Post Hole & Sonotube Concrete Calculator

Q: How many bags of concrete per fence post?

For a typical 10 in hole 3 ft deep with a 4x4 post and 6 in of gravel, about 2 bags of 80 lb concrete (1.15 cu ft per hole ÷ 0.60 cu ft per bag = 1.9). A wider or deeper hole needs more; use the calculator with your sizes.

Q: Does the post really reduce the concrete needed?

Yes. A 4x4 post takes up 12.25 sq in of the hole's cross-section; in a 10 in hole that is about 16% of the volume.

Enter your hole or form-tube size and how many you are pouring. The calculator subtracts the gravel at the bottom and the space the post takes up, then converts the concrete volume to 40, 60 or 80 lb bags of premixed concrete, or to cubic yards if you are ordering ready-mix.

Post holes & sonotubes calculator

How it works

Each hole or tube is a cylinder. The concrete fills the part above the gravel, less the post:

Hole area = π × diameter² ÷ 4 (square inches)
Concrete height = depth below grade − gravel + height of tube above grade
Concrete per hole = (hole area × concrete height − post area × post length in concrete) ÷ 1,728 → cubic feet
Bags = total cubic feet × (1 + extra %) ÷ bag yield, rounded up

Bag yields are the manufacturer’s figures for premixed concrete: about 0.30 cu ft from a 40 lb bag, 0.45 cu ft from a 60 lb bag and 0.60 cu ft from an 80 lb bag. One cubic yard is 27 cubic feet.

Worked example

Four fence post holes, 10 in across and 36 in deep, with 6 in of gravel in the bottom and a 4×4 post in each:

Number of holes or tubes: 4
Hole or tube diameter: 10 in
Depth below grade: 36 in
Tube height above grade: 0 in
Gravel in bottom of hole: 6 in
Post set in the concrete: 4×4 post (3.5 x 3.5 in)
Bag size: 80 lb bag (about 0.60 cu ft)
Extra for waste and spillage: 10%

The calculator gives:

Bags of concrete: 9 bags (80 lb)
Concrete volume: 4.60 cu ft (0.170 cu yd, 0.130 m³)
Per hole: 1.151 cu ft
With 10% extra: 5.06 cu ft (0.19 cu yd)

Each hole holds (78.54 − 12.25 sq in) × 30 in = 1,989 cubic inches, or 1.15 cu ft. Four holes need 4.60 cu ft, and with 10% extra that is 5.06 cu ft, or 8.4 bags, so buy 9.

Material and waste notes

Hole sizes are rarely exact. Augered holes flare at the top and hand-dug ones are usually wider than planned, so measure a finished hole before you buy the last few bags.
The extra percentage is an allowance you choose for spillage and oversized holes. It is not a figure from a standard.
For a form tube that stands above the ground, enter the above-grade height. The tube is filled to the top.
Round steel posts displace very little concrete, so the result is close to a solid pier.
Post depth, hole diameter and whether you need a footing below the pier are set by your local code, the fence or deck design, and the frost depth where you live.

Frequently asked questions

How many bags of concrete per fence post?

For a typical 10 in hole 3 ft deep with a 4×4 post and 6 in of gravel, about 2 bags of 80 lb concrete (1.15 cu ft per hole ÷ 0.60 cu ft per bag = 1.9). A wider or deeper hole needs more; use the calculator with your sizes.

How many 80 lb bags make a cubic yard?

At 0.60 cu ft per bag, 27 ÷ 0.60 = 45 bags per cubic yard. With 60 lb bags it is 60, and with 40 lb bags 90.

Does the post really reduce the concrete needed?

Yes. A 4×4 post takes up 12.25 sq in of the hole’s cross-section; in a 10 in hole that is about 16% of the volume.

How do I work out concrete for a sonotube?

Enter the tube diameter, how deep it goes below grade and how far it sticks up, set the post to “None” and the gravel to whatever you place under the tube.

Sources

Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.