Deck Footing Concrete Calculator
Enter how many footings you are pouring, their diameter, and their depth. The calculator gives the concrete volume and the number of premixed bags to buy.
It does not size the footings. The diameter, depth and frost-depth requirement depend on the loads, your soil and your local code, so confirm those with your building department first.
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Worked example
Inputs: number of footings 8; footing diameter 12 in; footing depth 36 in; waste allowance 10.
- Radius: 12 in ÷ 2 = 6 in = 0.5 ft. Depth: 36 in = 3 ft.
- One footing: π × 0.5² × 3 = 2.356 cubic feet.
- Eight footings: 8 × 2.356 = 18.85 cubic feet.
- Add 10% waste: 18.85 × 1.10 = 20.73 cubic feet = 0.77 cubic yards (÷ 27).
- Bags: an 80 lb bag yields about 0.60 cubic feet, so 20.73 ÷ 0.60 = 34.6, round up to 35 bags. A 60 lb bag yields about 0.45, so 47 bags.
Result: 0.77 yd³ concrete to order (with waste).
Where the formula comes from
Volume of a cylinder. Bag yields are typical published figures for premixed concrete (about 0.60 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag); check the yield printed on your bag. Footing requirements: IRC Section R507.3 and local frost depth.
Estimates only. Results are planning figures, not engineering or code advice. Check your local building code and confirm quantities with your supplier.
Frequently asked questions
How deep should deck footings be?
At least to the frost depth where you live, and deeper if the soil or loads require it. Frost depth varies widely by location, so ask your local building department.
Do I need a tube form?
Many builders use cardboard tube forms to keep the hole clean and the diameter consistent. If you use one, enter its inside diameter here.
Why add waste?
Holes are rarely perfectly round and spillage happens. 10% is a common allowance; use more if the holes are rough or oversized.
Should I order bags or a truck?
Hand-mixing dozens of bags is a lot of work. As a rough guide, many people consider ready-mix delivery once the volume approaches a yard or more, but delivery minimums vary, so ask your supplier.