How to Calculate Concrete Volume for Columns and Footings

Two shapes cover almost every footing

A round pier is a cylinder: volume = π x radius² x depth. A square or rectangular pad footing is a box: volume = length x width x depth. Everything else — bell-bottom piers, stepped footings — is a combination of the two.

Work in feet throughout. Convert inches by dividing by 12 before you multiply anything.

Cylinder volumes at a glance

Per foot of depth, a round footing consumes:

Diameter	Cu ft per ft of depth	80 lb bags per ft	Typical use
8 in	0.35	0.6	Fence posts, light pergola
10 in	0.55	0.9	Deck piers, small posts
12 in	0.79	1.3	Standard deck footing
16 in	1.40	2.3	Heavy deck / porch column
18 in	1.77	3.0	Beam pier, carport post
24 in	3.14	5.2	Structural column, addition pier

Frost depth drives your quantity

Footing depth is set by your local frost line, not by preference. Southern Florida and coastal California may allow 12–18 inches; the Upper Midwest and northern New England commonly require 42–60 inches. Doubling depth doubles concrete, so confirm the local requirement before you estimate.

Step-by-step

1. 1

   Confirm footing size and depth with your building department

   Call or check the online code page for two numbers: frost depth and minimum bearing area. Most jurisdictions publish a table matching tributary load to footing diameter. A typical 12x16 residential deck needs 12-inch piers at frost depth with 16-inch bell bottoms or pad footings in poor soil.

2. 2

   Convert every dimension to feet

   A 12-inch tube is 1 ft in diameter, so radius = 0.5 ft. A 42-inch depth is 42 ÷ 12 = 3.5 ft. Doing this before you touch the formula eliminates the most common source of order errors.

3. 3

   Calculate one pier

   Volume = π x r² x depth = 3.1416 x 0.5² x 3.5 = 2.75 cubic feet. At 0.60 cu ft per 80 lb bag, that is 4.6 bags — call it 5 bags per pier.

4. 4

   Multiply by the pier count and add the pads

   Nine piers x 2.75 cu ft = 24.75 cubic feet, or 0.92 cubic yards. If code requires a 24 x 24 x 8 inch pad under each pier, add 2 x 2 x 0.667 = 2.67 cu ft each, another 24 cubic feet — which nearly doubles the pour to 1.8 cubic yards.

5. 5

   Add waste and decide bags vs truck

   Add 10% for over-dug holes and soil sloughing: 1.8 x 1.10 = 2.0 cubic yards. That is 90 bags of 80 lb mix — well past the practical DIY limit. Order ready-mix and place it with a wheelbarrow or a pump.

6. 6

   Set rebar and anchors before the pour

   Most codes require two to four vertical #4 bars in piers over 12 inches in diameter, with 3 inches of concrete cover. Set post base anchors in the wet concrete while it is still plastic, check them against a string line, and re-check after 20 minutes — they drift.

7. 7

   Cure before loading

   Concrete reaches roughly 70% of design strength in 7 days and 100% at 28 days. You can set posts after 48 hours in warm weather, but do not load the structure with framing and decking until day 7 minimum.

Safety note: this guide is educational. Always follow local building codes, pull required permits, and hire a licensed pro for structural, electrical, gas or plumbing work when in doubt.