Deck piers, fence posts and pergola footings are all cylinders. This calculator finds the concrete in each one and the total for the job, so you buy the right number of bags in one trip.
Formula
Volume of a cylinder = π × r² × depth, where r is half the diameter. A 12" hole that is 42" deep holds π × 0.5² × 3.5 ≈ 2.75 ft³, or roughly 5 bags of 80 lb premix.
Setting the footing depth
Footings must reach below the local frost line so freeze-thaw cycles do not heave them. Frost depth ranges from a few inches in warm climates to more than 48" in cold ones; your local building department publishes the number. Bell-shaped bottoms and wider footing pads may also be required for deck loads.
Layout first
Pier locations must form a square grid or the beams will not sit straight. Set string lines on batter boards, square them with 3-4-5, and mark the hole centres where the strings cross. Our batter board guide walks through it.
Watch: how it is done
Video by Contractor John on YouTube. More in our video library.
Frequently asked questions
How many bags of concrete per fence post?
A typical 4x4 post in a 10" × 36" hole needs about 1.6 ft³ including the post, which is two to three 50 lb bags or two 80 lb bags.
Should the post sit in concrete or on gravel?
Many builders put a few inches of gravel in the bottom for drainage, then concrete. Deck posts are often set on top of the pier with a metal base instead of being buried.
Does the calculator subtract the post volume?
No. It returns the full hole volume; a 4x4 post displaces only about 0.11 ft³ per foot of depth, which is covered by the waste allowance.
Results are planning estimates. For structural work, confirm requirements with your local building authority and a qualified professional. See our disclaimer.