How to use the concrete footing calculator
- Choose the footing type: a square or rectangular pad, a round pier (such as a tube form), or a continuous strip footing under a wall.
- Enter the dimensions shown for that type, the depth (thickness) of concrete, and how many identical footings there are.
- Add a waste allowance and choose the bag size you plan to buy.
- Optionally enter a price per bag or per cubic yard to compare costs.
How the calculation works
Rectangular pads and strip footings are rectangular prisms: length × width × depth. Round piers are cylinders: π × (diameter ÷ 2)² × depth. The volume for one footing is multiplied by the quantity, the waste allowance is added, and the total is converted to cubic yards and cubic feet.
Bag counts divide the total cubic feet by the yield of the chosen bag and round up. Yields are the typical figures printed on standard pre-mixed concrete bags: about 0.30 ft³ for 40 lb, 0.375 ft³ for 50 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb.
Formula
Worked examples
Four 2 × 2 ft pads, 12 in deep
Each pad: 2 × 2 × 1 = 4 ft³. Four pads: 16 ft³. With 10% waste: 17.6 ft³ = 0.65 yd³.
With 80 lb bags: 17.6 ÷ 0.60 = 29.3 → 30 bags.
Six 12 in round piers, 4 ft deep
Each pier: π × 0.5² × 4 = 3.142 ft³. Six piers: 18.85 ft³; with 10% waste 20.73 ft³ (0.77 yd³), or 35 bags of 80 lb mix.
Assumptions and limitations
- This tool calculates volume only. Footing size, depth below frost line, reinforcement and bearing capacity must follow your local building code or an engineer’s design.
- Bag yields are typical values; check the bag you buy, as yields vary by product.
- Belled or flared pier bottoms and stepped footings are not modelled; calculate those sections separately.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
A cubic yard is 27 ft³. At about 0.60 ft³ per 80 lb bag, that is 45 bags. With 60 lb bags (0.45 ft³) it is 60 bags.
How deep should a footing be?
It depends on your local code, the frost depth in your area, the soil and the load. This calculator does not determine footing depth. Check with your building department or a structural engineer.
How much concrete does a 12 inch tube form hold per foot?
A 12 in diameter round form holds π × 0.5² = 0.785 ft³ per foot of depth, a little more than one 80 lb bag per foot.