Concrete footing calculator
For strip footings under walls and foundations, and square pad footings under posts. Enter the total length of the trench, its width and its depth, and the calculator gives cubic yards to order, bag counts and cost.
Calculator
Your estimate
Enter your measurements and select Calculate to see how much to order.
- Concrete to order
- cu yd
- Exact volume
- cu yd / cu ft
- Bags of 80 lb mix
- Cost with bags
- Cost with ready-mix
- Best way to buy
How it is calculated: length × width × thickness in feet gives cubic feet. Divide by 27 for cubic yards, then add your extra for waste and round up to the next quarter yard.
How the footing calculator works
A footing is a long, narrow slab, so its volume is length times width times depth. Length is measured in feet along the trench, while width and depth are small enough that inches are the natural unit, so both are divided by 12.
Volume (cu ft) = length (ft) × width (in) ÷ 12 × depth (in) ÷ 12
Cubic yards to order = volume ÷ 27 × (1 + waste %), rounded up to the quarter yard
Worked example
A strip footing around a 20 × 20 ft shed, 16 in wide and 8 in deep. The trench runs 80 ft in total:
- Width in feet: 16 ÷ 12 = 1.333 ft. Depth in feet: 8 ÷ 12 = 0.667 ft
- Volume: 80 × 1.333 × 0.667 = 71.1 cu ft
- Cubic yards: 71.1 ÷ 27 = 2.63 cu yd
- With 10% waste: 2.90 cu yd, so order 3 cu yd, or buy 131 bags of 80 lb mix
Footing trenches are dug, not formed, so the sides are rarely straight and the bottom is rarely level. Many pours use more than the math says, which is why the 10% matters more here than on a formed slab.
What to enter
- Total length. For a perimeter footing, add the four sides. Measure along the centre of the trench. For several pad footings, enter the length of one and use the count field.
- Width. The trench width, or the pad width. Footings are often wider than the wall they carry; a common rule is twice the wall thickness, but your code or engineer sets it.
- Depth. The thickness of the concrete. If the trench is deeper than the footing and the rest will be backfilled or built up with block, enter only the concrete depth.
- Number of footings. For identical pad footings under deck posts or a pole barn, enter how many.
Limitations
- Footing size and depth are structural decisions set by soil bearing capacity, frost depth and the load above. This calculator estimates material only. Check your local code’s frost line and minimum footing size.
- Stepped footings on a slope, footings with a keyway, and footings with a stem wall need to be calculated as separate pieces.
- Rough trench walls can add 10% or more to the real volume. Measure the actual trench, not the drawing.
- Round footings and piers use the column calculator.
Frequently asked questions
How much concrete do I need for a footing?
Multiply the total length of the trench in feet by the width and depth in feet, then divide by 27 for cubic yards. A 100 ft footing that is 16 in wide and 8 in deep is 100 × 1.33 × 0.67 = 88.9 cu ft, or 3.3 cu yd before waste.
How deep should a footing be?
Deep enough to sit below the frost line in your area and on undisturbed soil, with a concrete thickness that suits the load. Both are set by your local building code, which is the place to check before you dig.
Can I pour a footing with bags?
For a few pad footings or a short wall, yes. For a full perimeter footing of several cubic yards, a ready-mix truck is the usual choice because the whole footing should be poured in one continuous operation.