Calculate exact bags or cubic yards of concrete for round footings, sonotube forms, fence posts, and deck piers.
Our standard Concrete Bag Calculator is built for rectangular slabs. But fence posts, deck footings, and sonotube piers are cylinders, not boxes. A cylinder's volume is calculated as π × radius² × depth, not length × width × depth. Skip this step and you'll either order way too much concrete or run short mid-pour.
Footing depth is dictated by your local frost line, not preference. In cold climates, footings must extend below the frost depth or seasonal freeze-thaw cycles will heave the post out of the ground over a few winters.
No — a sonotube (the cardboard concrete form) doesn't add or remove volume from the math. It just gives the wet concrete a smooth, straight-sided shape above the frost line so the post sits cleanly instead of bulging out into the surrounding dirt. Measure the inside diameter of the tube, or the diameter of the auger bit if you're pouring directly into a bare hole.
An 8" diameter sonotube at 30 inches (2.5 ft) deep needs about 0.87 cubic feet of concrete, which works out to roughly 2 standard 80lb bags per hole.
A 12" diameter hole at 2 feet deep needs about 1.57 cubic feet, or roughly 3 bags of 80lb concrete per hole.
Yes. Add 4-6 inches of compacted gravel at the bottom of any post hole before pouring concrete. This allows water to drain away from the base of the post instead of pooling and rotting wood or rusting metal posts.
For small jobs like a single mailbox post, yes — many contractors pour the dry bag directly into the hole, add water, and mix in place with a stick or shovel. For multiple holes or larger piers, pre-mixing in a wheelbarrow or mixer gives more consistent results.