How much concrete for tubes and posts?

How much concrete does a Sonotube take?

Multiply the tube's cross-section by its height: π × radius² × height. A 12-inch tube holds about 0.79 cubic feet per foot, so a 4-foot tube takes about 3.1 cubic feet — six 80 lb bags of concrete mix.

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

For a 4×4 post in a 10-inch hole 2 feet deep, over 6 inches of gravel, about 0.7 cubic feet: two 50 lb bags of fast-setting mix. That matches Quikrete's own rule of thumb of two 50 lb bags for a 4-inch post in a 10-inch hole 2 feet deep.

How big and deep should a fence post hole be?

Quikrete recommends a hole three times as wide as the post — about 10 to 12 inches for a 4×4 — and 1/3 to 1/2 as deep as the post stands above ground, so at least 2 feet for a 6-foot fence. Put about 6 inches of gravel in the bottom for drainage, and go below the frost line where your local code requires it.

What size Sonotube do I need?

Builder's tubes come in 6, 8, 10 and 12 inch diameters, and commercial tubes go much larger. 8 to 10 inches is typical for a 4×4 post and 12 inches for a 6×6. Deck footings are sized for the load and your soil, so check your local code or the deck plans; many piers also sit on a wider footing at the bottom.

Why subtract the post from the hole?

A post set in the hole takes up room the concrete doesn't have to fill. A 4×4 in a 10-inch hole takes up about a sixth of it, and a 6×6 in a 12-inch hole more than a quarter, so ignoring the post means buying bags you won't use.

Should I use fast-setting concrete or regular mix?

Fast-setting mix is made for posts: pour it dry into the hole around the post, add water, and it sets in 20 to 40 minutes, so the post stands on its own. Regular mix costs less per cubic foot and suits bigger pours such as tube footings. A 50 lb bag of either yields about 0.375 cubic feet.

Concrete per foot of Sonotube

In metric, a 25 cm tube holds about 49 liters per meter and a 30 cm tube about 71.