36 in deep	1.38 ft³	4 bags	3 bags
12 in wide, 24 in deep	1.40 ft³	4 bags	3 bags
12 in wide, 30 in deep	1.75 ft³	5 bags	3 bags
12 in wide, 36 in deep	2.10 ft³	6 bags	4 bags

Fast-Setting or Concrete Mix?

Fast-Setting. According to its data sheet, you pour the dry mix into the hole until it is 3 to 4 inches from the top, add about 1 gallon of water per 50 lb bag, fill the top with soil or sod and wait 4 hours before the post takes any strain. The mix sets in 20 to 40 minutes. For holes deeper than 30 inches, you add the water in two steps.
Concrete Mix. The data sheet says to place 6 inches of dry mix in the bottom of the hole, position the post, and add concrete that you mixed with water first. Wait 24 hours before the post is subjected to any strain.
Bags are rounded up for all the holes together, so one bag can serve two holes. The result also shows the bags per hole before rounding.

For a post inside a cardboard tube, see the Sonotube calculator. For round columns without a post, see the concrete calculator. QUIKRETE’s own tube sheet (QUIK-tubes) puts about 2 inches of gravel in the bottom of the hole. For footings under walls and decks, use the concrete footing calculator. For the rails and pickets of the fence, use the fence calculator. This page does not show prices, because they change by region and supplier.

Frequently asked questions

How many bags of concrete do I need for a fence post?

It depends on the hole and the post. A 4x4 post (3.5 inches across) in a 12 inch wide and 30 inch deep hole needs 1.75 cubic feet of concrete once the post is subtracted. That is 3 bags of 80 lb QUIKRETE Concrete Mix, or 5 bags of 50 lb Fast-Setting Concrete Mix, rounded up. The calculator does this for any hole and counts all the holes together.

How big should a post hole be?

QUIKRETE’s data sheets say to dig the hole about 3 times the diameter of the post, with a depth of 1/3 of the overall post height. For a 4x4 that measures 3.5 inches, three times is 10.5 inches. A 9 foot post goes 3 feet into the ground.

Do I subtract the post from the concrete volume?

Yes. The post takes up room in the hole, so the concrete is the volume of the hole minus the volume of the post. QUIKRETE’s own calculator does it that way. Choose “Do not subtract the post” in the menu if you want the full volume of the hole.

How much water does Fast-Setting Concrete need?

For the dry-pour method, QUIKRETE’s data sheet says to pour the dry mix to 3 to 4 inches from the top and add about 1 gallon of water for each 50 lb bag, depending on soil conditions. For holes deeper than 30 inches, fill to 30 inches or less, wait until the water soaks into the mix, then add the rest. It sets in 20 to 40 minutes, and the data sheet says to wait 4 hours before the post is under any strain.

Should I put gravel in the bottom of the hole?

It depends on your plans, and we did not find a manufacturer figure for loose posts. The QUIKRETE Concrete Mix data sheet says to place 6 inches of dry concrete mix in the bottom of the hole for fence posts. For its own QUIK-tube, QUIKRETE says to place about 2 inches of gravel. If you use gravel, type its depth in the gravel box and the calculator takes it off the concrete depth.

Why does the QUIKRETE data sheet say two bags for a hole that the calculator says takes three?

The Fast-Setting data sheet says two 50 lb bags will set a 3 inch square post in a 9 inch hole that is 30 inches deep. The hole minus the post is 0.95 cubic feet and a 50 lb bag yields about 0.375 cubic feet, so the volume works out to 2.53 bags, which rounds up to 3. The formula in QUIKRETE’s online calculator also gives 3 bags for a 3 inch round post in that hole. We do not know how the data sheet arrives at two, so the calculator rounds up.

How many bags do I need for 10 fence posts?

Work out the concrete for one hole and multiply by 10, then add waste. With 4x4 posts in 12 by 30 inch holes and 10% waste that is 19.26 cubic feet, which is 33 bags of 80 lb Concrete Mix or 52 bags of 50 lb Fast-Setting. Type 10 in the number of holes and the calculator gives the same figures.