How the concrete is worked out
- Round: π × (diameter ÷ 2)² × depth
- Square: side × side × depth
- Multiply by the number of footings, add your extra, then divide by the bag yield and round up.
Example: six 12" footings, 36" deep
Each holds about 2.4 cu ft. Six of them plus 10% extra is 15.6 cu ft (0.58 cubic yards) — 26 × 80 lb bags or 35 × 60 lb bags.
Coming from the framing calculator? The number of footings is filled in for you.
Frequently asked questions
How many bags of concrete for a deck post?
A 12" round footing 36" deep holds about 2.4 cu ft — about 4 bags of 80 lb premix. Enter your size, depth and number of footings for the full count.
How deep should deck footings be?
Below your local frost line, and at least as deep as your code requires. Frost depth varies from a few inches to several feet across the US, so check with your building department.
How big should deck footings be?
Footing size depends on the load each post carries (its tributary area) and your soil. The IRC deck tables size footings by tributary area; county deck guides reproduce them (example). This calculator works out concrete for whatever size you choose.
Premix bags or ready-mix?
For a handful of footings, bags are easiest. Once you need more than about a cubic yard, ready-mix delivery can be cheaper and faster — the calculator shows cubic yards so you can compare.